Understanding the potential for biological control and pest management in industrial hemp cropping systems

1. Insect pests and biological control agents in Pennsylvania hemp crops throughout the growing season.

Description of Hemp Fields 

To ensure representative sampling across various ecological conditions during the insect survey in the 2022 growing season, we selected seven hemp fields across six counties in Central and Southeastern Pennsylvania. Four farms grew hemp for fiber/grain; the other three were planted with CBD hemp. According to the farmer's practices, the hemp fields had single or mixed varieties within the same plot. The number of samplings at each farm varied based on harvest time, with a more extended sampling period for CDB fields (See Table 1). Every week from June until October, we visited the fields to observe insect damage signs on the plants and record, count, and collect the insects directly interacting with these crops. We also installed yellow sticky traps to give a better sense of the insects flying between crop plants over more extended periods of time.

Table 1. Hemp farms surveyed for insect communities in Central and Southeastern Pennsylvania during the 2022 growing season.

The hemp fields presented diverse crop management practices across cultivars. Most were established in agricultural lands, often surrounded by corn crops, with a few located near urban zones. Fiber and grain fields were planted with high densities either using no-till or conventional tilling methods. In contrast, CBD plots were planted with lower densities in smaller areas using black plastic mulch beds, fabric mulch, or natural soil covers for weed control. Harvest timing and production cycle also differed. CBD generally had a more extended growing period than fiber hemp (Figure 1).

In addition to our project funded by the Northeast SARE program, we secured additional funding from the Pennsylvania Department of Agriculture to enhance the scope of our research across the state. This allowed us to conduct a second year of insect sampling throughout the 2023 growing season in the central and southeastern counties. To explore variations in the insect community composition among cultivars, locations, and hemp types, we included twenty individual variety plots established within two experimental and three commercial farms.

We selected the same six fiber/grain varieties for surveying at each experimental farm, resulting in 12 sampling sites. In the commercial farms, six plots were established with different CBD cultivars, while two of the plots cultivated fiber varieties (See Table 2). We conducted vacuum sampling in each site, yellow sticky and pitfall traps. Due to the increased number of samples, the identification process is still ongoing; however, the results of this second sampling year will be shared through Penn State Extension in field days, conferences, and a scientific publication.

Table 2. Hemp farms surveyed for insect communities in Central and Southeastern Pennsylvania during the 2023 growing season.

Insect Survey 2022: Visual Sampling and Yellow Sticky Traps

During the 2022 growing season, we used two sampling methods to increase our knowledge of the insect communities in hemp crops in the Northeast region. Visual sampling allowed us to detect insects directly interacting with the hemp plants and observe their populations, behaviors, damage, and impacts on the crop. We recorded and identified 20,915 arthropods, including insects and arachnids (Table 3). Several weaknesses were noted using visual sampling. This method may not capture all insect species, particularly small parasitoid wasps and leafhoppers that can be easily disturbed and may evade observation while approaching and manipulating the plants. In addition, This method can be time-consuming, making it less practical and more costly to sample across a larger number of sample sites (hemp farms. 

The yellow sticky traps allowed us to monitor a broad range of flying insects for longer periods during sampling and to complement the visual sampling, addressing some of these potential biases. These traps captured significantly more specimens than visual sampling (128,368 specimens). Both methods captured a similar number of families and morphospecies. Hemiptera and Thysanoptera were highly detected in both types of sampling; however, visual sampling was more effective in detecting spiders, while sticky cards were more successful in capturing Hymenopterans and Dipterans (Table 3).

When categorized by functional groups, herbivorous insects were the most predominant group observed in yellow sticky traps and visual sampling, representing between 67.6% and 89.8% of the collected specimens, respectively. Natural enemies were the second most abundant insect group, better represented on the sticky cards than in visual sampling. Incidental species—not directly related to the crop—accounted for 1%, and pollinators contributed less than 1% (Figure 2).

Table 3. Summary of specimens captured in hemp crops using visual sampling and yellow-sticky cards during the 2022 insect survey.

Differences in insect community composition in hemp crops

Exploring the differences between the insect communities found in Pennsylvania hemp crops can help us understand how herbivores and natural enemies vary across different hemp types, varieties, locations, and other factors that might influence their populations within the crops. To assess these differences, we first visualized how distant these insect communities are between samples using an NMDS plot (Non-Metric Dimensional Scaling). Then, we assessed whether these differences were statistically significant using a permutational multivariate analysis of variance (PERMANOVA).

Figure 3 shows the community composition in CBD and fiber crops. Each point in the graph represents the insect communities sampled in CBD (teal points) and fiber crops (red points). The proximity of the points indicates how similar the communities are. The samples for CBD and fiber form separate and distant clusters, indicating distinct differences in the insect communities between the two types of hemp. The PERMANOVA analysis confirmed that the composition of insect species significantly varied between CBD and fiber crops (F = 8.5261, p = 0.001). This can be explained by the differences in plant structure, cultivation practices, and chemical composition between these cultivars. Additionally, the fiber samples appear more dispersed from the central point than those in CBD, indicating greater variability in the insect communities within the fiber crops.

From this, we visualized the insect communities across hemp farms. While each farm seems to have distinct communities, these overlap in CBD farms. Interestingly, the arthropod communities across our fiber plots did not overlap. The PERMANOVA analysis confirmed that insect communities are significantly different between hemp farms (F = 5.6232, p = 0.001), indicating that aside from the type of hemp, there are factors specific to the sample sites that are shaping the composition of the insect community, especially in the fiber farms (Figure 4). 

Key differences in our fiber farms include the landscape type—urban versus agricultural—the surrounding vegetation, prior crops, and practices such as tillage or non-tillage, mixed varieties, and varying plant densities. The upcoming analysis will delve deeper into these communities, focusing on their distribution across sampling sites, types of hemp, occurrence over time, and diversity.

Prevalent Insect Pests Interacting with Hemp Crops in Pennsylvania

The most prevalent insect pests interacting with hemp crops in Pennsylvania were determined based on the herbivore species with the highest relative abundance. Relative abundance measures the proportion of each herbivore species to the total number of individuals from all species collected during the insect samplings. To better understand the ecological roles of herbivore species with similar niches and functions, we grouped the specimens by families, calculated the relative abundance of herbivore families for each sample (plant or sticky card), and estimated the mean relative abundance in each sampling event, farm, hemp type, and crop phenological stage to observe patterns. 

Based on the visual samplings, aphids (Aphididae) and thrips (Thripidae) are the insect families with the highest mean relative abundance; together, their abundance represents 50% of the herbivores feeding on hemp. Leaf beetles (Chrysomelidae), leafhoppers (Cicadellidae), stink bugs (Pentatomidae), and plant bugs (Miridae) were also abundant but less prevalent than aphids and thrips (Figure 5). In the sticky traps, leafhoppers (Cicadellidae) were the most abundant herbivore group, representing 40%. These insects are more likely to be detected in sticky traps than in visual samplings due to their distinct jumping behavior to evade predators and move between plants. Thrips were the second most prevalent pests, with a relative abundance of 25%, consistent with the visual observations, followed by whiteflies (Aleyrodidae), leaf beetles (Chrysomelidae), and plant bugs (Miridae). The aphid population was still significant but less abundant in this sampling method. The aphid population in sticky traps was represented mainly by alate aphids (Figure 5).

Herbivore Families in Hemp Crops

The composition of these prevalent insect pests varied across hemp farms and sampling methods. In the visual samplings, aphids (Aphididae), thrips (Thripidae), leaf beetles (Chrysomelidae), and leafhoppers (Cicadellidae) were widely distributed across all hemp farms, while stink bugs and plant bugs were found in 86% of the sampling sites. Their relative abundance showed different patterns. In Lazy Moon, Rock Springs/CBD, Shaffer, and Cedar Meadow farms, the insect pest communities were dominated by aphids, followed by thrips and leaf beetles. Meanwhile, in Coexist, Rock Springs/fiber, and SEAREC, the populations of insect pests were more evenly distributed, with thrips being slightly more abundant. On the other hand, the yellow sticky traps confirmed that leafhopper and thrips populations play an important role in the composition of insect pests across hemp farms (Figure 6).

Prevalent Insect Pest in Hemp Farms

The group of prevalent insect pests was the same in CBD and fiber crops; however, their relative abundance had different degrees of relevance for each type of hemp during the 2022 growing season. In the visual samplings, aphid populations were considerably more important for CBD hemp than they were for fiber, where aphids held second place. Thrips were more relevant for fiber crops than they were for CBD hemp. Leafhoppers and leaf beetles had the same level of representation for both types of hemp, while plant bugs and stink bugs were slightly more relevant for fiber crops. Interestingly, the sticky traps showed consistency regarding the importance of these herbivores for CBD and fiber (Figure 7). This indicates that the interactions of these herbivores with the crop will have a different impact on CBD and fiber, which has implications for pest management and helps to refine pest monitoring priorities for both crop systems.

Prevalent Insect Pest in CBD and Fiber Hemp

Dynamics of Common Pests Throughout the Crop Cycle in CBD and Fiber Hemp.

Recognizing the differences in pest insect communities in hemp helped us identify the dominant herbivores in each type of hemp. Not every prevalent pest performs or interacts with CBD and fiber plants in the same way, resulting in different degrees of pest dominance and varying impacts of herbivory. To understand the extent of the risks that these prevalent pests pose to each type of hemp and to plan accordingly for the measures to mitigate their impact, the next step was to determine when in the lifecycle of the crops these prevalent insects occur, dominate, or increase their populations. Figure 8 shows the mean relative abundance of each prevalent pest for CBD and fiber crops at each phenological stage. This information will be used to inform pest management practices, targeting those herbivores that have the potential to be more problematic.

• Aphids (Hemiptera: Aphididae)

During the visual sampling conducted in 2022, the cannabis aphid (P.  cannabis) (Figure 9) emerged as the most prevalent pest affecting hemp crops in Pennsylvania. This exotic pest specializes in plant species within the hemp family (Cannabaceae) and was first documented as a pest of hemp in the United States in 2016. Initial reports indicated its presence in hemp grain cultivars in Colorado and indoor cultivation facilities in Virginia (Cranshaw et al., 2018). Since then, the cannabis aphid has disseminated across several states where hemp is cultivated.

In Pennsylvania, P. cannabis was detected in both CBD and fiber hemp crops and was consistently present at our sampling sites across central and southeastern counties (Figure 6). The aphid populations were significant, with their presence first observed during the vegetative phase of the crops. However, in CBD hemp, the highest populations occurred throughout the post-flowering stage, persisting until harvest (Figure 8). Aphid infestations were notably more abundant in CBD hemp compared to fiber hemp (Figure 7). Typically, the production cycle for CBD is longer than that for fiber, resulting in an extended period of vulnerability for CBD plants, where aphid infestations can be problematic. This highlights the importance of understanding their impact on crop yield and plant metabolites, such as THC. These sap-sucking insects can cause direct damage to plants by extracting nutrients and indirect damage, such as the production of honeydew, which promotes the growth of sooty mold and the transmission of plant viruses, including cucumber mosaic virus (CMV) and alfalfa mosaic virus. (AMV).

• Herbivore Thrips (Thysanoptera: Thripidae) 

Three generalist herbivore thrips species were frequently observed in hemp crops: onion thrips (Thrips tabaci), western flower thrips (Frankliniella occidentalis), and soybean thrips (Neohydratothrips variabilis). These rasping-sucking insects consume the cellular contents of leaves, resulting in punctures and scrapes that can diminish the foliage's appearance and increase the plants' susceptibility to pathogens (Gill et al. 2015) (Figure. 10). The presence of thrips in hemp is of particular concern due to their ability to feed on a wide range of host plants, survive through winter in soil and plant debris, and attack hemp crops since the establishment. Despite a significant abundance of thrips in most of the farms where we conducted the insect samplings, we did not observe detrimental damages that could affect the quality of the crops; however, in our experience growing hemp indoors for experiments, we observed significant thrip damage, especially in smaller plants, resulting in reduced plant vigor and quality.

Thrips populations were notably prevalent during the vegetative stage of the crops, especially for fiber,  and declined as they progressed toward the post-flowering stage (Figure 12). In relation to the type of hemp, thrips populations were more abundant in fiber hemp than in CBD (Figure 7).

• Leaf Beetles (Coleoptera: Chrysomelidae): Flea Beetles and Spotted Cucumber Beetles.

Various chrysomelid species were often observed in hemp crops, particularly several flea beetle genera, such as Altica sp., Systena sp., and Crepidodera sp., and spotted cucumber beetles (Diabrotica undecimpunctata) (Figure 11). These beetle populations were similarly prevalent in CBD and fiber hemp (Figure 7), reaching peak levels during the flowering periods  (Figure 8).

Flea beetles are known to cause significant damage by feeding on leaves, creating characteristic small holes that could reduce the plants'  photosynthetic capacity and stunt growth if infestations are severe. In addition to foliage damage, spotted cucumber beetles are known vectors of plant pathogens, such as bacterial wilt caused by Erwinia tracheiphila in plants from the family Cucurbitaceae and other essential crop families (Bragard et al. 2020). The presence of beetles could represent a potential threat; however, until now, this bacterial disease has not been reported in hemp. Furthermore, the effect of this group of chewing herbivores on plant metabolites such as THC and CBD has not yet been investigated.

• Leafhoppers (Hemiptera: Cicadellidae)

Leafhoppers were represented by diverse species that are common pests in other crops in the region, such as the potato leafhopper (Empoasca fabae), corn leafhopper (Dalbulus maidis), and the aster leafhopper (Macrosteles quadrilineatus), among others. Although no significant damage was observed in the crops, it’s important to note that many of these species have the potential to harm hemp by introducing pathogens or causing direct feeding damage, especially when conditions favor population growth (Figure 12). In Colorado and Arizona, there have been reports linking leafhoppers to the transmission of plant viruses in CBD and fiber hemp crops (Giladi et al. 2020; Hu et al. 2021; Stats et al. 2023).

• Stink Bugs (Hemiptera: Pentatomidae) and Plant Bugs (Hemiptera: Miridae)

Stink bugs and plant bugs, especially tarnished plant bugs were frequently found among the piercing-sucking insects in hemp crops (Figure 13). These pests were particularly prevalent in fiber hemp farms (Figure 7), with their populations becoming more pronounced during the flowering and post-flowering stages (Figure 8). This indicates that these growth stages are critical for monitoring and managing infestations. This seasonal increase aligns with their feeding preferences and the nature of the damage they inflict. 

Stink and plant bugs have been reported feeding on flowers, seeds, and delicate plant tissues. This feeding can lead to challenges like seed deformation, bud abortions, and distorted flower structures. These issues might lower germination rates and alter the seeds' physical appearance for hemp growers focusing on seeds and grain. Similarly, such insects can diminish the crop's commercial value in flower production (Britt et al. 2019). While we’re learning more about the roles of stink bugs and plant bugs in hemp cultivation, there’s still much to discover about their long-term impact on yield and the levels at which they start to cause economic harm.

• Importance of Less Abundant Herbivores

Other species, like Japanese beetles (Coleoptera: Scarabaeidae), were less abundant, but when present, they were observed causing significant defoliation and skeletonization in fiber/grain cultivars (Figure 14), which could lead to reduced photosynthetic capacity and crop yield. They were also observed in mating aggregations on male flowers, suggesting that hemp could become a hospitable host for these damaging beetles. Japanese beetles can feed on various plants and are commonly present across the Eastern United States, making them a potential emerging pest for hemp cultivation.

The corn earworm (Lepidoptera: Noctuidae) is recognized as a significant pest of hemp crops, particularly for CBD and grain varieties, where larvae feed on flowers, buds, and developing seeds (Figure 15). While its abundance was low during our pest monitoring in 2022, and no significant impact was observed in the sampling sites, its presence became more prominent at the end of the 2023 growing cycle. It is critical to conduct studies to understand how the planting season, as well as the proximity of hemp fields to corn crops, could play a role in the impact of corn earworm infestations, especially in CBD varieties. Planting CBD crops late in the season, between July and October—beyond the corn harvesting period—may increase the risk of corn earworms migrating into hemp fields due to this spatial and temporal overlap.

Incidence and Severity of Insect Pest Damage on Hemp Crops

Insect damage in the hemp crops was categorized based on the feeding behavior of the different pests interacting with the plants. Chewing damage is caused by herbivores with strong mandibles, including caterpillars, inchworms, leaf beetles, and grasshoppers, primarily affecting leaves and flower buds. Scraping damage results from piercing-sucking insects, such as thrips, aphids, whiteflies, leafhoppers, plant bugs, and stink bugs, which extract fluids from the plant foliage, stems, and seeds. Boring damage is caused mainly by lepidopteran stem borers that tunnel into the stems, affecting the plant structure.  Mining damage is often inflicted by dipteran and lepidopteran leaf miners who create tunnels inside the leaf tissue while completing their larval stage—lastly, bud rot is attributed to the corn earworm feeding and excrement within flower buds (Figure 16).

Figure 17 illustrates the overall proportion of plants with insect damage across CBD and fiber hemp types. In CBD hemp, the incidence of insect damage was higher, with approximately 85% of plants affected, whereas in fiber hemp, the proportion was lower, at around 68%. Variations in the proportion of insect-damaged plants were also observed across hemp farms. Regardless of the severity of insect damage and its impact on the crop, in CBD farms (Lazy Moon, Rocksprings, and Shaffer Farm), insect feeding signs were consistently high, with over 81 % of herbivory incidence on the plants. In contrast, the proportion of plants with insect feeding signs was lower in fiber hemp farms, except at Cedar Meadow farm, which had a similar damage incidence to the CBD farms (Figure 18).

Figure 19 shows the proportion of plants at each damage level (0= none, 1= low, 2= moderate, and 3= severe) for each type of damage (chewing, scraping, boring, mining, and bud rot) in CBD and fiber hemp crops.  The damage caused by chewing insects was the most prevalent among both hemp types, followed by piercing-sucking insects. Approximately 78% of CBD plants showed chewing damage, with most of this damage rated at low levels. Similarly, about 64% of the fiber plants exhibited chewing damage, mainly concentrated at low levels. Scraping damage was less frequent, occurring in 30% of CBD plants and 15% of fiber plants, predominantly at low levels. Boring, mining, and bud rot damages were insignificant across CBD and fiber hemp crops.  Examining the type of damage and severity across hemp farms, chewing and scraping damages were widespread, with a higher incidence on CBD farms. However, these damages occurred mainly at low severity in all sampling sites (Figure 20)

These results show that while CBD hemp experiences a higher incidence of insect damage than fiber, the severity remains predominantly low across all observed damage types. Chewing and piercing-sucking insects are the primary herbivores affecting hemp crops in Pennsylvania. Despite their widespread presence, the low severity of damage suggests that current pest pressure does not significantly impact crop yield. Therefore, intensive pest management interventions may not be necessary under current conditions. Continued monitoring is recommended to detect changes in pest damage incidence and severity.

Prevalent Natural Enemies in Pennsylvania Hemp Farms

The visual sampling indicated that lady beetles (Coccinellidae), spiders (Arachnids), and minute pirate bugs (Anthocoridae) were the predominant groups of natural enemies foraging on hemp crops. Predatory thrips (Aelothripidae), ants (Formicidae), hoverflies (Syrphidae), and lacewings (Chrysopidae) were represented in moderate abundance. Additionally, several families of parasitic wasps (Hymenoptera) were observed; however, it’s possible that they are not fully represented in the visual sampling since some species are tiny and exhibit rapid flight, making their detention difficult (Figure 21).

The yellow sticky traps proved highly effective at capturing parasitoids belonging to a diverse array of hymenopteran families. This group accounted for 40% of the natural enemies collected using this method. Alongside lady beetles, arachnids, and hoverflies, frequently seen in visual sampling, the sticky traps also captured notable families such as Dolichopodidae, Tachinidae, and predatory frit flies (Chloropidae). However, these were less abundant than the parasitoid group (Figure 21). 

Prevalent Natural Enemies in Hemp Crops

• Lady beetles (Coleoptera: Coccinellidae)

Lady beetles made up 25% of the predator observations, showcasing a rich diversity of native species, including the pink lady beetle (Coleomegilla maculata), the minute lady beetle (Diomus terminatus), the 14-spotted lady beetle (Propylea quatuordecimpunctata), and the seven-spotted lady beetle (Coccinella septempunctata), which were commonly observed. Additionally, the introduced multicolored Asian lady beetle (Harmonia axyridis) was also abundant in our samples (Figure 22); however, its populations did not dominate over native species such as the pink and minute lady beetles.

These coccinellid species serve as generalist predators, preying on several soft-bodied insect pests, such as aphids, scale insects, mealybug mites, tiny larvae, and insect eggs found on hemp. Some of them also consume pollen and nectar. Their presence in the crops suppresses populations of prevalent pests, such as cannabis aphids, highlighting the importance of preserving them in the hemp ecosystem to avoid detrimental pest effects.

• Spiders (Araneae)

Collectively, the spider families Thomisidae (crab spiders), Salticidae (jumping spiders), Araneidae (orb-weaver spiders), and Cheiracanthiidae (sac spiders) contribute an additional 24% to the total natural enemy population, emphasizing their vital ecological role in hemp crop systems. These spiders regulate pest populations and contribute to ecosystem stability through their diverse hunting strategies and habitats. As generalist predators, they prey on a wide range of arthropods, including pests and occasionally beneficial insects; their impact is particularly significant against abundant pests.

• Minute Pirate bugs (Hemiptera: Athocoridae)

Minute pirate bugs (Orius sp.)  (Figure 23) rank as the third most prevalent general predator detected in hemp crops, accounting for approximately 18% of beneficial control agents. These voracious insects are typically released in biological control programs in field and greenhouse environments to mitigate significant agricultural pests, including aphids, whiteflies, spider mites, and thrips. In hemp crops, we frequently observed them preying on thrips. They can also occasionally consume pollen in instances of prey scarcity.

•  Ants (Formicidae), hoverflies (Syrphidae), and lacewings (Chrysopidae)

These predator families are essential components of the natural enemy community in hemp crops, playing a role as natural pest controllers and contributing to crop health and productivity through their ecological functions. Ants are generalist predators and scavengers, feeding on pests such as caterpillars, aphids, and insect eggs while disrupting pest colonies through their foraging behavior. Hoverflies, in their larval stage, are predators of aphids, scales, and thrips, while the adult flies also serve as pollinators. Lacewings are effective predators of small pests such as aphids, mites, and whiteflies.

• Parasitoid wasps (Hymenoptera: Ichneumonoidea, Chalcidoidea)

Parasitoid wasps exhibited considerable diversity in morphology and dimensions. The larger specimens were classified within the superfamily Ichneumonoidea, whereas most of the smaller examples were categorized under the superfamily Chalcidoidea. These parasitoid wasps are effective natural enemies used in biological control programs against significant agricultural pests such as caterpillars, aphids, whiteflies, mealybugs, scale insects, and beetles. We observed a high abundance of aphelinid specimens in sticky traps, a specialized group of endoparasites of aphids in the family of Aphelinidae.

The visual inspections of insects in Central Pennsylvania allowed us to discover an exotic wasp, Aphelinus maculatus Yanosh 1979 (Hymenoptera: Aphelinidae), parasitizing adults of the cannabis aphid P. cannabis in CBD hemp (Figure 24). There are very few records of A. maculatus worldwide; it was first described in Russia in 1979 (Yasnosh 1981) and most recently in China in 2016, attacking green peach aphids Myzus persicae (Chen and Li 2016). This is the first time that A. maculatus has been naturally found in Pennsylvania, attacking the cannabis aphid in cultivated CBD hemp. This increases the list of natural enemies to conduct biological control in hemp crops.

• Predatory flies (Diptera: Dollycopodidae, Tachinidae, Chloropidae)

Dolichopodidae, Tachinidae, and Chloropidae represent significant natural enemies in hemp ecosystems, often found on yellow sticky traps. Dolichopodidae, known as long-legged flies, help reduce populations of aphids, mites, and thrips, particularly during high-density outbreaks. Tachinidae, or tachinid flies, act as parasitoids that are vital in biologically controlling common pests such as caterpillars, beetles, and true bugs. Among the chloropids, Thaumatomyia sp. emerged as the predominant species, recognized as a specialized predator of root aphids during its larval stage.

The presence of these beneficial arthropods holds promising implications for biological control in this new crop system. They have the potential to mitigate the impact of detrimental herbivores. Moreover, their representation serves as valuable indicators of ecosystem health, fostering the adoption of sustainable farming practices.

Diversity index of insect pests and natural enemies in Hemp Crops

Understanding the diversity of the insects that interact with hemp crops serves as a measure of the stability and resilience of these agroecosystems to prevent pest outbreaks and autoregulate pest populations. The diversity index considers the number of species in a specific ecosystem (species richness) and how evenly the populations are distributed among those species (species evenness). Crops with high diversity support a broad array of natural predators that help maintain herbivore populations at levels that are not harmful to the crop, decreasing reliance on chemical pesticides. We can use this index in hemp crops to gain insights into crop management practices that can better support natural enemy communities, inform potential pest outbreak risks, evaluate the effectiveness of pest control interventions, and help identify when these interventions are needed.

The Shannon-Weiner Diversity Index was calculated for each functional group (herbivores and natural enemies) in CBD and fiber hemp. Despite the herbivores having a higher relative abundance among the functional groups, they were less diverse than natural enemies in both CBD and fiber crops, indicating that few families within the insect pests are dominating the herbivore communities. On the contrary, natural enemies were less abundant, but their communities were more diverse and more evenly distributed.  Comparing the diversity of herbivore communities between hemp types, CBD had a lower index (1.813) than fiber (4.168), indicating a more uneven distribution of insect pest populations in CBD, dominated by cannabis aphids.  Conversely, in the predator group, a greater diversity of species was observed in CBD hemp (10.07) compared to fiber hemp (6.247) (See Table 5).

Table 5. Total Shannon- Weiner Diversity Index for insect functional groups observed in CBD and fiber hemp during the 2022 visual survey in Pennsylvania.

To better understand how diversity changes as different species colonize and develop in plants at different times throughout the crop cycle, we calculated the Shannon-Weiner Diversity Index of herbivores and natural enemies for each sampling event.   

In CDB crops, a variety of natural enemies have established rapidly since the early vegetative stage and consistently exhibited a more diverse community composition than the herbivore group throughout the entire growing season. This demonstrates the extraordinary capacity of CDB hemp crops to attract and sustain populations of diverse natural enemies, helping to mitigate pest pressure. Conversely, herbivore populations established more gradually during the vegetative stage, peaked in diversity during the flowering period, and then declined towards the end of the cycle, as cannabis aphids became the dominant insect species. A higher diversity of herbivores can be harmful when many damaging species affect the crop, or beneficial when balanced communities prevent any species from becoming overly dominant, allowing the crop to tolerate damage from various herbivores without hampering productivity (Figure 25a).

In fiber hemp crops, the diversity of pests and natural enemies increased at similar rates since the early vegetative stage; however, as the plants moved towards flowering, the natural enemy population decreased, while the diversity of herbivore insects rapidly surged. This indicates that lower populations of beneficial insects can lead to a higher number of pests and potentially more crop damage, requiring interventions with other control methods. During post-flowering, the diversity of natural enemies remained stable and higher than that of insect pests (Figure 25b).

2. The impact of insect pests on plant defensive metabolites and yield in industrial hemp cultivars 

The initial phase of this research has provided valuable insights into the key pest species affecting hemp crops in the region. Notably, the cannabis aphid (P. cannabis) has emerged as the most prevalent and widely distributed pest in CBD and fiber hemp crops throughout Pennsylvania. Significant populations of these aphids tend to establish early in the vegetative phase, with numbers peaking as the plants approach the flowering stage, especially for CBD hemp. However, the true impact of cannabis aphids on hemp production, particularly regarding plant yield, crop health, and the potential effect of aphid populations on important plant compounds such as THC and CBD — an area of critical concern for farmers given the crop legal regulations— remains unclear. One of the challenges in assessing the extent of aphid damage is the difficulty in quantifying or observing visible signs of harm, as these pests can infest plants at any phenological stage without obvious external symptoms. This lack of visible damage makes it difficult to gauge their actual effects on crop performance and determine thresholds to conduct control interventions with different methods.

The second goal of our broad hemp research at the Lab of Arthropod Ecology and Trophic Interactions at Penn State University is to assess the long-term impacts of cannabis aphid infestations on locally used hemp cultivars, particularly regarding yield loss and modifications in cannabinoid and volatile terpenes, as well as the plant's response to aphid feeding. With this proposal sponsored by the Sustainable Research and Education program (SARE), we began examining these effects on three different fiber varieties: Henola, Joey, and Bialobrzeskie, through greenhouse experiments conducted independently for each cultivar. To broaden our research scope, we also included one auto-flowering CBD variety for indoor cultivation and one regular photoperiod CBD variety (Abacus) in the greenhouse studies. In addition, to investigate aphid herbivory under more realistic conditions of a cropping system, we established a fiber plot with Bialobrzeskie and a CBD plot with the Abacus variety at the Russell E. Larson Agricultural Research Center during the 2024 growing season.

Aphid development is affected by the plant's phenological stage.

Aphids’ reproductive capacity is affected by the plant’s phenological stage. This effect was consistently observed between the aphid-infested treatment plants across all the fiber varieties during greenhouse evaluations. Fifteen days after we induced aphid herbivory stress by resealing five cannabis aphid adults in vegetative, flowering, and post-flowering plants, significant differences were observed in the aphid populations that grew at each phenological stage. In vegetative plants, the number of aphids was twice as high as the aphid population in flowering plants. In the Bialobrzeskie cultivar, we were able to include post-flowering plants, and the number of aphids in these plants was significantly smaller than in flowering and vegetative plants (Figure 26).

In only two weeks, five single aphids were able to form a colony with up to 3,800 aphids in vegetative plants—depending on the variety—highlighting the fast reproductive potential of P. cannabis in the absence of natural enemies, low cannabinoid compound concentrations in plants, competitors, and stable environmental conditions. In flowering and post-flowering plants, the same five aphids raised colonies with populations between 800 and 2,800—still quite a large population for a single plant. However, this indicates that older plants offer less favorable conditions for aphid reproduction. This could be due to chemical changes during plant maturation, combined with leaf toughness, which may prevent aphids from reaching their full potential.

Aphid herbivory seems not to affect the photosynthetic potential of hemp plants

The yield of photosystem II (PSII) reflects plant health by assessing how effectively the plant converts sunlight into the energy needed for growth and achieving maximum productivity. A low PSII yield indicates that plants are under stress due to water and nutrient deficiencies or other environmental issues that may hinder their photosynthetic capacity. Given the significant reproductive capacity of cannabis aphids, if large populations of aphids are feeding on the nutrients and sugars from the phloem of hemp plants, a disruption in the normal nutrient flow is expected, which could indirectly affect the plants' photosynthetic potential.

We didn’t find significant differences in the PSII yield between aphid-infested plants and aphid-free plants at any of the phenological stages that could demonstrate detrimental effects of aphid herbivory on the photosynthetic potential of the fiber varieties. While bialobrzeskie exhibited, in general, lower PSII yields than the rest, none of the vegetative and flowering plants displayed values that would indicate plant stress (which would be lower than 0.7). The lower photosynthetic potential observed in post-flowering plants could be attributed to the natural senescence of plants at this stage (see Figure 27). 

Figure 27. Effect of cannabis aphid herbivory on the plant photosynthetic potential of three industrial hemp varieties.  

Hemp varieties responded differently to the effect of aphid herbivory on plant yield.

The impacts of aphid herbivory on plant biomass and the ability of the plants to withstand high aphid populations varied across the three fiber varieties assessed in the greenhouse. These differences highlight the different levels of resistance and tolerance among hemp varieties to aphid damage.

In Joey, no differences were observed between infested and non-infested plants at the vegetative and flowering stages. This suggests that aphid feeding did not affect plant biomass, indicating that this variety could exhibit higher tolerance to aphid infestations (Figure 28).  

In Bialobrzeskie, while aphid-infested plants tended to reduce biomass compared to aphid-free plants at all phenological stages, these differences were not significant (Figure 28). This suggests that this variety might have a moderate tolerance to aphids, with plants possibly showing a slight decrease in growth, but not to the extent of impairing plant development.

In contrast, Henola showed a significant reduction in dry weight when flowering plants were exposed to aphid infestations for two weeks (Figure 28). This suggests that Henola had a lower tolerance for aphid attack during the reproductive stage. It is possible that aphid infestations during the flowering stage impact plant processes such as nutrient allocation and reproductive energy.

The greenhouse experiment provided valuable insights into the impact of aphids on plant biomass in a controlled environment. However, it’s important to recognize that these results may not fully reflect the interactions between aphids and plants in natural settings. While we can carefully regulate factors such as temperature, humidity, and light in the greenhouse, these conditions may not accurately represent the variability and challenges present outside. In our study, Joey demonstrated remarkable tolerance to aphid infestations, whereas Henola experienced a significant decline in biomass during the flowering phase. Nonetheless, it’s crucial to assess these effects in field conditions, where real-world elements like weather fluctuations, soil health, pest-predator dynamics, and competition from surrounding plants could significantly influence how plants respond to aphid herbivory.

Plant defenses against aphids: Phytohormones, Plant Volatiles and Cannabinoid compounds

We are making significant progress in identifying plant chemical compounds that play a crucial role in defending against key pests like aphids, as well as compounds that may influence the attraction of natural enemies to hemp crops. However, conducting experiments with a new crop like hemp has presented several challenges, including adapting methods, the need for proper infrastructure for growing hemp, and dealing with insect and pathogen contamination in plants grown for experimentation, which can delay the expected outcomes. Despite these obstacles, we have successfully collected volatiles from about 400 samples from Henola, Bialobrzeskie, Joey, Abacus, and Auto-flower varieties in our greenhouse experiments, and these samples are currently being processed for further analysis.

We have found that, in general, flowering plants release approximately three times more volatiles than vegetative plants, as observed in Figure 29.  In the chromatogram of volatiles analysis, we have consistently identified peaks corresponding to around 16 chemical compounds, including green leaf volatiles, terpenes, and sesquiterpenes, present in both phenological stages. Some of these volatiles include alpha- and beta-pinene, myrcene, limonene, ocimene, caryophyllene, beta-farnesene, and humulene. Among vegetative plants, particularly within the sesquiterpene group, we observed lower concentration peaks in aphid-infested plants (Figure 30), suggesting the potential suppression of these compounds. We are currently conducting further analyses to determine whether aphids suppress specific chemical defenses in the plants to facilitate colonization. Additionally, flowering plants exhibited significant variability in the concentration of the same sesquiterpenes between aphid-infested and aphid-free plants, highlighting a dynamic chemical response to aphid herbivory (Figure 31).

In addition to the volatile compound analysis, we have collected the same number of samples for phytohormone analysis and cannabinoid profiling, both of which are currently in the chemical analysis process. These analyses will provide a comprehensive understanding of the chemical interactions between hemp plants, pests, and natural predators, which are important for developing sustainable pest management strategies. We are working to make these results available to farmers to provide valuable insights into the management of important pests like cannabis aphids in hemp crops.

Works Cited:

• Bragard C, Dehnen-Schmutz K, Serio FD, et al (2020) Pest categorisation of Diabrotica undecimpunctata 18:. https://doi.org/10.2903/j.efsa.2020.6358
• Britt KE, Pagani MK, Kuhar TP (2019) First Report of Brown Marmorated Stink Bug (Hemiptera: Pentatomidae) Associated With Cannabis sativa (Rosales: Cannabaceae) in the United States. J Integr Pest Manag 10:17. https://doi.org/10.1093/jipm/pmz014
• Chen Y, Li C-D (2016). Three new species and new distributional data for five rare species of Aphelinus (Hymenoptera: Aphelinidae) from China. Zootaxa 4092:258–272. https://doi.org/10.11646/zootaxa.4092.2.8
• Cranshaw WS, Halbert SE, Favret C, et al (2018) Phorodon cannabis Passerini (Hemiptera: Aphididae), a newly recognized pest in North America found on industrial hemp. Insecta Mundi
• Giladi Y, Hadad L, Luria N, et al (2020) First Report of Beet Curly Top Virus Infecting Cannabis sativa in Western Colorado. Plant Disease 104:999–999. https://doi.org/10.1094/PDIS-08-19-1656-PDN
• Gill HK, Garg H, Gill AK, et al (2015) Onion Thrips (Thysanoptera: Thripidae) Biology, Ecology, and Management in Onion Production Systems. Journal of Integrated Pest Management 6:6. https://doi.org/10.1093/jipm/pmv006
• Hu J, Masson R, Dickey L (2021) First Report of Beet curly top virus Infecting Industrial Hemp ( Cannabis sativa ) in Arizona. Plant Disease 105:1233–1233. https://doi.org/10.1094/PDIS-11-20-2330-PDN
• Stats AK, Sweat KG, Masson RN, et al (2023) The Desert Whale: the boom and bust of hemp in Arizona. J Cannabis Res 5:19. https://doi.org/10.1186/s42238-023-00187-8
• Yasnosh VA (1981) Aphelinus maculatus. (Hymenoptera, Aphelinidae) - a parasite of aphids in the south of the Maritime Territory. Tr Vsesoyuznogo Entomol Obshchestva 61:164–165