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

1. The composition of insect pests and biological control agents throughout the crop growing cycle in Pennsylvania.

The evaluation of the arthropod community within industrial hemp fields throughout the 2022 growing season, was conducted weekly in seven commercial plots situated across six counties in the Central and Southeastern regions of Pennsylvania. Four plots grew industrial hemp for fiber and grain production while the remaining three farms grew hemp for CBD extracts. The selected plots contained either individual varieties or a mix of varieties within the same plot, depending on the growers' preferences. The number of samplings conducted at each farm varied based on harvest time (See Table 1).

To enhance the scope of our research, we conducted a second year of insect sampling throughout the 2023 growing season in the Central and Southeastern counties of Pennsylvania. We included twenty individual variety plots established within two experimental and three commercial farms to explore variations in the insect community composition among cultivars, locations, and hemp types. This expansion increased the number of sampling sites and reduced the sampling frequency, enabling a more comprehensive collection of samples across geographic areas. At each of the experimental farms, we selected the same six fiber/grain varieties for surveying, resulting in a total of 12 sampling sites. In the commercial farms, we had no control over variety matching among locations, six plots were established with different CBD cultivars, while 2 of the plots featured fiber varieties (See Table 2).  

Table 1. Location of field sites, hemp type, and number of samplings conducted during the 2022 insect surveys in Pennsylvania hemp crops.

Table 2. Location of field sites, hemp type, and number of samplings conducted during the 2023 insect surveys in Pennsylvania hemp crops.

Insect survey 2022:

During the visual sampling conducted in the 2022 growing season, 20,915 arthropods were recorded and identified including both insects and arachnids. This diverse group of arthropods was systematically classified into 10 orders, 42 families, 62 morphospecies, and a separate category for unclassified insects. Hemiptera, Coleoptera, Thysanoptera, Araneae, Diptera, and Hymenoptera were the most predominant orders among the specimens observed, encompassing both herbivores and natural enemies (Table 3). When categorized by functional groups, herbivore insects emerged as the most abundant group representing 90% of the specimens collected, followed by natural enemies with 9% of representation, while incidental species accounted for 1% and pollinators contributed to less than 1% (Fig 1)

Table 3. Summary of specimens observed during the 2022 visual sampling in hemp crops across Pennsylvania, classified by orders, families, and morphospecies.

Herbivore Insects

Within the herbivorous pest category, Aphididae (aphids), Thripidae (thrips), Chrysomelidae (leaf beetles), Cicadellidae (leafhoppers), Pentatomidae (stink bus), and Miridae (plant bugs) were the insect families with the largest number of specimens observed (Table 4). Aphids, particularly the cannabis aphid (P. cannabis), emerged as predominant pests, constituting 86% of the herbivore species feeding on hemp. These sap-sucking insects were detected in both CBD and fiber crops since the vegetative phase of the crops and were commonly present in the sampling sites across central and southeast counties of Pennsylvania. In the case of fiber cultivars, aphid populations were higher during the flowering stage while conversely, for CBD hemp, there was a population decline during the flowering period, followed by a substantial surge throughout the post-flowering stage until the harvest. Typically, the production cycle for CBD is longer than for fiber resulting in an extended period of vulnerability for CBD plants where aphid infestations can be problematic, highlighting the importance of understanding their impact on crop yield and plant metabolites such as THC (Table 5).

Table 4. List of herbivore insects and their relative abundance observed during the 2022 visual sampling in CBD and fiber hemp crops across Pennsylvania field sites.

The second most prevalent group of herbivores found in hemp crops were thrips comprised mainly of three generalist species: onion thrips (Thrips tabacci), western flower thrips (Frankliniella sp.), and soybean thrips (Neohydratothrips sp.). These rasping-sucking insects extract cell contents from the leaves producing punctures and scraping signs that may affect the appearance of the foliage and render the plant susceptible to pathogens (Gill et al. 2015). Thrips can overwinter in the soil, plant debris, or weeds and were observed causing potential impacts in hemp crops beginning in the early vegetative stage.

Various chrysomelid species, including flea beetles from the genus Altica sp. And Crepidodera sp., along with cucumber beetles (Diabrotica undecimpunctata), were more abundant in CBD than in fiber varieties and were frequently observed chewing small holes in the foliage throughout the flowering stage of the crops. Among the hemipterans, leafhoppers, stink bugs, and plant bugs (Lygus sp.) represented the most prevalent families within this insect group. Although no significant damage was observed in the crops, leafhoppers might pose a threat to hemp since reports have associated them with plant virus transmission in Colorado and Arizona states (Giladi et al. 2020; Hu et al. 2021; Stats et al. 2023). Other species like Japanese beetles (Popillia japonica) were less abundant but when present, they were observed causing significant defoliation and skeletonization in fiber/ grain cultivars which could lead to reduced photosynthetic capacity of the plant and reduced 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 a wide variety of plants and are commonly present across the Eastern United States. During the 2022 growing season, the corn earworm (Helicoverpa zea), recognized as a significant pest for hemp crops, did not exhibit any notable impact due to its low abundance (Tables 4 and 5).

Table 5. The abundance of the most common insect pests throughout the crop growing cycle during the 2022 visual sampling of CBD and fiber hemp across Pennsylvania.

Natural Enemies

Among the predominant families of natural enemies, Coccinellidae (lady beetles) constituted 28% of the total observed predators, showcasing a diverse assembly of native species such as the pink lady beetle (Coleomegilla maculata) and the 14-Spot lady beetle (Propylea quatuordecimpunctata). Additionally, the introduced Multicolored Asian lady beetle (Harmonia axyridis) was abundantly represented in our samples. All these ladybeetle species are generalist predators, feeding on a variety of insect pests and arthropods including aphids, scale bugs, mites, small larvae, and insect eggs. Collectively, the spider families Thomicidae (carb spiders), Salticidae (jumping spiders), Araneidae (orb weaver spiders), Cheiracanthiidae (sac spiders), and Opiliones (harvestmen) contributed to an additional 26% of the total natural predator population. Their abundance and diversity highlight their important role in the regulation and stability of ecosystems due to their multiple ecological functions. Minute pirate bugs (Athocoridae) ranked as the third most crucial general predator, constituting 14% of beneficial control agents. These voracious insects were observed throughout the entire hemp growing season preying on thrips and aphids. Ants (Formicidae), hoverflies (Syrphidae), lacewings (Chrysopidae), and different families of parasitic wasps were equally represented in terms of relative abundance (Table 6). In the group of parasitic wasps, we found the first specimens of Aphelinus maculatus reported for Pennsylvania attacking P. cannabis. Overall, CBD hemp exhibited higher individual abundance across these species compared to fiber/grain cultivars. The presence of these beneficial arthropods holds promising implications for biological control in this new crop system with their 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.

Table 6. Total and relative abundance of predator families observed during the 2022 visual sampling in CBD and fiber hemp crops across Pennsylvania.

We used non-metric multidimensional scaling - NMDS to visualize differences in the communities of arthropods on hemp plantings. First, we visualized observed communities across each sampled field site (Figure 2). While each site seems to have distinct communities, there are a few that overlap (CBD plots). From this, we visualized the communities of organisms by hemp type and see that the community composition is distinct between fiber and CBD (Figure 3). These dissimilarities in insect communities were assessed between the two types of hemp cultivars using the permutational multivariate analysis of variance (PERMANOVA). The analysis revealed that the composition of insect species significantly varied between CDB and fiber crops (F= 8.5261, p=0.001). Because the communities of arthropods across our fiber plots did not overlap, we ran a PERMANOVA which revealed that they are indeed significantly different from one another (F= 5.6232, p=0.001), indicating that other factors are shaping the insect communities between those sample sites (Fig. 3). 

The Shannon-Weiner Diversity Index was computed for the arthropod functional group (herbivores versus natural enemies) and hemp type, revealing a higher diversity and more even distribution of herbivore species in fiber hemp (4.168) compared to CBD hemp (1.813). 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 7). In summary, for both types of hemp, natural enemy communities demonstrated greater diversity and more balanced distribution compared to insect pests.

Table 7. Total Shannon- Weiner Diversity Index for insect functional groups observed in CBD and fiber hemp during the 2022 visual survey in Pennsylvania.

The diversity index estimated at each sample event reveals the functional group dynamics for each type of hemp over time. In the case of CBD hemp, the diversity of herbivores and predators displays a consistent increase until the end of the flowering stage. During post-flowering the dominance exhibited by few herbivores (e.g. aphids) might have reduced the diversity of this group.  Presumably, certain predatory species may assume greater dominance in response to pest behavior (Fig. 4a). For fiber hemp, the dynamic of diversity across time exhibited opposite trends between herbivores and predators indicating possible interactions between those communities (Fig. 4b).

To enhance our comprehension of the dynamics influencing interactions between predator and herbivore communities, and to determine the major players in the crops, further analysis will be conducted using general linear models.

In addition to the visual samplings, yellow sticky traps were installed to passively sample arthropod communities in the crop, we monitored flying insects by installing yellow sticky traps throughout the 2022 growing cycle. The identification process of the specimens in the sticky cards is in progress.

Throughout the 2023 hemp growing season, we utilized different sampling methods including visual inspections, vacuum sampling, yellow sticky cards, and pitfall traps. The collected samples are currently undergoing processing in the lab for subsequent statistical analysis.

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

Laboratory experiments have been conducted on two varieties of fiber hemp, Henola, and Joey to evaluate how the most prevalent insect pest, the cannabis aphid (Phorodon cannabis), affects plant growth and photosynthetic potential in hemp plants during the vegetative and flowering stages by exposing plants to aphid herbivory and comparing them with non-infested plants.

 In this research, we are only including a few hemp cultivars where we have found differences in their tolerance response to insect attack. We consider it important to evaluate the susceptibility or resistance to insects and pathogens of the cultivars that are available for growers in the market and to determine how these varieties can be utilized according to each grower’s necessities.

Aphid populations were significantly lower in flowering plants compared to vegetative plants in both varieties indicating that aphid development was affected by the plant phenological stage (Fig. 5). Regarding the photosynthetic potential of hemp plants, there were no differences detected between aphid-infested and aphid-free-infested plants in the Joey variety for any of the phenological stages (Fig. 6a). However, in Henola, only the aphid-infested-flowering treatment plants exhibited a lower photosynthetic yield with no significant differences found between the treatments at the vegetative stage (Fig. 6b). Despite these values of photosynthesis for infested flowering plants in Henola, none of the treatments indicated plant stress. The photosynthetic potential Y(II) values for stressed plants, as determined by the method employed, are lower than 0.7.

Plant biomass was not affected in the Joey variety when both vegetative and flowering plants were infested with aphids. However, in the case of Henola, a significant reduction in dry weight was observed when flowering plants were exposed to aphid infestations for a two-week period (Fig. 7). This variation in response between the two hemp varieties to aphid herbivory suggests that Joy demonstrates better tolerance to high aphid infestations, as evidenced by the absence of a reduction in dry weight. This is particularly important when looking at varieties that could tolerate insect damage.

To identify volatiles emitted by plants in response to P. cannabis herbivory, preliminary experiments have been conducted using vegetative and flowering plants from the Henola variety exposed to aphid herbivory for 14 days, while another set of plants for each stage remained free from aphid infestation.

In general, flowering plants released approximately three times more volatiles than vegetative ones, as illustrated in Figure 8. Chromatograms revealed peaks corresponding to around 16 compounds, encompassing green leaf volatiles, terpenes, and sesquiterpenes, which were consistently present in both phenological stages. Among vegetative plants, distinctions were observed, particularly within the sesquiterpene group, indicating lower concentration peaks in aphid-infested plants (Figure 9). Flowering plants exhibited significant variability in the concentration of the same sesquiterpenes between aphid-infested and aphid-free plants (Figure 10).