Progress report for GNE20-236

Project Type: Graduate Student
Funds awarded in 2020: $14,955.00
Projected End Date: 07/31/2021
Grant Recipient: University of Maryland
Region: Northeast
State: Maryland
Graduate Student:
Faculty Advisor:
Dr. Cerruti R. R. Hooks
University of Maryland
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Project Information

Summary:

Drawbacks of excessive pesticide usage in cantaloupe have created a demand for alternative pest control strategies that can reduce producers’ reliance on chemical inputs. The economic burden of growing cucurbits such as cantaloupe can be considerable due to the cost of frequently applied chemicals to protect cucurbits with low pest tolerance. Further, chemical sprays can have a negative impact on natural enemies inhabiting cucurbit fields and consequently cause secondary pest outbreaks. Interplanting cash crops with cover crops, also known as companion planting, has been used for centuries to improve soil quality and suppress weeds. However, interplanting a live cover crop (living mulch) into a cucurbit crop can be used to also help manage insect pests and diseases that they vector. Recent research has shown that interplanting a red clover living mulch into cucumber plantings can be an effective strategy to reduce insect pest and increase natural enemy populations without negatively impacting crop yield. This is a promising finding for other crops in the Cucurbitaceae family that share similar insect pests. This study will determine if similar benefits can be conferred to a cantaloupe system. Specifically, the objective is to determine whether interplanting cantaloupe into two perennial living mulches (i.e., alsike clover or Virginia wildrye) systems that are structurally dissimilar can be used to reduce insect pest and increase natural enemy populations in cantaloupe. Additional objectives are to compare the ability of these living mulches to enhance cantaloupe yield and serve as overwintering refuge for natural enemies.

Project Objectives:

The overall objective is to investigate and disseminate an alternative tactic for managing insect pests in cantaloupe plantings. Specific study objectives are as follows:

  • Compare the impact of a monoculture cantaloupe treatment with two additional treatments: 1) cantaloupe interplanted into a perennial bunch grass (Virginia wildrye, Elymus virginicus) and 2) interplanted into a perennial legume (alsike clover, Trifolium hybridum) on herbivore abundance,
  • Compare treatment impact on the abundance of natural enemies,
  • Determine treatment impact on yield and fruit quality, and
  • Determine the potential of different treatments to serve as overwintering refuge for natural enemies
Introduction:

The purpose of this project is to investigate living mulches in cantaloupe plantings as a sustainable alternative to insecticides for managing insect pests. Insect pests pose a challenge for Northeast cantaloupe growers; and if left unmanaged, can cause significant yield reductions and lost revenue. However, using an intensive chemical regimen for their mitigation can contribute to environmental and human health problems (Geiger et al., 2010; Relyea, 2016; Weisenburger, 1993). For example, a common pest mitigation practice used in cantaloupe is the application of systemic neonicotinoids to protect seedlings which is typically the most vulnerable stage (CMCC, 2003). Unfortunately, these products have received public scrutiny worldwide because of their potential contribution to pollinator decline (Van der Sluijs et al., 2013). Further, the high susceptibility of cantaloupe to pest damage and insect vectored diseases contributes to many growers administering prophylactic sprays. Cantaloupe is the second most susceptible cucurbit crop to insect damage (ATTRA, 2008), and tolerance to insect feeding damage is low in cantaloupe due to the threat of bacterial wilt. As such, some extension programs suggest foliar application of pyrethroids when only one cucumber beetle per plant is observed (Brust & Foster, 1999). In the year 2000, insecticides were used on 74% of US cantaloupe crops. Most treatments were applied to plants after crop emergence to protect them after fruit-set (CMCC, 2003). Concerns for cucumber beetles alone cause some growers to apply pesticides as many as seven times over a single season to prevent significant lost (Brust & Foster, 1999).

In recent studies, performed in our lab, crops in the Cucurbitaceae family (e.g., zucchini and cucumber) experienced a significant reduction in insect pest and increase in natural enemy abundance when interplanted with a living mulch (Hinds & Hooks, 2013; Kahl et al. 2019). Increasing the vegetational diversity within a crop field by adding multiple plant species can confer associational resistance to cash crops. Associational resistance is when neighboring plant species decrease the likelihood that vulnerable plants will be discovered or exploited by insect herbivores. This phenomenon can be explained by two non-exclusive hypotheses: 1) The resource concentration hypothesis states that increased plant diversity makes it more difficult for herbivores to detect their host plants, and 2) the natural enemy hypothesis suggest that natural enemies can more effectively colonize and reduce herbivores on crop plants in more vegetationally diverse environments due to increased structural diversity and the presence of alternate prey (Barbosa et al., 2009).  This research will advance our knowledge on living mulch usage by investigating responses of pests and natural enemies to their presence in cantaloupe plantings. Further, I will compare the effect of two perennial living mulches with structural differences on populations of pests, beneficials, over wintering natural enemies, and crop yield. Past studies have shown that leguminous cover crops (clover) can be used to reduce pest colonization and tenure time and that perennial grasses can serve as refugia for overwintering natural enemies (Bowers, Toews, Liu, & Schmidt, 2020).

Research

Materials and methods:

Experimental layout and design

To fulfil my objectives, field trials were conducted at the University of Maryland’s Central Maryland Research and Education Center in Upper Marlboro, MD. The experiment consisted of three treatments: 1) cantaloupe interplanted into alsike clover living mulch, 2) cantaloupe interplanted into Virginia wildrye living mulch, and 3) monoculture cantaloupe (grown in bare-ground). The experiment was replicated four times and arranged in randomized complete block design. Each plot measured 12.1 m by 12.1 m and was separated by 7.6 m alleys. Within each plot there were eight rows of cantaloupe with plants spaced 91 cm apart within rows and a between row spacing of 1 m.  Each row consisted of 13 cantaloupe plants each.

(Figure 1).

Alsike clover and Virginia wildrye were seeded in their respective treatment plots with a no-till drill in late September 2019 for the 2020 field season. Monoculture plots were left fallow over the winter. Sugar cube was the variety of cantaloupe used, partially due to its high level of disease resistance. Cantaloupe seedlings were grown out in a greenhouse for two weeks before they were transplanted into the field on June 30th. One week before the cantaloupe was transplanted, monoculture plots were be rotary tilled, and living mulch treatment plots were be mowed with a rotary mower to reduce potential early season competition with the cantaloupe and strip tilled approximately 38 cm wide where cantaloupe rows were established. The remaining interrow areas consisted of the alsike clover of Virginia wildrye.

 

Objective 1: Compare treatment impact on insect pest (herbivore) abundance

Several monitoring techniques were used to quantify herbivore abundance. The primary monitoring techniques included visual counts on the cantaloupe foliage and the deployment of yellow sticky cards. For visual counts, a square meter quadrat was randomly placed in each interior cantaloupe row with a minimum of one meter from the border. All arthropods encountered on the foliage or visible on the ground within the quadrat were field-identified to the lowest taxonomic level possible and recorded. Insect herbivores that were expected to colonize the cantaloupe included striped and spotted cucumber beetles, squash bugs, and melon and green peach aphids. Visual counts were conducted every week initiating two weeks after planting up until the beginning of harvest. Visual counts were always conducted in the mornings close to the same time and the order of reps was randomized to maintain observational integrity.

Yellow sticky cards were used mainly for detecting aerial arthropods at or above the cantaloupe canopy. They were deployed in the center row of each plot in the inter-row areas. Cards were held in place on fiberglass poles with a pair of clothes pins at canopy height. Sticky cards were deployed two weeks after planting and remained in place for one week prior to collection. Cards were deployed on 3 occasions spaced 3 weeks apart to provide an image of insect abundances in the early, middle, and late stages of the cantaloupe development. Sticky card specimens were counted and identified to the lowest taxa possible. An advantage of using sticky cards is that they can remain in the field for an extended period of time and continue to capture insects throughout the day and evening. Once collected, cards were placed in zip-tied bags and stored in the lab freezer until their contents could be identified using a stereomicroscope.

Objective 2: Compare treatment impact on the abundance of natural enemies

Natural enemies were counted along with herbivores during sampling tasks described in objective 1. However, in addition to foliar and sticky card counts, pitfall traps were be used to quantify the activity density of ground dwelling predators, including wolf spiders, rove beetles and ground beetles. Two pitfall traps were installed in each plot, one in the within (intra) and between (interrow) cantaloupe row areas. Pitfall traps consisted of two nested plastic cups placed flush with the surface of the ground. The top cup was removable and partially filled with propylene glycol as the trapping/killing agent. A 30×30 cm square black plastic cover supported by three large screws was placed over each trap with two to three centimeters of clearance to prevent traps from being flooded by rain or disturbed by wildlife. Pitfall traps were deployed and retrieved on the same dates as sticky cards. Once trap contents were retrieved, samples were brought back to the lab and vacuum filtered to separate them from the propylene glycol. Specimens were then transferred to a 70% ethanol solution to be preserved until they could be identified under a stereomicroscope.

Analysis (Objective 1 and 2)

Important or frequently appearing arthropod taxa observed across the sampling period are being analyzed. Visual counts were converted into number per square meter densities. All quadrats within a plot were averaged to represent the estimated density of arthropods for that entire plot for the given sampling period. Densities of notable arthropod species were compared over time between treatments using a linear mixed-effect model, and where treatment effects were detected pairwise comparisons of each treatment on each date were conducted to determine where the treatment effects were arising from. Results of visual counts and yellow sticky cards were also compared with action thresholds of an average of one cucumber beetle per plant or 15 beetles per sticky card. If there is no effect of pitfall trap location (intra vs inter-row) content of both traps will be pooled otherwise comparison will be made by trap location.

Objective 3: Determine treatment impact on yield and fruit quality

During each harvesting event, the total number and weight of harvestable and marketable fruit more than 4ft from the plot border was measured. Harvested fruit was evaluated by quantifiable measures of fruit quality by sampling 4 fruit of equal ripeness from each experimental plot during a single harvesting event. These measured characteristics included weight, polar and equatorial diameter, soluble solid (sugar) content, titratable acidity, skin and flesh hue, chroma and lightness, as well as texture characteristics including firmness, adhesiveness, springiness, cohesiveness, chewiness, and resilience. Treatment effects were checked for using a one-way ANOVA, and when treatment effects were detected a Tukey-Kramer test was completed to determine which treatments were statistically unique from each other.

Objective 4: Determine the potential of different treatments to serve as overwintering refugia for natural enemies 

After harvest, the living mulch will remain undisturbed in the plots through the fall and winter into 2021. In March 2021, two 60cm x 60cm soil emergence traps (cages) will be place in two randomly selected interrow areas in each plot. Trap will last until sometime in May (approximated 10 weeks). Collection jars in the top of each emergence cage will be partially filled with propylene glycol. Trap content will be collected and replaced weekly. After collection, contents will be filtered and transferred to a 70% ethanol solution for storage until they can be identified to the lowest possible taxa. Arthropod counts will be converted into number per 0.36 m2 (area covered by the trap). Traps within the same plots will have their weekly values averaged. Weekly insect trap content for each treatment will be compared using a linear mixed effect model and total seasonal emergence will be compared between treatments.

 

Research results and discussion:

The study is currently ongoing, and data and trap captures are still being analyzed. However, insect detections and captures from within the cantaloupe rows (intra-row samples) from the summer 2020 field season have largely been identified and analyzed.

Objective 1: Compare treatment impact on insect pest (herbivore) abundance

Overall, the 2020 field season saw very little abundance of pest insects across the cantaloupe growth period. Detections remained low for nearly all taxa throughout the summer regardless of treatment. The majority of important pest insects didn’t appear in large numbers or show a treatment effect on their abundance. Striped cucumber beetles (Acalymma vittatum), the most damaging cucurbit pest, never surpassed their action threshold of 1 insect per square meter in any of the experimental treatments, including the control. Striped cucumber beetles did however appear to respond  somewhat to the treatments.  A linear-mixed effects model revealed a treatment response for striped cucumber beetles using both foliar counts and sticky cards, and pairwise comparisons of treatments on each date revealed what and when the response was. Striped cucumber beetles were detected at reduced abundances via foliar counts in mid July, while the control and Virginia wildrye treatments remained statistically similar across the season. Detection via sticky card captures revealed more beetles being detected in the conventionally tilled control plots, through the season, with statistically lower beetle captures in clover and wild-rye treatments respectively  on the final date of trap collection. (figure 2)


Figure 2. Detections of striped cucumber beetle (Acalymma vittatum) via foliar counts and sticky card captures in summer 2020.

 

Objective 2: Compare treatment impact on the abundance of natural enemies

Like herbivores, the abundance of natural enemies was unusually low across the 2020 field season as well. Spiders were the only group of predators that demonstrated a treatment effect. Spiders counted through foliar observation were not identified to any lower taxa, though next year I would like to field identify them to family. Spiders captured in pitfall traps were exclusively wolf spiders (lycosidae). Analysis was carried out in an identical manner to pest species, with a linear mixed effects model determining if treatment effects were present, and pairwise comparisons determining the nature of those effects. Spiders were detected at higher densities in the alsike clover treatment than in the wildrye and conventional treatments by foliar counts, though the season ended with spider counts in both the control and clover treatments being similar and statistically higher than in the wildrye treatment. In the pitfall traps wolf spider captures in alsike clover were higher than in the control and wildrye treatments at the beginning and end of the season. The middle date saw all treatments have statistically identical and very low capture rates, but this could be attributable to weather as mid August was very wet and some of the traps flooded. At no point did the ryegrass treatment yield statistically greater captures than the conventional treatment. (figure 3)

Figure 3. Spider detections via foliar counts and pitfall trap captures in summer 2020.

Objective 3: Determine treatment impact on yield and fruit quality

There was a significant yield reduction in the Virginia wildrye plots. This could potentially be due to early-stage competition with weeds more so than the wildrye itself. After the initial mowing, the wildrye canopy did not become sufficiently thick enough to shade out weeds again until later in the season. The clover plots faired better but they were still on the cusp of yielding a statistically lower total average weight between it and the control plots. (Figure 4)

Figure 4. The Virginia wildrye treatment had reduced yield compared to the control treatment. The alsike clover treatment was intermediate between the two.

For the other quality parameters there was little, if any major difference between the quality of the fruit produced by each treatment. Weight, dimensions, soluble solids, and acid content were statistically identical, as were most parameters of texture and color. Fruit from the ryegrass treatment were slightly firmer than the other treatments, and there were slight differences between the flesh hue of the control and clover fruits, and skin lightness between the ryegrass and clover, and ryegrass and conventional fruit. These differences were minor however, and would be near imperceptible to any end consumer of the fruit.

Objective 4: Determine the potential of different treatments to serve as overwintering refugia for natural enemies 

There currently isn’t any data related to the overwintering communities of natural enemies in the plots, but overwintering traps will be deployed in March contingent on how weather develops this winter, with warmer weather potentially pushing up the deployment date of the emergence traps to ensure that data representative of the overwintering communities in the plots can be collected.  Data will be analyzed using similar methods to the insect surveys conducted via foliar counts and sticky and pitfall trap captures over the past summer.

Discussion

Research is still ongoing and it is still premature to come to any broad conclusions. However, it is encouraging to see reductions of striped cucumber beetles, an important pest, along with an increase in abundance for such an important group of predators as spiders in the intra-row space of the alsike clover treatments. Until insect captures from traps placed in the inter-row areas are fully identified and analyzed however, my analysis of the impacts these treatments on pest and natural enemy abundance in the cantaloupe plots is incomplete. 

The consistence in quality of the fruit across the treatments is encouraging as well. The yield reductions however, are an element that need to be addressed. It is most likely that yield reductions were a product of competition with weeds in the early growth stages of the cantaloupe. The first 30 days of the cantaloupes’ growth cycle is essential for determining the ultimate yield of the plant. When the living mulch was mowed prior to transplanting the cantaloupe to the field it took much longer than expected for it to recover and reestablish its weed suppressing characteristics. For the upcoming summer, I had the seeding rate of the living mulch treatments increased by 50 percent to improve their density and reduce the chances of weeds growing among them when I mow the fields ahead of the next transplanting date. I also plan to mow them to a greater height, and earlier in the year to give them a better shot at recovering ahead of the cantaloupe transplanting. I also plan to slightly widen the planting rows for the cantaloupe when I strip till to reduce the chances of direct competition with the living mulch through the season either through shade, or competition between their the roots for water and nutrients.

With these changes yield reduction in 2021 may be reduced, as it didn’t appear that insect damage had anything to do with the yield reductions from 2020. If not, it would serve to demonstrate a potential drawback of this particular living-mulch system. Increased integrity of the living mulch, combined with a hopefully more normal and insect rich summer, may be able to provide a much clearer picture of the viability of this system for real world application this coming summer.

 

 

 

Participation Summary

Education & Outreach Activities and Participation Summary

3 Webinars / talks / presentations

Participation Summary

Education/outreach description:

While the Covid-19 pandemic of the last year has disrupted virtually all in-person meetings, I took advantage of several successful virtual outreach opportunities to share the objectives and current findings of this ongoing project with members of the agricultural and scientific community.

In place of the annual University of Maryland Central Maryland Research and Education Center’s annual twilight wagon tour event which was canceled due to the ongoing pandemic, I gave a short talk on the progress of the project for the University of Maryland Extension – Charles County Youtube channel,  which is titled Developing a perennial living mulch system to manage cantaloupe pests. I also virtually presented the project’s current progress to the Northeast IPM Research Update Conference on November 17, 2020 and the Entomological Society of America 2020 Virtual Meeting. In the latter I was entered in the student 10-minute paper presentation competition, where my presentation on this project was awarded first place in the IPM-Horticulture category. My presentation was available on-demand to attendees in recorded format for the duration of the conference from November 11-25, which was widely attended by scientists from across the country. I also intend to submit a poster presentation of my project to the Mid-Atlantic Fruit and Vegetable Convention’s poster session from February 8-11 to further spread awareness about the ongoing project.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.