The role of cover crop diversity on key generalist predator attraction and pest suppression

The role of cover crop diversity on key generalist predator attraction and pest suppression

Summary

Insect natural enemies play a critical role in suppressing agricultural pests, but they also rely on plants for alternative food and shelter when prey is scarce. Many agricultural landscapes are simplified and lack alternative plant resources. Natural enemy survival in these landscapes may be reduced because the resources on which they depend are absent. Addition of resource-producing insectary plants in the landscape represents an effective tactic to promote natural enemies. These insectary plants provide nutrient rich nectar and pollen to predators and parasitoids and can therefore be used to bolster the abundance of natural enemies and enhance their ability to suppress insect pests. Using insectary cover crops to promote the presence of beneficial insects in the field represents a sustainable approach to pest management that can result in reduced reliance on pesticides which can safeguard grower health and the environment.

Objectives/Performance Targets

The specific objectives the study are:

1. 1. Evaluate the effects of plant-based cover crop resources on the survival and longevity of two key generalist predators (Ongoing).

1. 1. Determine cover crop growth, development, and plant resource quality established in the field (Completed).

1. 1. Assess the foliar arthropod community associated with buckwheat, cowpea and a buckwheat-cowpea biculture compared to a fallow control (Completed).

1. 1. Monitor predation on pest insects within the cover crop strips and adjacent corn crop (Completed).

1. 1. Investigate dispersal of arthropods between the cover crop borders and cash crop plots (Completed).

Overall Progress:
This past field season, the second trial of the field study was conducted similar to the previous year. However, a few adjustments were made to the initial project design. To achieve a healthier, more representative stand of cowpea in its respective treatments, seeding rates were increased. The size of the project was also increased to observe the influence of larger insectary borders on the insect community. Similarly, the corn main crop plot sizes were increased to observe dispersal at further distances. Another change to the project was the addition of color-coded fluorescent dyes added to the egg white marking solution to better track insectary visitation by the two study insects. This year, the study was flagged and planted in early June. Data collection continued through September.

Objective 1: Natural Enemy Longevity
Progress toward this objective is ongoing. We are currently learning how to most efficiently rear and harvest the study insects in large enough numbers to conduct the proposed greenhouse studies (seen in Allen 2012). The rearing colonies have been established, but we are still figuring out the best ways to harvest eggs.

Objective 2: Insectary Establishment
This year, the weedy fallow, buckwheat monoculture, cowpea monoculture and buckwheat-cowpea biculture treatments were established in the field in early June. Three weeks after planting , we began sampling to assess the insectary cover crop establishment. Sampling included weekly measurements of cover crop plant density, height, leaf number, total number of buckwheat floral inflorescences and cowpea extrafloral nectaries and the presence of weed species. We also recorded accumulated growing degree days.

Objective 3: Insect Community Effects
Assessment of the insect community was conducted similar to the previous year. Insects were collected weekly using sweep net sampling. Timed counts were done in standing corn plots to assess the abundance of natural enemies. This year’s samples are currently being identified and analyzed.

Objective 4: Predation Effects
Predation was assessed using European corn borer sentinel insect eggs placed in the field every two weeks. Currently, all collected eggs have been examined, recorded, entered and is awaiting formal statistical analysis and interpretation.

Objective 5: Predator Dispersal
To observe dispersal between insectary borders and the corn main crop, insectary borders were marked using egg white protein and an additional color coded fluorescent dye to determine the particular insectary treatment that a marked insect has visited. Recaptured specimens are currently being processed for the presence of fluorescent dye and will later be tested for the presence of the egg white protein.

Accomplishments/Milestones

This year we’ve worked toward each of the objectives and training outlined in my initial proposal. Here, I will summarize some of the finding from the data collected in 2014.

1. Effects on Longevity
No data is currently available from this study on the direct effects of floral provisioning on natural enemy longevity as we are currently rearing Coleomegilla maculata and Orius insidiosus for use in the small scale green house study.

2. Cover Crop Establishment.
After planting, cover crops in each of the treatments emerged 5 days after planting and data collection began two weeks after planting. Figure 1 depicts cover crop resource production, measured as the number of inflorescences (left) or extrafloral nectaries (right) per 0.25m², each week after planting. Buckwheat in both the monoculture and mixture treatments began to flower 5 weeks after planting, reaching peak blooms of ~700 and ~400 inflorescences per 0.25m² at 7 weeks after planting, respectively. Cowpea nectary structures begin developing shortly after planting in the stipels of each trifoliate leaf. Cowpea nectary production reached ~250 and ~50 nectaries per 0.25m² before termination 7 weeks after planting in the monoculture and mixture treatments, respectively. Buckwheat inflorescences and cowpea extrafloral nectaries were lower in the mixture treatments due to lowered seeding rate of each. Cowpea development was slowed in the mixture treatment likely due to vigorous growth by buckwheat leading to increased competition with cowpea. Moving forward, this effect can be minimized by further lowering the seeding rate of buckwheat, while increasing the rate of cowpea in the mixture. The weedy fallout treatment contained no observable floral or extra floral nectary resources.

3. Effects On Insect Community
Predators
Sampling for arthropods began just before floral bloom. The most abundant predatory arthropods found in our sweep net samples were crab spiders (Araneae: thomisidae), pink spotted ladybeetles (Coccinellidae: Coleomegilla maculata) and insidious flower bugs (Anthocoridae: Orius insidiosus). Crab spiders appeared in increasing numbers in the buckwheat and mixture treatments, peaking at ~2.5 and ~2 spiders per ten sweeps, respectively. Crab spiders were significantly more abundant in BW and MX treatments compared to weedy fallow and cowpea treatments (Figure 2a). Lady beetle abundance was relatively low across all treatments until week 7 where there were significantly more lady beetles in buckwheat and biculture compared to weedy fallow and cowpea. Weedy fallow contained significantly more lady beetles than cowpea at 7 weeks after planting (Figure 2b). Insidious flower bug abundance increased steadily in buckwheat and biculture treatment over the course of the bloom period. Insidious flower bugs were significantly more abundant in BW and MX treatments at 6 and 7 weeks after planting (Figure 2c). Flower bug abundance remained low in weedy fallow and cowpea treatment throughout the insectary bloom period.

Pests
The insectary borders impacted pest arthropods as well. The most abundant pests found in the insectary borders were aphids (Aphididae), tarnished plant bugs (Miridae: Lygus lineolaris) and potato leafhoppers (Empoasca fabae). Aphid abundance was sporadic throughout the growing season in each treatment. By week 7, aphids appeared to be more abundant in the weedy fallow, buckwheat, and mixture treatments, the differences were not statistically significant different (Figure 2d). Tarnished plant bugs appeared to be highly attracted to buckwheat as their abundances were significantly higher in buckwheat and mixture treatments throughout the course of the study. By week 7, tarnished plant bugs abundance in weedy fallow were similar to those seen in BW and MX treatments (Figure 2e).Potato leafhoppers were highly attracted to cowpeas and peaked at roughly 50 leafhoppers per 10 sweeps in cowpea treatments (Figure 2f). Leafhopper abundance was low in BW, MX, and weedy fallow throughout the season.

Summary
Buckwheat appears to be attractive to crab spiders, lady beetles, and insidious flower bugs which can be economically important predators. However, buckwheat can also be attractive to tarnished plant bugs, which can often be an economically important pest. Cowpea did not appear to be attractive to either of the predatory species. This could potentially be due to poor stand establishment during the study period. Cowpea appears to also attract an economically important pest species, the potato leafhopper.

4. Predation Effects
To test for predation, we deployed sentinel European corn borer eggs within the insectary borders and at increasing distances away from the insectary border in the corn regions (5ft, 15ft, 25ft and 40ft). Figure 3 shows the cumulative predation by chewing and sucking predators on European sentinel corn borer eggs placed throughout the study. No differences were found comparing distances within corn, so they were combined and compared to the insectary border. Approximately 60% of eggs, on average, placed in the corn portion of plots were attacked by predators and did not differ depending on the type of insectary border treatment. When placed in the insectary borders, sentinel eggs were less likely to be attacked. Predation rates in insectary borders varied from 30%-45% and were significantly lower compared to those place in main corn. Looking at predation within the insectary borders, predation was significantly lower in buckwheat, cowpea, and the mixture suggesting that the presence of the resources in these insectary borders distract the natural enemies from feeding on the targeted pest insects.

5. Dispersal Effects
We captured 1052 pink spotted ladybeetles and 1291 insidious flower bugs to test for the presence of the protein mark. However, few individuals were positively marked with either protein. Due to low recovery of positively marked predators, it is difficult to determine how insectary treatment and/or transect influence predatory abundance in corn cash crop after termination of the resource rich border. The positive chicken marks, 22 (2.09%) pink spotted ladybeetle and 15 (1.16%) insidious flower bug, confirm that there is some degree of movement by predators that visited insectary borders into corn after termination of the insectary border. Positive milk marks indicate 15 (0.99%) C. maculata and 37 (2.86%) O. insidiosus preyed on marked sentinel eggs. No predator tested positive for both marks which would have confirmed that an individual both dispersed and preyed on marked sentinels.

• Figure 1. Resource density measured in 2014 as mean number of inflorescences (left) and cowpea extrafloral nectaries (right) per 0.25m2
• Figure 2. Mean number of (A.) Araneae, (B.) Coccinellidae, (C.) Anthocoridae, (D.) Aphididae, (E.) Miridae, (F.) Cicadellidae collected from sweep net samples collected from flowering insectary strips in 2014.
• Figure 3. Cumulative proportion of European corn borer sentinel eggs attacked by predators during the 2014 growing season. Blue bars represent the proportion of eggs attacked in the insectary borders while red bars represent eggs attacked within the corn plots in weedy fallow (WF), buckwheat (BW), cowpea (CP), and buckwheat-cowpea mixture (MX).
• Table 1. Total number and proportion of C. maculata and O. insidiosus that were collected and tested positive for the presence of a dispersal and predation protein.

Impacts and Contributions/Outcomes

Data from this study was presented at this year’s annual Entomological Society of America’s Eastern Branch meeting and again at the annual national meeting. In addition to these two conferences, this research was also presented at three different occasions at farmer-focused advisory board meetings and field day events. These events each focused on grower adoption of cover crops mixtures aimed at improving ecosystem functions such as weed suppression, pest management and soil fertility. These events attracted both local and international farmers interested in learning of the costs and benefits cover crop use.

Collaborators: