Introducing Beneficial Entomopathogenic Nematodes for Biological Control and Enhanced Plant Resistance to Improve Pest Management in Cucurbit Crops

View the project final report

Commodities

• Vegetables: cucurbits

Practices

• Pest Management: biological control, integrated pest management

To combat heavy reliance on synthetic insecticides among conventional growers, and improve pest management options for organic growers, we plan to evaluate the potential for biological control with beneficial entomopathogenic nematodes (EPNs) to help manage cucumber beetles and other pests of cucurbit crops.

Previous research trials using EPNs for biological control have yielded promising results against a variety of insect pest species (Koppenhöfer et al., 2020). This includes a trial with cucumbers demonstrating EPNs can be used to control cucumber beetles in both conventional and organically managed systems, with strong potential to reduce insecticide inputs (Ellers-Kirk et al., 2000). Our recent research has explored the ecological basis for EPN biocontrol of cucumber beetles, adding further support for its potential applications (Grunseich et al., 2020). Furthermore, we recently documented additional benefits of EPN biocontrol that could further enhance its effectiveness for pest management (Helms et al., 2019). Building on previous efforts to advance biocontrol with entomopathogenic nematodes, our recent findings suggest that soil applications of EPNs can directly boost plant resistance against foliar-feeding pests like adult cucumber beetles (Acalymma vittatum) and melon aphids (Aphis gossypii), as well as the fungal pathogen powdery mildew (Podosphaera xanthii) (unpublished data). A preliminary biocontrol trial on our cooperator’s farm also suggested EPNs contributed to better plant protection. By documenting robust and reliable benefits of EPN-based biocontrol, and communicating these findings with growers, our long-term goal is to encourage greater use and improve sustainability in pest management by reducing insecticide inputs.

The specific objectives of this study are to: 1. Determine the effectiveness of EPN biocontrol in cucurbit crops for reducing numbers of pest insects and plant damage, while preserving beneficial insect populations 2. Assess how introducing EPNs affects plant growth and yield. We hypothesize that introducing EPNs will reduce insect damage and boost yield of cucurbit crops through two mechanisms, including both direct mortality of cucumber beetle larvae by EPNs and increased plant resistance against pests following plant exposure to EPNs. Furthermore, because EPNs will only be applied to soil near roots, we predict that they will not reduce numbers of beneficial insects visiting plants. We plan to evaluate EPN introductions for 3 different cucurbit crops, including cucumbers (Cucumis sativus), watermelons (Citrullus lanatus), and summer squash (Cucurbita pepo). This will inform control potential in 3 important crops for our cooperator and the U.S. cucurbit market, and will particularly benefit growers on diversified farms seeking better management options.

An On-farm Research Grant will also allow us to coordinate education and outreach events to communicate and promote our research findings. Taken together, this project offers an exciting opportunity for on-farm assessment of EPN biocontrol and for developing an education and outreach program to encourage greater use of biological pest control among vegetable growers.   

Project Objectives
We plan to evaluate the potential for entomopathogenic nematodes (EPNs) to provide biological control and boost plant resistance to improve pest management in cucurbit crops following two specific objectives:

1. Determine the effectiveness of EPN biocontrol in cucurbit crops for reducing numbers of pest insects and plant damage, while preserving beneficial insect populations.
2. Assess how introducing EPNs affects plant growth and yield.

Experimental design
This project will be conducted at Ronin Farm, which is a 15-acre diversified vegetable farm located in Bryan, TX. This project was developed in close collaboration with the owner, Mr. Brian Light, who expressed strong interest in improving pest management on his farm through sustainable strategies and incorporating biological control with beneficial nematodes as part of his management plan. Ronin Farm sells produce to a local “farm to table” distributor, in a CSA, and in the Ronin Restaurant. Mr. Light selected the plant varieties for this project based on previous experience with produce quality and plant performance in Brazos County, TX. The cucumber variety will be Southern Delight and the summer squash variety, Grey Zucchini (Kitazawa Seed Co., USA). The watermelon variety will be Ali Baba (Baker Creek Seeds, USA). The entomopathogenic nematodes selected for this project are Steinernema riobrave, which is a heat-tolerant species originally isolated from the Texas Rio Grande Valley and is suitable for southern climates and cucumber beetle control (Arbico Organics, USA).

This project will be conducted over two field seasons in 2021 and 2022. We will use a randomized complete block design, similar to a previous EPN study (Ellers-Kirk et al., 2000). Each block will be divided into 2 rows and each row will receive one treatment: 1) plants treated with EPNs and 2) untreated control plants. Blocks will be replicated 4 times each for cucumbers, watermelons, and summer squash. Ronin Farm, and other growers in this area, typically plant the first cucurbit crops in early March and make additional plantings through May. We will conduct our trials mid-season, starting in early April of each year. Plants will be grown and maintained following typical cultivation practices on Ronin Farm. Seeds will be planted directly into raised rows and thinned as needed. For cucumber, we will plant ~4 m long rows with 10 plants each, spaced 30 cm apart. For Summer squash, we will plant ~7 m long rows with 10 plants each, spaced 60 cm apart. For watermelons, we will plant ~9 m long rows with 5 plants each, spaced 1.5 m apart. Crops will be adjacent, and all rows will be 1 m apart. Cucumber vines will be trained onto trellises to promote airflow to leaves and facilitate easier surveys and harvest. Rows will be covered with straw mulch to suppress weeds and retain moisture and plants will be irrigated as needed. Ronin Farm does not apply any insecticides and none will be used during the project.

EPN treatments will be applied twice per year to trigger plant resistance and target soil-dwelling pests like cucumber beetles. Plants will be treated with EPNs at 1 true leaf (~2 weeks after planting) and when they begin to flower and set fruit (~6 weeks). Each treatment plant will receive ~200,000 free-living EPNs in 100 mL water and control plants will each receive 100 mL water added to soil at the plant base.

Insect surveys and damage assessments
To quantify numbers of pest insects, we will conduct weekly visual surveys of all experiment plants from germination until final harvest. Each plant will be observed for 60 seconds to search for insects. If detected, insects will be sight identified (to species, family, or order as possible) and counted (Skidmore et al., 2019). Examples of insects in surveys include striped (Acalymma vittatum), spotted (Diabrotica undecimpunctata howardi), and banded (Diabrotica balteata) cucumber beetles, melon aphids (Aphis gossypii), squash bugs (Anasa tristis), squash vine borers (Melittia cucurbitae), leaf footed bugs (Leptoglossus zonatus), and silverleaf whiteflies (Bemisia tabaci). We will also assess cucumber beetle control using traps to monitor adult beetle emergence from soil. Traps (25 cm², 12 cm tall) will be placed on the soil surface between plants in each row (4 per row for each plant species) 5 weeks after planting, allowing time for any adult beetles to lay eggs, and will be monitored twice weekly for 4 weeks (Ellers-Kirk et al., 2000). 

During surveys, we will also quantify numbers of beneficial insects present on plants, allowing us to determine whether EPN introductions influence the beneficial insect community. Examples of insects to be surveyed (sight identification) include pollinators like honey bees (Apis mellifera), squash bees (Peponapis pruinosa), and other native bees (Osmia, Agapostemon, Bombus, Megachile sp., etc.), as well as natural enemies like wasps (e.g. Ichneumonidae, Braconidae, Vespidae, etc.), flies (Syrphidae), and beetles (e.g. Coccinellidae, Carabidae, etc.).

In separate weekly surveys, we will estimate percent damage (insect or pathogen) area on leaves, flowers, and fruits.

Plant growth and yield data
To determine the effects of EPN introduction on plant biomass, we will quantify root length density following previously described methods (Ellers-Kirk et al., 2000). Briefly, soil cores will be taken near plant stems during the final week of harvest and cm root length per mm soil will be calculated. This will be replicated three times per row for all plant species. We will also record plant heights, numbers of leaves, and numbers of male and female flowers in weekly surveys.

Yield data will be collected throughout the season as fruits are harvested. Fruits will be graded for marketability and total weight and fruit number recorded. The estimated harvest durations are: ~6 for weeks cucumber, ~6 weeks for squash, ~4 weeks for watermelon (Skidmore et al., 2019).

Data analysis
Data will be analyzed using the software program R. Data on insect numbers, damage estimates, plant growth, and yield will be analyzed using generalized liner mixed models. Additionally, to characterize the insect pest and beneficial communities present on each treatment, we will use multivariate community analyses, including permutational multivariate analysis of variance and non-metric multidimensional scaling ordination (Oksanen et al., 2019).