The use of cover crops provides both direct and indirect benefits to agricultural ecosystems. Cash crops sown after cover crops directly benefit from better nutrient availability, increased moisture retention, and improved weed suppression. Although there is some evidence, we still lack a complete understanding on the effects of cover crops as an insect herbivory suppressor, and the mechanisms that mediate these effects, if any. To test this, we planted two commonly used leguminous cover crop species, sunn hemp, Crotalaria juncea, and cowpea, Vigna unguiculate in an organic farm in South Texas and observed their effectiveness in providing pest management while also attracting beneficial insects. Data was collected on plant height, insect damage, and plant biomass, in addition to insect community composition in the field. Our preliminary results on cover crops suggest that sunn hemp performed significantly better in all measures, and continuously showed significantly lower pest damage (caterpillars, true bugs, and aphids) along with a high prevalence of beneficial insects (Coccinellids, wasps and bees) when compared to cow pea. Sunn hemp also improved (higher species richness) the surrounding insect community in the area and thrived in the summer heat without much resource input. Since host location and host choice by insects are primarily mediated by plant volatiles, we are currently examining whether the two cover crop species differ in their volatile profile- both constitutively emitted, and under herbivory. Future experiments also include examining the residual effects of cover crops on the cash crop- which is currently underway.
The specific objectives of this project were to:
- Determine the impact of cover crops on the tri-trophic interactions of insects in organic vegetable systems.
- Examine the volatile emitted by plants under pest attack and how that impacts beneficial insect reaction.
Legume cover crops, sunn hemp and cowpea were planted in an certified organic farm in Edinburg, TX during the summer of 2018. We collected information on: cover crop height, insect damage, and cover crop biomass. We also collected insect samples in the cover crop treatments. Cover crop height was measured using measuring tape from the base of the plant, not including roots, to its tallest point. Damage was assessed using a visual scale of 0 (no damage) to 4 (extensive damage, little of the plant remains). From the visual insect damage estimation we determined the damage caused by insect pests. Cover crop biomass was collected just before cover crop termination. Cover crops were harvested and oven dried for 48 hours and total biomass for each cover crop was estimated. Flying insects in the cover crops were collected using the sweep net. We also estimated the pest damage in cash crops following the visual damage scale recording. Insects in the cash crops were collected using a sticky trap and pitfall traps.
Our preliminary results indicate that sunn hemp performed significantly better than cowpea in biomass, height, and had low insect damage. The biomass average for sunn hemp was ~15 grams and ~7 grams for cowpea. The average height of sunn hemp was ~35 centimeters and ~20 centimeters for cowpea. Sunn hemp showed significantly less damage than cowpea. This extensive damage cause by herbivores on cowpea is possibly due to its attractiveness (through volatiles) and/or higher nutritional value- with possible variation in resource allocation between defense and fitness. Lastly, insect samples were collected from the sunn hemp, cowpea, and control plots to determine insect community composition. Once analyzed, results showed higher insect diversity among sunn hemp, for both beneficial and herbivores, while cowpea in general had lower diversity.
Educational & Outreach Activities
Cover crops, beneficial insects, interaction between pest and beneficial insects