Performing Department
Entomology
Non Technical Summary
Billions of dollars are allocated to pesticides each year. Misues/overuse of pesticides can have negative environmental and human health consequences. Devloping alternative pest management strategies can improve agro-ecosystem health and stability. Cover crops are typicaly non-commodity crops that are grown in between plantings of the main crop (e.g. corn, soybean). Cover crops provide many ecosystem benefits, but little is known about how they affect soil communities, such as beneficial microbes. Arbuscular mycorrhizal fungi (AMF) are a group of beneficial soil organisms that form mutualistic associations with most agricultural crops. These beneficial fungi increase nutrient uptake in their host plant but can also affect how that plant defends itself against insect pests. This study will link cover crops to AMF colonization in the following crop (corn) and will further investigate the impact on plant defenses to different insects. Results from this study can be used by growers to intentionally select cover crops based on their pest management needs while contributing toreduced reliance on chemical inputs, such as pesticides, and improved soil and environmental health.
Animal Health Component
(N/A)
Research Effort Categories
Basic
100%
Applied
(N/A)
Developmental
(N/A)
Goals / Objectives
The goal of this project is toexamine how cover crops with varying levels of AMF associations influence resistance to multiple insect pests in corn. Additionally, to providefundamental information that uncovers the mechanisms linking cascading interactions between cover crops, soil microbe activity, and herbivore resistance. It will also contribute to a growing body of knowledge that can be utilized by farmers to intentionally select cover crops for improved pest management.Objective 1: Quantify how diverse cover crops facilitate AMF colonization, affect plant defenses,and alter herbivore performance.Objective 2: Evaluate the behavioral responses of three differentpests to host-plants with cover crop facilitated AMFcolonization.Objective 3: Determine whether common mycorrhizal networks communicate defense-relatedsignaling between plants and how this impacts pest dynamics.
Project Methods
Herbivore Performance:Separate experiments will be conducted to evaluate defense responses and performanceof the different herbivores. Surface sterilized corn seed will be sown in soil collected from fieldsof each of the cover crop plots andgrown in a greenhouse. Once plantshave developed to the V3 stage, theywill be exposed to one of threeherbivore treatments (FAW, WCR,or CLA) or left undamaged in orderto compare constitutive and inducedplant defenses in response to thesevarious herbivore feeding strategies.FAW treatments: Five neonates willbe placed on each plant. After 24 hr,leaf tissue will be collected fromdamaged and undamaged plants inorder to measure plant responses.Herbivore performance will be evaluated by comparing the number and weight of the survivinglarvae. Leaf area removed will be quantified using imaging software (Image J).WCR treatments: Twenty neonates will be applied to the roots of each plant and left to feed.Root tissue will be collected 4 d post-infestation from half of the damaged and all undamagedplants. Because I found in my preliminary study that 4 d was not sufficient to detect differencesin larval performance, the remaining herbivore-infested plants will be left intact for an additional6 d. Larvae will then be recovered, counted, and weighed to compare survival and weight gain.CLA treatments: Five viviparous aphids will be placed on each plant. After 4 hours, leaf tissuewill be collected from half of the damaged and all undamaged plants18. The remaining aphidinfestedplants will remain intact. After 1 wk, the total number of aphids will be counted on eachplant.Plant measurements: Collected plant tissue will be flash frozen in liquid nitrogen forphytohormone analyses, gene expression, and protease inhibitor activity. Jasmonic (JA) andsalicylic (SA) acid are two phytohormones that are induced by herbivore damage. Damage bychewing herbivores is most commonly associated with induction of JA, while SA is usuallyassociated with pathogens and phloem-feeding insects. Colonization by AMF has also beenlinked to increased SA signaling. Induction of phytohormones leads to downstream plantdefenses, making them good indicators of plant defense responses. Transcript abundance ofgenes associated with JA and SA biosynthesis and known marker genes involved in corn's directdefenses will be measured using cDNA generated by qRT-PCR. Insect feeding also inducesanti-nutritive defense enzymes, such as cysteine and trypsin protease inhibitors. Proteaseinhibitor activity will be measured in plant tissue by assessing percent inhibition of enzymaticactivity. Root tissue will be stainedand percent AMF colonization will be calculatedmicroscopically by counting the proportion of intersections colonized by AMF.Herbivore Preference:Adults: FAW, WCR, and CLA females will be individually assayed in choice arenas. Controlsand plants will be randomly placed equidistantly in a mesh cage. A single gravid female FAW,WCR, or winged CLA will be released into the center of each cage. Initial host-plant choice willbe recorded for CLA.After 48 hr, the number of egg masses and total number of eggs on eachplant (FAW) or in soil (WCR) will be recorded.Larvae: Choice arenas will be constructed by arranging plants around a circular boxboardplatform. The platform will reach the base of each plant and enable herbivore access to each hostplant. An individual first instar FAW larva will be placed in the center of the platform andobserved until a choice is made. A 6-arm olfactometerwill be used to evaluate WCR larvalbehavior. The central chamber of the olfactometer will be filled with sterilized soil andconnected to chambers containing individual treatment plants and controls. Fifty neonates will bereleased into the central chamber. After 48 hrs, the olfactometer will be disassembled and thenumber of larvae recovered on each plant will be recorded.To identify differences in volatile signaling, root and shoot volatiles will be collectedsimultaneously using glass guillotine collection chambers32. Air will be pulled over roots andshoots, and collected on Super-Q adsorbent traps. Using GC-MS, I will assess how volatileprofiles change in response to cover crop treatments.Mycorrizal Netowrks:Mesocosms will be established in agreenhouse using pots that are divided in half by astainless steel mesh barrier (40 μm aperture). This willprevent root-root contact and WCR movement, butallow hyphal connectivity. To establish CMNs, soilwill be sterilized to remove any prior fungal spores.Ten surface sterilized triticale seeds will be sown ineach half of every mesocosm. Half of the mesocosmswill be inoculated with AMF. Triticale will be grownin the mesocosms to encourage the formation ofCMNs for 4 months and removed prior to the start ofthe experiment. Two-week old corn seedlings will betransplanted, one in each half of the mesocosm andgrown for 4 weeks to allow established AMF tocolonize the roots. Twenty WCR neonates will be applied to one plant (the "donor") in eachmesocosm, while the other plant (the "receiver") receives no damage. This design allows fordamaged and undamaged pairs with and without CMNs. The aboveground plant tissuewill be bagged in polyethyleneterepthalate to prevent aboveground plant signaling. Donor andreceiver plant root and shoot tissue will be sampled after 4 d for phytohormone analysis, geneexpression, and protease inhibitor activity.