Arbuscular mycorrhizal fungi alters soybean resistance against herbivores in a feeding guild specific manner

Cover crops provide many benefits for soil health and fertility, but also leave a soil legacy for the subsequent cash crop by changing microbial communities. Some cover crops facilitate the proliferation of arbuscular mycorrhizal fungi (AMF), while others can diminish fungi by not forming mycorrhizal associations, or by acting as a bio-fumigant. AMF has been historically viewed as beneficial to crops--priming plant defenses against insect herbivores, and increasing water and nutrient uptake. However, we now know this relationship is context-dependent, and the direction of the interaction may depend on biotic factors (insect feeding guild, plant species, fungal species) and abiotic factors (nutrient and water availability). We evaluated the effect of commercial AMF inoculant on plant defenses with insect performance assays. Insect herbivores were chosen from two feeding guilds (chewing and piercing-sucking). The piercing-sucking herbivore (Aphis glycines, soybean aphid) had lower population growth on AMF-inoculated plants as compared to control plants. In contrast, weight gain of the leaf-chewing herbivore (Chrysodeixis includens, soybean looper) was higher when feeding on AMF-inoculated plants compared to larvae feeding on non-inoculated plants. To better understand if cover crop legacy influenced field crops, we examined whether AMF provides a similar response in plants grown in a field after a mycorrhizal (triticale) vs non-mycorrhizal (radish) cover crop, and what role natural enemies play in soybean aphid population control.