Manure decreases herbivore performance but increases early-season damage on corn in the greenhouse and field

Many farmers use manure as an alternative to inorganic fertilizer. Previous research has shown that manure can decrease plant susceptibility to herbivores, but the mechanisms remain unclear. To determine how manure affects herbivore performance in a greenhouse setting, we fertilized corn with stacked cow manure or an equivalent amount of NPK fertilizer and measured caterpillar development, plant nutritional content, and defenses. After four weeks of growth, we allowed fall armyworm (Spodoptera frugiperda) or black cutworm (Agrotis ipsilon) caterpillars to feed on these plants for 6 d. Manure reduced mass of black cutworm caterpillars and smaller fall armyworms compared to inorganic fertilizer. We paired this greenhouse experiment with a three year field experiment, which incorporated a wheat cover-crop treatment crossed with the fertilizer treatments in a 2x2 factorial design. We measured plant damage early in the season from naturally-occurring herbivores and measured neonate fall armyworm performance on field-collected leaf tissue. In 2017, corn in manure-fertilized plots sustained more herbivore damage, primarily driven by a higher incidence of slug damage. Fall armyworm performance, however, was lower on leaves collected from manure-fertilized plants. In contrast to previous studies, we did not find increased micronutrients or enhanced defenses in manure treated plants. While manure can offer resistance to some herbivores, our results suggest that this resistance can be overshadowed by habitat conditions which may favor herbivores that are more tolerant to decreased plant quality.