Across a variety of agricultural systems, We evaluate context-dependent environmental drivers of natural pest suppression and crop productivity. Much of our work examines how agricultural biodiversity influences pest and weed control services, yet, we approach biodiversity through many different lenses and at many different scales. For example, we examine how biodiversity in soil microbe communities affects interactions between plant health, herbivore pressure, and the recruitment of predators who provide biological control services. We also examine how cover crops and tillage operations influence non-crop plant diversity at various scales, and how this in turn shapes predator community structure and subsequent biological control of pests and weeds. We combine experimental research in the field, lab, and greenhouse with a powerful observational approach, collaborating with farmers to complete on-farm biodiversity surveys of plants, insects and soil communities.
Weed ecology and conservation biological control of crop pests
Typically, weeds are aggressively managed in crop systems because they often compete with crop species and limit yields, but weeds also contribute to non-crop plant diversity that can deliver important ecosystem services. For example, the structural complexity of living plant material can provide refuge and a suitable microclimate for predatory insects that consume crop pests. Weeds can also provide nectar and pollen resources as alternative food for omnivorous predators, which can enable them to survive in agroecosystems during times of prey scarcity. Despite the perception that all weed growth is detrimental to crop production, many vegetable varieties are relatively competitive, and tolerating some weed growth may not necessarily come at a yield cost. Moreover, weed growth is often tolerated unwillingly by vegetable farmers who lack access to the labor resources required to cultivate frequently. To capitalize on biological control services conferred by weeds and to help farmers make more informed decisions about allocating limited labor resources for weed control, we are examining yield costs and pest control benefits over a gradient of weed pressure across a variety of crop families. Check out a poster on this work made by Melina Madden, an undergraduate researcher.
Also, using a large observational data set I collected as a postdoc, I am evaluating direct and indirect links between weed growth, predator biodiversity, and natural pest suppression in broccoli crops using structural equation models. We use this system to test two classic hypotheses about mechanisms by which non-crop plant diversity is thought to reduce pest pressure: (1) by providing additional resources to natural enemies that bolster their numbers and strengthen biological control (the ‘enemies hypothesis’), or (2) by disrupting host-location by specialist herbivores (the ‘resource concentration hypothesis’).
Organic fertilizers mediate herbivore resistance and biological control
Fertility management can have important implications for pest pressure in crop systems by increasing recruitment and retention of natural enemies and by altering host plant quality. Organic fertility amendments, animal manures in particular, are hypothesized to simultaneously promote plant vigor, herbivore resistance, and top-down suppression, but few studies test this complex series of interactions in the field. Further, while biodiversity in soil communities and predator communities can lead to greater plant growth and pest suppression, respectively, these biodiversity/productivity/biocontrol concepts have not yet been applied to fertility management. For example, diverse cocktails of fertility materials might lead to more diverse microbe communities that enhance production of anti-herbivore defenses by plants, but little is known about the communities of beneficial soil microbes that are present in various fertility materials (poultry manure, green manure, worm castings, etc). In ongoing work, I am comparing herbivore growth, inducible plant defensive chemicals, and biological control on plants grown in simple and diverse fertility cocktails.
Underground predator-prey interactions
Pests that develop in the soil, like flea beetles and cucumber beetles, are among farmers’ greatest pest management challenges. They evade chemical control underground as larvae, and often escape control as adults due to rapid mobility. Despite the importance of these pests, surprisingly little is known about their ecology during vulnerable immature stages, due to their cryptic larval habits. We’re working to better understand subterranean food webs to help farmers leverage cultural and biological control tools that specifically target key pests in their subterranean larval stages.
Predator invasion, antagonistic interactions, and weed seed biological control
Invasive predators are common in disturbed agroecosystems, and while they may perform biological control services by consuming pests, they also interfere with pest suppression by less dominant predators. Using red-imported fire ants as a model keystone predator, my students are examining how invaders alter habitat selection and trophic links between native species in a seed-based food web. Using seed removal assays, insect sampling and video surveillance, we found that fire ants largely exclude invertebrate seed predators in no-till crop systems. Moreover, because fire ants preferentially use habitat with vegetative cover, we find that typical preferences for vegetated habitat by the native species reversed: native seed predators negatively associated with cover in our system! Check out a poster on this work made by Annie McElvenny, an undergraduate researcher.
Omnivory, structural complexity, and top-down control
Most of the predators in agroecosystems are omnivores. Broad diets enable them to persist in highly disturbed environments where prey availability is not particularly stable. That means the food webs in crop environments are thrillingly complex (to a trophic ecologist) or frustratingly unpredictable (to a farmer). Furthermore, structurally complex refuge habitat, like cover crops, changes the foraging behavior of many predatory insects! To top it all off, predator foraging patterns and refuge use are both shaped by fluctuating cycles of predation risk (after all, predatory insects are often prey themselves). I use seed-based food webs to examine how omnivores shift their diets in response to predation pressure (“the stress diet”), and how this in turn affects biological control services.
Mixed signals: environmental drivers of tri-trophic interactions
Recent work from a rapidly-growing field suggests that pest control may be improved by indirect plant defenses, particularly herbivore-induced plant volatiles (HIPVs), which predators and parasitoids use to locate prey. Growers need clear information about how these induced plant defenses may enhance biocontrol and protect yield, but we currently have limited knowledge of the agriculturally-relevant impacts of HIPVs on biocontrol in working farm systems. A main reason for this poor understanding is that volatile production is a complex process mediated by many interacting ecological and abiotic factors that impede our ability to predict HIPV function in dynamic agroecosystems. For example, the composition of induced volatile profiles can be altered by the herbivore community, soil nutrition, microbes, drought stress, temperature, and humidity. Because so many exogenous variables ‘muddy up’ indirect plant defenses, relatively few plant volatile experiments are performed in field environments. However, without evidence from the field in naturally-diverse systems, community-level consequences and applied implications of tri-trophic interactions remain unclear. We combine observational and experimental approaches to examine volatile-mediated predator-prey interactions across diverse field environments.
Photos Megan Asche