Cornell University takes its horticultural protection duties very seriously. They consider every angle when it comes to helping growers, including scaring pests to lessen plant damage.
“Pests are attacked by a lot of different predators,” noted Jennifer Thaler of Cornell’s Department of Entomology. Their reactions – will they keep feeding? Will they hide? Will they run away? – give researchers the information they need to influence biological controls.
“Normally when we think of these systems, we think about how predators kill prey to protect plants – the consumptive effect. We also want to think about this other pathway: predators can scare prey and change their behavior and physiology – the non-consumptive effect,” she explained. Across more than 100 different studies, fear caused 85% of the effect of predators on the prey’s resource.
Prey organisms – usually those that feed on your plants – can’t grow rapidly and invest in defending themselves at the same time. When prey detect a predator, they can either stop feeding or not respond. If they stop eating, they’re not as fit; if they do nothing, they may end up as meals. It’s the classic fight-or-flight response, with an added “hide” option. Thaler said thrips will fight, Colorado potato beetles will take flight and silverleaf whiteflies will hide. “Prey have short- and long-term responses that affect how much they eat and where they eat,” she added.
So an experiment was conducted. They manipulated predation risk with stink bugs and potato beetles. They cut off the last bit of stink bugs’ beaks so they could hunt and press their beaks into the sides of prey, so the prey was attacked but not eaten. They found potato beetle larvae feed less in response to predation risk – and larval exposure to predators decreased the number of eggs they laid as adults. “Adults are generally invulnerable – predators don’t affect where they colonize in a field,” Thaler said. “However, they do eat less and lay fewer eggs in the presence of predators. That made us think of other ways we could influence predation risk.”
The researchers wondered how potato beetle larvae detect predators, because if they knew that, they could influence the larvae without actually having the predators around. (Growers can already manipulate “the scent of death” – ladybirds leave odor tracks on leaves, and that odor reduces aphid acceptance.) “We wanted to know if beetles detect stink bug aggregation pheromone,” Thaler said. In the lab, they blew air over containers with stink bugs or empty containers onto the beetles, and they found larval feeding decreased if they were exposed to air from male bugs. The next step was to see if that scent affected host plant acceptance, so the researchers put the male odor on leaves – and the beetle larvae were not fans.
So what comes next? Nicholas Aflitto, also of Cornell’s Department of Entomology, said they’re working on developing a blend of odors to find which cues are responsible for the greatest prey response, whether that’s a male-female combination, single or aggregated predators, the predators’ diets or something else. They discovered that specifically, the stink bugs’ dorsal abdominal scent gland created the prey response.
“Predators can decide when they release these pheromones, so we wanted to see what influenced the release,” Aflitto explained. Out in the field, they wanted to measure plant damage, potato beetle presence and where they laid eggs. Across two growing seasons, they saw a 22% reduction in damage in 2016 (an extreme drought year) but no difference in 2017 (a very wet year) while using the stink bug smells. The weather did affect predator behavior. And the bugs tend to release more pheromones in the evenings to early mornings.
The researchers measured potato beetle behavior across all life stages and plant damages. When plants were pre-exposed to stink bug pheromones for 72 hours and then the leaves were fed to potato beetles, they saw similar results in two tested varieties – a 31% reduction in larvae and a 46% reduction in plant damage. (A bonus: pheromone exposure also marginally increased the yield in those plants.)
It turns out the smell alone can scare a number of the pests. The next steps in this research will be trialing additional crops, testing out a synthetic pheromone, developing release devices and measuring different plant responses to the pheromones.