Pesticide makes cockroaches slow down and forget

Abstract

Farmers have been waging war against pests since humans first started planting crops. More recently, advances in pesticides have curbed many of the crop losses that farmers have been experiencing, which has helped the planet's population to grow to 8 billion people. However, some of the pesticides come with a cost; they inadvertently kill pollinator species which the agriculture industry relies upon to fertilize their crops. Not only do many pesticides have unintentional victims but they also can remain in the groundwater for some time, thereby affecting insects that don't come into direct contact with the treated plants. Though the amount of pesticide that makes its way into the water supply is much less than the amount that is sprayed onto a plant, these smaller doses could still have dire consequences for other insects. With this in mind, Maria Rosa of the Universidade Federal do Pampa (UNIPAMPA), Brazil, and Cháriston Dal Belo of the Universidade Federal de São Paulo, Brazil, turned to the speckled cockroach (Nauphoeta cinerea) to help them answer whether smaller doses of the pesticide fipronil affected these non-target insects.Working with other scientists from UNIPAMPA, the Universidade Estadual de Campinas, Brazil, and the University of Oslo, Norway, Rosa and Dal Belo first gave the cockroaches different amounts of fipronil before measuring how many times they cleaned their legs and antennae in 30 minutes. The cockroaches didn't groom their legs as much when they were given fipronil. According to Rosa, ‘grooming in insects is associated with body cleanliness and courtship’, among other behaviours, and not spending time grooming could lead to a shorter lifespan and fewer offspring. In addition, the researchers measured how far the insects walked versus how much time they spent stationary in a 10 minute video. When given the pesticide, the cockroaches didn't explore as much and spent more time stationary, even at the lowest doses of fipronil. Not spending time moving or exploring could cause these insects to be unable to find food or escape from predators. As the cockroaches weren't moving as much, Rosa and Dal Belo wondered what was causing this lack of locomotion.To get to the root of this, the duo measured how hard the muscles of the cockroach's legs contracted. With the highest dose of fipronil, the legs had trouble contracting, therefore reducing the animal's mobility and potentially hampering their ability to find a meal. But it isn't just movement that fipronil affects. Because of the way that fipronil interacts with the insect's brain, the duo were also interested in how the pesticide might affect how well the cockroaches remembered certain smells. Normally, cockroaches are repulsed by the smell of citronella, but they can learn to associate this odour with the reward of a sugar solution, causing them to move towards the smell. However, if the insects were given fipronil beforehand, they couldn't learn to associate citronella with the sugary reward. Being unable to learn to associate smells with a reward could have devastating effects on an animal's ability to find food or mates.While Rosa and colleagues utilized the nearly indestructible cockroach to study the effects of this particular pesticide, they are quick to point out that their findings are likely true for other insect species as well. While using pesticides definitely helps our crop yields, it could also have the unintended yet devastating side-effect of decimating the number of pollinator species such as bees and butterflies, which could be even more disastrous for humans than allowing a few crops to be eaten by pests.