Sticky pesticide to protect crops from insects

Dutch scientists have engineered a sticky substance that protects plants from diseases and pests by trapping them when they land on plant leaves, incapacitating them. The researchers hope that this ‘insect glue’, which they described in the journal PNAS, will help to reduce the use of toxic chemical pesticides.

In the search for alternatives to potentially harmful chemical pesticides, scientists from Wageningen University & Research (WUR) and Leiden University turned to nature for inspiration. As explained by Thomas Kodger, an associate professor at WUR, “The carnivorous sundew plant has so-called glandular hairs that secrete a sticky substance to catch insects. We wanted to mimic this to protect our plants and crops in a natural way.”

The researchers transformed vegetable rice oil into a yellow, sticky substance by blowing air over it and grinding it into small particles using a laboratory blender. This resulted in beads of about 1 mm in diameter that are as sticky as duct tape, and similar in size to Californian thrips (Frankliniella occidentalis) — common pest insects that cause major problems worldwide in greenhouse horticulture.

The pesticide was sprayed as thick, sticky drops on plant leaves, with these drops catching the thrips inside them. By catching these insects, plants stay healthier and are less likely to become infected with fungi that the thrips carry with them. And the larger the drops, the greater the catch.

“The large drops are clearly more successful,” said Thijs Bierman, a PhD candidate at the Institute of Biology Leiden. “Presumably, the thrips need to get stuck with a minimum body surface area. This is also observed in carnivorous plants.”

So far the scientists have mainly focused on thrips, but the insect glue may also work against other pests, such as the Suzuki fruit fly that currently threatens cherry cultivation. At the same time, the drops are small enough that beneficial insects, such as pollinators, do not get stuck. Furthermore, pest insects are unlikely to develop resistance against this adhesive.

“Insects have already evolved so that they avoid adhesion, for example through hairs on their body and a bumpy surface, [so] increasing their body size remains one of the few escape methods from this sticky trap,” Kodger explained. This evolution is not nearly as easy as developing tolerance to a chemical substance; it would take many generations, if it happens at all, and would only occur if the insect glue is used on a large scale.

After application, the sticky substance remains on the leaves for three months, which is long enough to control pests until harvest. By spraying crops before the fruits develop, farmers minimise the chance of the pesticide getting onto the food — although contact with food cannot be ruled out.

“The advantage of our pesticide over chemical pesticides is that you can see the small, yellow drops; you can wash it off with water and dish soap,” Kodger said. If the substance is ingested, Kodger expects it to be no more harmful than frying fat, given that it is derived from vegetable oil, but scientists still need to investigate how (un)healthy it is exactly.

In the future, farmers may be able to spray the sticky pesticide on their fields using existing chemical sprayers, as a special additive in the mixture ensures that the beads do not stick to each other or the machine. The researchers must, however, assess the environmental impact of this, as they would rather not dump large amounts of cooking oil onto the fields. They are also looking to understand how the substance will affect the natural enemies of thrips, which are often used by farmers for biological control.

In addition, the scientists are exploring possibilities to incorporate repellent or attractive scents into the pesticide to make it even more effective, and to utilise various waste oils based on availability, adapting their process accordingly. They aim to launch a spin-off company to further develop and market their innovation, with plans to start up by the end of the year.

Image caption: Thrips caught in sticky droplets on a chrysanthemum leaf. Image credit: Thijs Bierman.