Insecticide Sprays Can Target Feeding Habits Of Pests

Researchers here have developed a method to predict the combination of insecticide concentration, droplet size, and droplet number that is most lethal for a particular species. In computer simulations and laboratory experiments, manipulating these factors boosted insect mortality by 60 to 70 percent.

Some insects nibble at leaves, while others chomp mouthfuls or suck out the sap. They may hop, fly, or scamper about. But all species leave a unique “footprint” on the plants they eat, said Franklin R. Hall, professor of entomology at Ohio State, and farmers could kill more pests if they matched their spray delivery parameters to that footprint.

“We want farmers to be able to do more than just reach for the bag of insecticide in the barn,” said Hall. “That’s the easy solution, but it costs money -- increasingly so -- and it can create environmental risks and concerns about food safety. If we can optimize the way they apply the insecticide, we can achieve both safety and good pest control.”

Hall and co-authors Timothy A. Ebert and Roger A. Downer, both research associates, and Robin A.J. Taylor, a research scientist, conducted the study at Ohio State’s Laboratory for Pest Control Application Technology in Wooster, Ohio.

Two papers in the journal Pesticide Science describe the researchers’ new method for analyzing the interaction among droplet size, droplet number, and insecticide concentration of a spray.

Farmers could already adjust these factors, Hall explained, but nobody could calculate the most deadly mix of all three factors for a particular species. With this information, he said, farmers could reduce the amount of insecticide they use or spray less often.

“With our new ability to manipulate a pesticide spray, we can change how it is presented field to field, or even within a field, so we need to be able to find the optimum presentation,” said Hall.

The current project builds upon work the researchers started in 1993 when they created the Pesticide Dose Simulator (PDS), a computer model that simulates the feeding and movement of an insect on a leaf. They originally used the simulator to study how diamondback moth larvae feed on cabbage treated with the pesticide Bacillus thuringiensis, commonly known as BT.

In their new study, Hall and his colleagues re-examined the feeding habits of the diamondback moth on BT-treated cabbage with the computer simulation. They simulated 13 pesticide treatments, each with a different combination of droplet size, droplet number, and insecticide concentration.

The simulations showed that varying the three factors caused insect mortality to vary by as much as 70 percent. The highest mortality, 90 percent, resulted from a mixture of many small droplets per leaf and a low dose of insecticide per drop.

Next they tested the new methodology by using an actual insecticide on the cabbage looper. In the laboratory, the researchers fed the larvae cabbage leaves treated with another commercially-available insecticide, fipronil.

For this experiment, varying the droplet size, droplet number, and insecticide concentration caused insect mortality to vary by about 60 percent. Mortality was highest -- approximately 70 percent -- for a spray with a small number of small drops per leaf and low dose of insecticide per drop.

Hall said different sprays are effective for different species because of the way the insects move and feed on their host plants. Some insect species tend to chomp leaves -- that is, take large, infrequent bites. Conversely, some cabbage loopers nibble at leaves -- take small, frequent bites -- while others of the same species chomp.

Other nibbling insects include thrips and mites; chomping insects include leaf-cutters and most butterfly and moth larvae. Aphids are among those that pierce leaves and suck out the sap.

The researchers originally wanted to develop a method for tailoring insecticide sprays, but along the way they found that killing insects and saving crops aren’t always the same thing.

They found that altering the pesticide spray influences both insect mortality and life span. More insects die if the population acquires the poison over a long period of time, which means the insects have time to eat a large portion of the crops before they die. On the other hand, killing fewer insects but killing them early in the pesticide cycle means less vegetation gets eaten.

“Now we have to decide on the objective -- insect mortality or crop protection,” said Hall. “This raises the question of why a particular pest is a pest. Because of its potential to increase to large numbers? Or because of the amount of crops it eats? And what is the best way to decrease its potential to be a pest?”

The researchers are continuing this work, which was funded by state and federal funds, as well as a grant from the Environmental Protection Agency.