The mode of action of neonicotinoid insecticides is to essentially paralyze the target insect. Plectris grubs, as do most white grubs, go through a grooming process which eliminates many naturally occurring fungal and nematode pathogens.

More of these pathogens are eliminated as grubs move through the soil.

By slowing the movement of plectris grubs, neonicotinoid insecticides give biological pathogens a chance to attach to these insects where they penetrate the body eventually causing death.   

In her research Arrington used both naturally occurring pathogenic soil fungi and nematodes in treatments with and without neonicotinoids, each alone, and the combination of the fungi and nematodes.

The tests were done in Columbus County, in the heart of plectris grub damage to sweet potatoes. Insect levels were extremely high, and in non-treated plots, damage to sweet potatoes was as high as 80 percent.    

For her tests, Arrington found a nematode species that actively moves in the soil and actively seeks a host. This particular nematode was applied in a soil drench in July, along with a neonicotinoid insecticide.

The number of roots damaged by plectris grubs did not differ significantly from one treatment to another.

However, there was a significant impact on the severity of damage when a combination of a neonicotinoid insecticide, in this case Admire, and pathogenic nematodes were applied.

“The results showed that early-season application of nematodes and insecticide worked better than when the products were applied later. This may be because younger grubs are more susceptible to the treatments than older grubs,” she says.

“We know the adults of this species don’t feed at all, and we documented that the biological parasites would only infect third instar grubs.

“So, we know growers won’t need to apply these nematodes late in the season and it is likely one application per season is adequate to reduce damage caused to sweet potatoes by plectris grubs,” she says.

“This is just one test in one year, so it’s hard to make sweeping conclusions, but it is clear that some combination of a neonicotinoid insecticide and pathogenic nematodes can significantly reduce damage caused to sweet potatoes by plectris grubs, she adds.

“Obviously, we need more than one year of data from one site. Things like how environmental conditions affect the movements of these biological pathogens through the soil and the persistence of these pathogens within the soil will be critical to refining their use in combined management practice with neonicotinoid insecticides.”