By Chris Patrick
The invasive kudzu bug (Megacopta cribraria) is a gleaming glob of an insect. The adults, about the size of a lady beetle, are square-ish and shiny olive-green, resembling a squatty armor breastplate. The immature nymphs are football-shaped, lime-green, and hairy.
They came from Asia. And yes, they earned their moniker because they eat kudzu — the invasive, rapidly spreading vine from Asia shrouding foliage in the southeastern quarter of the United States.
Kudzu bugs are beneficial when they eat kudzu, but it’s difficult to control the vine with the bugs because “there is no way to get them to stay,” said Nick Seiter, assistant professor and extension entomologist at the University of Arkansas. When it gets too crowded in kudzu, the bugs leave for kudzu’s close relative: soybeans.
Dr. Seiter and his colleagues from Clemson University, North Carolina State University, and the University of Georgia monitored soybean fields for three years to determine an action threshold for managing kudzu bugs. Their findings appear in an article called “Action Thresholds for Managing Kudzu Bug, Megacopta cribraria (Hemiptera: Plataspidae), in Soybean Based on Sweep-net Sampling” in the Journal of Economic Entomology.
Kudzu bugs become a pest when they infiltrate soybean crops. They pierce plants and suck out nutrients and moisture with their sharp, tube-like mouthparts, ultimately causing crop yield loss.
But because kudzu bugs are so new, there is no established way to treat them. They were only found in the U.S. six years ago, in 2009.
In pest management, thresholds are used to determine when action should be taken to control a pest. There are different types of thresholds, but in this case and similar ones, it has to do with numbers. When the number of kudzu bugs in a field reaches a certain point, it’s time to control them. Economic thresholds, like this one, weigh the cost of pest-caused damage against the cost of action (applying pesticides, for example), and they help growers figure out when and how they should deal with pests.
To sample the number of kudzu bugs in a soybean field, the researchers used sweep-nets. They walked between soybean rows, swinging heavy canvas nets on poles from side to side, swooshing through the upper canopy of the leaves. When they reached the end of a row, they counted the number of kudzu bugs (and other insects) in their nets.
But don’t think sweep-net sampling is akin to merrily catching butterflies. When bothered or crushed, kudzu bugs secrete a chemical that can discolor and burn skin. Seiter has a burn on the back of his neck from kudzu bugs.
The same chemical also makes them stink, which isn’t so surprising since they’re closely related to stink bugs.
“They have a powerful smell,” said Seiter. “Difficult to describe. Similar to stink bugs, but a little bit stronger.”
Stink, burns, and stains are hard to avoid in a soybean field full of adult kudzu bugs. However, after testing insecticides at different thresholds in their fields, the researchers found the most cost-effective control efforts targeted nymphs, not adults.
Adult kudzu bugs may show up in soybean fields in large numbers, they often do so in staggered waves. If insecticide is applied before the nymphs appear, it is likely that adults will continue to arrive and future applications will be necessary.
The researchers found that waiting until nymphs appear and adhering to their recommended threshold of one nymph per sweep decreases the likelihood of requiring multiple insecticide applications. Targeting the immature insects may increase the chances of more adults migrating into the fields, but it decreases the numbers of the more damaging nymphs. Nymphs feed more on soybeans than the mobile adults, which fly around searching for mates and laying eggs.
A single pesticide application — at the right time — is key. Seiter and the other researchers found that multiple applications cost more, but they don’t prevent more damage or increase crop yield.
“Early on, a lot of multiple applications were occurring because we didn’t know how damaging [kudzu bugs] would be,” Seiter said. “A big implication of this [study] is helping to show that multiple applications of insecticide are not necessary.”
Further research is required to determine if the actual threshold is higher than one nymph per sweep, but that is the researchers’ recommendation for now.
“I don’t think [this action threshold] will lead to unnecessary application,” said Seiter. “I’m pretty comfortable with it where it is. It will give growers a tool to manage insects economically and prevent yield losses without spending too much.”
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Chris Patrick is a graduate student in the science writing program at Johns Hopkins University. Click here to read her blog.