By Leslie Mertz
The Mexican bean beetle (Epilachna varivestis) has flown under the research radar too long, despite the fact that it has been ravaging U.S. crops for almost a century, according to Louis Nottingham, lead author of a newly published article in the Journal of Integrated Pest Management. The article shines a light on the Mexican bean beetle and describes possible management strategies, including so-called “cultural methods” that can control this pest without insecticides.
“If you look through the literature on this pest, there are probably only 10 or so articles in the past 20 years, and nothing in terms of a profile-type article since the 1930s,” said Nottingham, a PhD student and graduate research assistant in the lab of Professor Tom Kuhar at Virginia Tech. “It was time to bring more attention to this beetle that is so significant in our growers’ lives.”
A type of lady beetle originally known from high elevations of western Mexico and Central America, the Mexican bean beetle showed up in the western United States in the late 1800s in connection with the spreading cultivation of beans. The beetle favors wax beans and green snap beans in the genus Phaseolus. It will, however, eat all types of legumes, including alfalfa, and as settlers began transporting that forage plant to the East Coast as animal feed, they brought the beetles along.
“It ended up making its way across the plains to Alabama, and once it got to Alabama in the 1920s, it just spread like wildfire,” Nottingham said. “Within five years, it was pretty much all across the East Coast and up into southern Canada.”
The biggest outbreaks came in the Mid-Atlantic United States during the 1960s and 1970s. A period of cooler temperatures (ranging from the mid-70s to mid-80s °F), consistent rainfall, and high humidity was perfect for the beetles and for bean crops, so the pests not only reproduced rapidly, but also had plenty of tasty plants for their burgeoning numbers.
“They were in their heyday,” Nottingham said. “In fact, they did so well on snap beans that the populations got huge and the beetles were able to move into the soybeans.”
The widespread plague on beans finally dwindled.
“In about 1981, temperatures started to go up, and there wasn’t as much precipitation or humidity, so the problem solved itself with a little help from global warming,” Nottingham said.
Although such a sweeping era of bean attack hasn’t repeated itself, the Mexican bean beetle is still a major pest species.
“We’ve determined from surveys and reaching out to as many growers and extension agents as possible that this pest is still incredibly severe, but just in a smaller range: western Virginia, western North Carolina, western Maryland, eastern Tennessee, southern Pennsylvania and eastern Kentucky,” Nottingham said.
To get an appreciation of exactly where the beetle is and where it isn’t today, he is conducting an online survey asking all bean growers to report the presence or absence of the beetles. Such information is important because many small and mid-sized growers have given up on growing beans because the beetle can wipe out an entire crop before making it to harvest.
The details of the Mexican bean beetle are especially important in helping smaller growers who refrain from using chemical pesticides.
“We need to understand the weak links — the limiting factors — in the beetle’s life history, and see if there’s some way we can use that against the beetles to prevent outbreaks instead of trying to deal them once they’re already in big populations,” Nottingham said.
One of those limiting factors led to an idea for the Virginia Tech research group.
“If you look back in the literature, you’ll see that if you shine a light on different sides of a bean plant, the beetles always abscond to the other side. That also explains why you find every life stage — eggs, larvae, pupae, and even the adults — almost always on the shaded side of the leaves,” Nottingham said.
After some thought about how they could manipulate light conditions, they decided to try metalized plastic mulch, which looks much like a sheet of aluminum foil with black backing. When placed on the soil around the plants, the mulch reflects light onto the bottoms of bean-plant leaves.
They contrasted small outdoor bean plots using the reflective mulch, black plastic sheeting, or plain soil, and observed beetle numbers, plant damage, and plant growth. Farm-scale research on the effect of reflective mulch remains to be conducted, but it looks good so far.
“We found significantly less of every life stage on plants in our plots with reflective mulch vs. black plastic and bare ground,” Nottingham said. “And the plants grew way better and had far less plant injury.”
Other options for organic growers, which make up the bulk of farmers in the middle-Atlantic states around Virginia Tech, include mechanical removal, row covers, and the use of parasitoids. However, mechanical removal is time-consuming in the best of years and nearly impossible when the weather favors beetles, and a row cover only works if a grower meticulously and continuously examines it to make sure beetles aren’t sneaking in through a small breach.
Mexican bean beetle parasitoid wasps (Pediobius foveolatus) are available from New Jersey’s Department of Agriculture, but they are expensive because they have to be reapplied every year. The parasitoids lay eggs in the beetle larvae, the eggs hatch, and the wasp larvae consume the beetle larvae from the inside out. In their native Asia, the wasps can overwinter in prey larvae, but the Mexican bean beetle overwinters as an adult, so the wasps have no safe harbor from the cold temperatures and die off.
“The parasitoids work great, but even if you can completely knock back the population, the beetles will eventually come back and you’ll have to buy the wasps again,” Nottingham said, adding that the parasitoids do poorly in cool, wet weather, which unfortunately are the best conditions for the Mexican bean beetle.
Management methods are especially important because Mexican bean beetle could soon skyrocket, Nottingham believes. The combination of a growing demand for and production of organic produce, and the cooler and wetter weather along the East Coast in recent years “could put the beetle back on the scoreboard,” he said, noting that he is continuing to look for options to thwart the bean-ravaging insects.
“I think the coolest way to look at pest management is to look at what limiting factors we can use against them and develop non-chemical management methods for the Mexican bean beetles,” he said.
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Leslie Mertz, PhD, teaches summer field-biology courses, writes about science, and runs an educational insect-identification website, www.knowyourinsects.org. She resides in northern Michigan.