By: Kristi Waterworth
Ladybugs are a gardener’s best friend, eating aphids and generally brightening up the place. Although most of the members of family Coccinellidae are useful garden allies, the Mexican bean beetle (Epilachna varivestis) can be devastating to plants. Keep reading for information on Mexican bean beetle control to prevent bean beetle damage in your garden.
Mexican bean beetles are found throughout the United States, east of the Rocky Mountains, but are believed to have originated in Mexico. These beetles thrive in locations where summers are wet or agricultural areas where a lot of heavy irrigation is required. Spotted, orange-red adults emerge by midsummer, seeking lima, snap, and soybean plantings where they lay their eggs in groups of 40 to 75 on the undersides of leaves.
Both adults and larval Mexican bean beetles feed on bean foliage, chewing the tender tissue between veins from the leaf’s underside. Upper surfaces may yellow and areas where tissues were chewed down to a very thin layer may dry up and drop out, leaving holes in leaves. When feeding pressure is high, leaves will drop and plants may die. Large populations of bean beetles spread out from the leaves to attack flowers and pods as their numbers grow.
A gardener faced with beans under heavy attack may wonder if control of bean beetles is possible, but there are several options suitable for every kind of garden. Organic gardeners wondering how to keep bean beetles off plants have options like floating row covers, installed before the beetles move into the area. Although row covers can become cumbersome during harvest, they prevent bean beetles from setting up shop on beans.
Selecting early season varieties of beans with bushing habits allows you to grow lots of beans before the Mexican bean beetles have started emerging from their winter’s rest. By the time the insects are seeking places to feed, your beans will have already been harvested. If you immediately plow spent plants, it will help to keep bean beetle numbers low by depriving them of food.
Insecticides often appear to fail because bean beetles migrate throughout the season, resulting in seamless waves of new pests despite treatment. If you choose to use insecticides, make sure to respray your beans before the residual effects of the prior poison application wear off, otherwise, the next immigration of beetles may destroy your beans. Labeled pesticides include acephate, acetamiprid, carbaryl, dimethoate, disulfoton, endosulfan, esfenvalerate, gamma-cyhalothrin, lambda-cyhalothrin, malathion, methomyl, and zeta-cypermethrin.
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So, you need to get rid of the bean beetles eating up your legumes, green beans, or pea plants.
These little buggers are driving you up the wall.
And every day you wake up to your leaves being chewed up- left with silver spots, holes, and skeletonized foliage.
How do you save your bean plants from the beetles?
In this article, you’ll learn about:
By the end of this guide, you’ll have a solid foundation of knowledge to control, manage, and eradicate bean beetles from your plants.
It’s quite detailed because it covers everything you need to know (or nearly).
So bookmark this page for easy reference later- you’ll probably be coming back a few times during the process.
And as always, feel free to post your questions at the end of the page if you’re confused or have a special circumstance.
Or contact me directly and I’ll try to help you out ASAP (I get a lot of emails so please be patient!).
Sound good? Let’s send those beetles back to the wild.
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Holes in the leaves of bean plants mean insects have moved in to share the harvest. Don’t fret there are some easy ways to manage these pests.
Several insects can feed on bean plants and their pods. The bean leaf beetle, Mexican bean beetle and spotted cucumber beetle are the most common.
The bean beetle is ¼ inch long, yellow-green to red with four black dots on its back. High populations can devastate a planting. Cover plantings with floating row covers to keep the insects off. Firmly secure the edges to prevent the beetles from crawling underneath.
The Mexican bean beetle is a bit larger and can be yellow or coppery brown with 16 black dots. The immature stage, larvae, is orange or yellow, fuzzy and rather hump-backed. Remove and destroy any of the insects and their bright yellow eggs that you find.
A thorough clean up in the fall will reduce future problems.
A bit more information: The spotted cucumber beetle can also be found nibbling on your bean plants. It is long and narrow, yellowish green with black spots. Remove insects as found or use one of the more eco-friendly products like Neem, if needed.
“History does not necessarily predict the future,” and “past performance does not necessarily predict future performance” are two clichés that I would like to have a nickel for every time I have heard them. Now in vegetable gardening, those words of wisdom could not be further from the truth when it comes to the never-ending battle with this pest in the vegetable garden. I know that every July, the Mexican bean beetle will show up and begin to devour my bean patch. So before the rush garden season gets revved up, I’ll take a little time to do a little planning and research on the annual opportunity the Mexican bean beetle affords me each year with its annual visit to my garden.
The Mexican bean beetle, Epilachna varivestis Mulsant, a relative of the ladybug, is an above-ground chewing pest which feeds on legumes (common green beans, shell beans, lima beans and soybeans) and is one of only two destructive North American species in the otherwise beneficial and economically important ladybird beetle family, which contains over 400 species. The other destructive North American relative is the squash lady beetle, Epilachna borealis Fabricius, which feeds primarily on cucurbits (cucumber, melon, gourds, squashes and pumpkins).
Native to the high elevations of western Mexico, the Mexican bean beetle was first identified in the United States in the mid 1860’s but was not recognized as a serious pests until 1883, when severe damage to wax beans was reported in Colorado.
In 1918 the Mexican bean beetle was first discovered in Alabama and by the late 1920’s, the beetle had spread as far north as Canada and west to Michigan. In the early 1930’s research was conducted in Connecticut to develop methods for “checking the ravages of the Mexican bean beetle in Connecticut.”
The conclusion of the Connecticut researchers proved to be interesting: that various sprays — both poisonous and non-poisonous to humans — were more effective when plants were given more space — at least 4-6 inches apart. Highly significant was the discovery that certain non-poisonous sprays and dusts had proven themselves to be just as satisfactory as the poisonous types. The use of non-poisonous sprays allowed the grower to spray the plants before and after the pods formed with no fear of finding poisonous residue on the beans at harvest. The poisonous insecticides used in the research included barium fluosilicate, calcium arsenate, and magnesium arsenate. The grower was instructed not to use materials containing arsenic or any fluosilicate compound once bean pods had formed. Non-poisonous insecticides utilized in the research were dusts and sprays containing pyrethrum and derris root, a tropical plant which contains the natural insecticide Rotenone. www.ct.gov/Circular 109 (“Control of the Mexican bean beetle in Connecticut,” 1935).
Unfortunately for those of us here in central Virginia, damaging populations of Mexican bean beetles are most common in the Mid-Atlantic and southern Appalachian Mountain regions of the United States and remain a devastating pest to common snap beans and lima beans.
Adult Mexican bean beetles are similar in size and appearance to their beneficial cousin, the ladybug. They are either yellow or copper with sixteen black spots arranged in three rows. Their bodies are oval and about ¼ inch long, a little larger than the ladybug. Adults can walk or fly but are generally sluggish once they locate a suitable plant — such as my green bean patch. Most of their adult life is spent feeding and mating in the host canopy, but adult beetles will fly long distances if food becomes scarce.
Adult bean beetles generally overwinter in groups, under leaf litter or pine needles and emerge in the late spring or early summer.
Adult Mexican bean beetle, Photo by James Castner, University of Florida
Photo: Scott Brown, USDA, ARS, AFRS
Eggs of the Mexican bean beetle are light yellow when first deposited but darken when they are close to hatching. They are generally deposited in clusters of 40-60 eggs per cluster on the underside of the bean leaves. Each female will lay an average of 460 eggs. In 5-14 days, depending on the temperature, the eggs will hatch.
Mexican bean beetle eggs, Photo by John Capinera, University of Florida
The larva are cylindrical and soft-bodied. They are yellow and covered in spines that are either black or yellow with black tips. They generally remain on the undersides of the leaves where they continuously feed on leaf tissue. The larva will go through four developmental stages or instar stages over an average of 20 days. The first instar stage develops in 4-6 days, the second instar lasts about 2-4 days, the third instar stage develops over 3-5 days, and the fourth and final instar stage develops over a 6-10 day period.
Mexican bean beetle larva. Photo by James Castner University of Florida
Pupae are similar in general appearance to larvae, however at this stage, the beetle attaches to a plant by its posterior end and becomes immobile. Pupae are often found aggregated on a single leaf in the lower half of the plant canopy. Pupation usually lasts for about nine days, and the life cycle begins again. Multiple generations occur annually in Virginia.
Mexican bean beetle pupae. Photo by North Carolina Cooperative Extension
The Complete Life Cycle of the Mexican Bean Beetle. Photo by Purdue University
Both adult and larva feed on plant tissue with chewing mouthparts. The majority of the feeding occurs during the third and fourth instar stages — about 12 to 20 days from when the eggs hatch.
Beetles generally feed on the lower or underside of the leaf, while avoiding veins, creating a lacy, skeletonized appearance of the remaining leaf. Beetles generally feed primarily on the foliage, but they will also feed on the bean pods and flowers once they become present.
Damage caused by Mexican bean beetle. Photo by James Castner, University of Florida
There are several management practices available to the gardener to aid in controlling the Mexican bean beetle.
Mexican bean beetle adult and larvae are deterred by direct light. Field experiments at Virginia Tech have shown that Mexican bean beetles are less likely to colonize and deposit eggs on beans mulched with metalized and white plastics, compared to bare ground and black plastic. There was also less foliar and pod damage and significantly greater yield when the beans were mulched with metalized and white plastics than beans grown on the bare ground or mulched with black plastic.
Planting beans on reflective plastic mulch may be effective in helping to control the Mexican bean beetle. Greater damage was detected on beans planted on the bare ground (left) and black plastic (center) than metallic plastic (Right). Photo by L. Nottingham, Virginia Tech.
Planting beans in the early spring or late summer can reduce crop damage from the Mexican bean beetle. In Virginia this pest generally hits its period of peak activity in July. By planting beans as early as possible or as late as possible, you can avoid the beetles’ peak period. However, this strategy does carry some risk: planting beans when the soil and air temperatures are still cool often results in slower germination and smaller plants in addition, planting early or late increases the risk that the bean plants will be exposed to frost, resulting in damaged or killed plants.
Physical or Mechanical
Polyester or floating row covers have been shown to successfully reduce the abundance of adult, larvae and pupae populations.
Gardeners commonly use mechanical or by-hand removal to reduce injury from Mexican bean beetles. Because beetles complete their entire life cycle within the bean canopy, this simple strategy can help control pest damage.
The Mexican bean beetle is an invasive pest, and like other invasive criters, has no natural predators in its new homeland. In addition, they are well protected from predatory organisms. They not only have protective spines that adorn the larvae, but also produce toxic, alkaloid secretions that are known to deter many predators on contact.
There have been observations that predatory stink bugs, ladybeetle, damsel bugs and assassin bugs have attacked the Mexican bean beetles during their various life stages. However, these insects have proven ineffective at controlling this pest.
In 1966 a tiny, exotic parasitoid wasp, Pediobius foveolatus, originally discovered in India, was imported into the United States to be tested for potential control of the Mexican bean beetle. Initial testing determined that this tiny wasp would readily parasitize the larva of the Mexican bean beetle, while leaving the native, predatory insects unharmed. In 1972, several mid-Atlantic states began releasing these wasps to control the Mexican bean beetle. The USDA branches in New Jersey, Maryland, Delaware, and Virginia released wasps throughout these states, focusing on areas with large soybean acreage and high Mexican beetle populations.
In the areas where these wasps were released, the results were pretty spectacular — reducing the bean yield loss 80-100%. The Pediobius foveolatus cannot overwinter in the United States due to the cold weather and the lack of an overwinter host. In the wasps’ native territory, the winter is either conducive to year-round exposure, or the wasps overwinter in their hosts (overwintering as larvae). Because the Mexican bean beetle overwinters as an adult, these beneficial wasps are without adequate winter refuge in the United States, and they die off during the winter thus, they must be released each year. The Pediobius faveolatus wasps are mass produced by the New Jersey Department of Agriculture and other commercial insectaries. They can be purchased from various on-line vendors that supply biocontrol agents. A list of commercial suppliers can be found at wiki.bugwood.org/Pediobius .
The female wasps lay about 20 eggs in a single bean beetle larvae. The wasp larvae hatching from these eggs kill the beetle larvae. The infected larvae eventually turn brown and die. Adult wasps emerge from the larvae after about 15 days, mate, and then search for more beetles to infect. These wasps will also parasitize the larva of the squash beetle, a closely related relative of the Mexican bean beetle that feeds on cucurbit plants. These black wasps are very small — about 1-2 mm long –and will not harm humans or beneficial insects.
A little planning is required to be successful with this biocontrol agent it is crucial to time the release. Ideally, the wasps should be released at both one and two weeks after the first instar beetles are discovered on the bean plants. Accurate scouting and timing of release is essential because the wasps reproduce within the third or fourth instar stage of the Mexican bean beetle larvae: so it is very important that the Mexican bean beetle instar larvae are present when the wasps are released. The general rule of thumb is to release the wasps as soon as the beetle eggs begin to hatch.
Pediobius foveolatus, an exotic parasitoid wasp of the Mexican bean beetle. Photo by L. Nottingham, Virginia Tech.
Pediobius wasps attacking Mexican bean beetle larva. Photo by State of New Jersey Department of Agriculture.
A Mexican bean beetle larva parasitized or mummified by a Pediobius foveolatus wasp. Photo by L. Nottingham
There are a number of insecticides approved to control the Mexican bean beetle. The common names for these approved insecticides include: acephate, carbaryl, malathion. A number of organic insecticides have been evaluated, including azadirachtin, pyrethrins, and spinosad, and all provided significant control of the Mexican bean beetle. You’ll find more information on these insecticides and their use at your local extension office.
The Mexican bean beetle has quite the history, and like other invasive pests, it arrived in our area without a natural enemy and has survived in generous numbers. Fortunately, the home gardener has a number of ways to control this pest in the bean patch. They include: plastic mulch, removing them by hand, row covers, biological controls, and chemicals, both organic and synthetic.
Thanks for stopping by The Garden Shed we look forward to your visit next month.
“Featured Creatures, The Mexican Bean Beetle,”University of Florida, Department of Entomology and Nematology. http://entnemdept.ufl.edu/creatures/veg/bean/mexican_bean_beetle.htm
“Control Of The Mexican Bean Beetle in Connecticut,” Connecticut Agricultural Experiment Station New Haven, November 1935, www.ct.gov/caes/lib/caes/documents/publications/circulars/c109.pdf
“Natural History, Ecology, and Management of the Mexican Bean Beetle (Coleoptera: Coccinellidae) in the United States,” Journal of Integrated Pest Management (Nottingham, Dively, Dively, Schultz, Herbert, and Kuhar, 2016) 7(1): 2 1–12, https://vtechworks.lib.vt.edu/bitstream/handle/10919/73788, /Reprint_Nottinghmam et al Mexican bean beetle JIPM.pdf?sequence=1
Our first bean plants to produce were nearly done producing, and yet most affected by the Mexican Bean Beetles. We chose to remove that plot altogether and burn the plants in hopes of keeping the beetles from spreading to the rest of the garden.
The beans next to them received a thorough spraying of pyrethrin. It’s hoped we’ve gained control and won’t see another round of infestation. Time will tell.
G. Coleman Alderson is an entrepreneur, land manager, investor, gardener, and author of the novel, Mountain Whispers: Days Without Sun. Coleman holds an MS from Penn State where his thesis centered on horticulture, park planning, design, and maintenance. He’s a member of the Phi Kappa Phi Honor Society and a licensed building contractor for 27 years. “But nothing surpasses my 40 years of lessons from the field and garden. And in the garden, as in life, it’s always interesting because those lessons never end!” Coleman Alderson
Consider these natural controls for Mexican bean beetles, listed in seasonal order:
In large plantings of more than a quarter acre, these organic interventions may be worth their substantial cost:
This is the strategy that has given me the most abundant harvests. This strategy requires that gardeners plant their bush beans early — right around the estimated last-frost-date for their yard. This strategy allows gardeners to begin harvesting beans as soon as possible.
Mexican bean beetles don’t typically become abundant until sometime in the first half of July in my garden, and my estimated last frost is around mid-April. Planting near this date, when it can still be very cool, lets me bring beans to the kitchen beginning at the end of May, through all of June, and into early July, before bean beetles destroy the crop.
When the Mexican bean beetles have shifted from just feeding on leaves to also feeding on the beans, it is time (or past time!) to remove the crop completely from the garden. Be sure to clear it all away. Plant something else in this spot.
In 2-3 weeks, in another part of the garden, you can then plant a second crop of bush beans. In my garden, this second crop is untroubled by Mexican bean beetles. It does, eventually, get bean leaf rollers, but those do a lot less damage to the bean plants and they never eat the beans.