The Mexican Bean Beetle
“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.
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.
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.
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.
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.
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 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.
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
“Mexican Bean Beetle,” Virginia Cooperative Extension, Publication ENTO-51, http://pubs.ext.vt.edu/content/dam/pubs_ext_vt_edu/ENTO/ENTO-51/ENTO-51-pdf.pdf
“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
“Mexican Bean Beetle,” Indiana Cooperative Extension, Purdue University, https://extension.entm.purdue.edu/fieldcropsipm/insects/mexican-bean-beetle.php
“Home Grounds & Animals 2017,” Virginia Cooperative Extension Publication, https://pubs.ext.vt.edu/content/dam/pubs_ext_vt_edu/456/456-018/section2-home-veg.pdf