common name: Mexican bean beetle
scientific name: Epilachna varivestis Mulsant (Insecta: Coleoptera: Coccinellidae)
Introduction - Distribution - Description - Life Cycle - Hosts - Damage - Economic Injury Level - Management - Selected References
The family Coccinellidae, or ladybird beetles, is in the order Coleoptera. This family is very important economically, since it includes some highly beneficial insects as well as two serious pests: the squash lady beetle, Epilachna borealis Fabricius, and the Mexican bean beetle, Epilachna varivestis Mulsant.
The Mexican bean beetle has a complete metamorphosis with distinct egg, larval, pupal and adult stages. Unlike most of the Coccinellidae which are carnivorous and feed upon aphids, scales and other small insects, this species attacks plants.
The Mexican bean beetle is believed to be a native of the plateau region of southern Mexico. The actual area inhabited by this insect includes a large section in the United States (in most states east of the Rocky Mountains) and Mexico. The eastern infestations are almost continuous, the pest being present wherever beans are grown, while the western infestation is composed of isolated areas, depending upon the local environment. The insect is not a serious pest over much of Guatemala and Mexico, but is very abundant in several localities in the western United States. The southern limit of the known distribution is in Guatemala and the northern limit is southern Canada and New England. It was reported and apparently eradicated from Florida in 1933, but was found in that State in 1938 and by 1942 was firmly established.
The native habitat of the Mexican bean beetle has a very wet climate during the summer months, and the insect is a severe pest in the eastern United States, where precipitation is heavy. The western infestation is largely confined to areas where moisture is added to the field by irrigation.
Egg: Eggs are approximately 1.3 mm in length and 0.6 mm in width, and are pale yellow to orange-yellow in color. They are typically found in clusters of 40 to 75 on the undersides of bean leaves.
Figure 1. Mexican bean beetle, Epilachna varivestis Mulsant, eggs. Photograph by John Capinera, University of Florida.
Larva: The newly-hatched larva is light yellow in color and not over 1.6 mm in length. The body is covered with rows of stout branched spines, arranged in six longitudinal rows on the backs. The spines at first are yellow, but later become darker at the tips and more conspicuous. The larva has a soft body that tapers posteriorly and has an anal segment having a sucker-like apparatus for attachment to feeding surfaces. The mature larva is from 6.0 to 9.5 mm in length, and greenish yellow. The larva molts four times during the time of development. A few hours after molting, the tips of the spines become darker, giving a general greenish or dirty yellow color. Larvae have a tendency to aggregate in considerable numbers for pupation.
Figure 2. Larva of Mexican bean beetle, Epilachna varivestis Mulsant. Photograph by James Castner, University of Florida.
Pupa: The larva, when mature, attaches itself by the posterior end of the body to the underside of leaves, stems, or pods of the bean plants and often to parts of nearby plants. In this position, the larva pupates. After attaching, the larval skin is pushed back from the thorax to the abdomen where it remains in a whitish, wrinkled mass. The black tips of the spines remain conspicuous on the cast skin. The pupa is yellow, spineless, and of about the size and shape of the adult.
Adult: The adult is oval in outline, and about 6 to 7 mm in length. The newly emerged adult is of a straw or cream-yellow color. Shortly after emergence, eight black spots of variable size appear on each wing cover, arranged in three longitudinal rows on each wing cover. The adults darken with age until they become an orange brown with a bronze tinge, at which time the black spots are less conspicuous. The males are slightly smaller than the females. Males can be distinguished from females by having a small notch on the ventral side of the last abdominal segment.
Figure 3. Adult Mexican bean beetle, Epilachna varivestis Mulsant. Photograph by James Castner, University of Florida.
The adult beetles come out of hibernation, where they have spent the winter months under collections of brush or leaves, as soon as warm weather arrives. Some may, however, delay their appearance until mid-summer. In mid-May adults tend to search out snap and lima beans, but by late June they begin ovipositing in soybeans. After feeding on the tender young bean plants for one to two weeks, the females start to lay their eggs, each depositing 500 to 600 of them in batches of 40 to 75 on the underside of the foliage. The eggs are carefully attached at the end so that they all stand vertically. They hatch in a week during warm weather but may require at least two weeks under more unfavorable conditions.
The larvae feed voraciously for two to five weeks, depending upon the temperature. When first hatched, they all feed together. If the leaf is somewhat dry, the first hatched may devour the remaining unhatched eggs. As they grow older, they still retain their gregarious habits but tend to split up into small, scattered groups. When pupating, the larva fastens the tip of the abdomen to a part of the plant and starts to wiggle out of the larval skin, not entirely shedding it but pushing it back until only the tip of the abdomen remains in the skin. The pupal stage lasts for five to ten days, but may drag out much longer in the cool weather of autumn. The adults are strong fliers and travel long distances hunting for new bean fields. The beetles overwinter in moist, protected places, remaining dormant until spring.
Bean is the preferred host, and includes most varieties of snap beans and lima beans, Phaseolus vulgaris L. and P. lunatus L. The common weed beggarweed or beggar tick (Desmonium sp.) is believed to be a natural host. The insect can also live on cowpea, black-eyed pea, and soybean, and may attack mung, adsuki, and velvet bean, alfalfa, and clover. In some areas this beetle is a very serious pest of snapbean, lima beans, and soybeans, and during years of high infestation total defoliation of these plants is quite common. Soybeans are especially vulnerable to insect defoliation during the latter period, when plants are in the podset-podfill stages.
The insect in both the larval and adult stages will feed upon the leaves, flowers and growing pods of the bean plant, but the greatest amount of injury is done to the leaves. The adult beetles are not responsible for as great a level of injury as are the larvae. They usually feed by clinging to the under surface of the leaves and eating irregular sections of the lower leaf surface. The upper surface of leaves quickly dries out after the lower section is injured, giving a lace-like, skeletonized appearance. Occasionally blossoms, and in many cases small pods, will be entirely destroyed or so badly eaten that they drop from the plant.
Figure 4. Damage caused by the Mexican bean beetle, Epilachna varnestis Mulsant. Photograph by James Castner, University of Florida.
Prior to the development of economic injury levels for soybean insects, applications of insecticides were often made at the mere sight of a pest population. With the calculation of economic injury levels in the early 1970s based on knowledge of insect feeding and development, the plant's response to defoliation, economic costs associated with insecticide application and price of soybean, growers gained the knowledge that moderate insect populations could be tolerated without insecticides being needed. The most important aspect of the relationship between insect injury and crop response is that soybean has a tremendous ability to compensate for low levels of defoliation. This natural tolerance allows growers to accept some injury knowing that yield losses will not occur.
When assessing bean beetle population in soybean, a sampling technique is used that is appropriate for the stage of the plant. Direct observation on the plant during early stages of growth in the spring is considered the best option due to the plant's small size. As the plant attains a large size, most IPM programs suggest to use a ground or shake cloth, or a sweep net. A ground or shake cloth, while more cumbersome than a sweep net often gives near absolute counts of Mexican bean beetle. A sweep net can also be used, wherein a net is swept through the plant canopy a given number of times and then the insects are counted. Sweep net sampling provides insect counts that vary with the size of the plants and person doing the sweeping, which can be a disadvantage, but sweep net sampling is usually considered the most appropriate insect-sampling technique in IPM programs because of the economy of use. Recommendations often advise at least weekly sampling during the growing season. Examinations of plant injury combined with insect sampling will allow for the identification of a potential pest population.
In estimating the economic threshold for this insect, the following criteria must be determined 1) the dollar loss associated with a specific number of Mexican bean beetle larvae per unit area; and 2) the cost of the control (insecticide and application costs) on the same unit area. When criterion 1 equals criterion 2, control becomes economically feasible and the economic threshold has been reached. Various yield loss-treatment cost analyses have determined the threshold level to range from 1 to 1.5 larvae/plant on beans. This varies depending upon the bean variety and growing conditions. Current pest management guidelines for control of Mexican bean beetle on soybean suggest applying controls when 30 to 35 percent defoliation is observed prior to full bloom and 15 percent during pod-set and podfill. Economic levels of infestation usually do not appear until the onset of the second generation in late July or August, when soybeans are most susceptible to insect feeding.
Rescue treatment with an insecticide is warranted when defoliation is greater than 40 percent at pre-bloom, greater than 15 percent from blooming to pod-fill, and greater than 25 percent from full pod to harvest. Treatments should be applied only when the observed level of defoliation and number of Mexican bean beetles both indicate that damage will increase.
Because immigration of beetles can occur while soybeans are still susceptible to insect defoliation, the inability of an insecticide to suppress a continuing influx of adults can result in further economic losses to the grower. To provide crop protection in these situations, a pesticide should have a high initial efficacy against Mexican bean beetle adults and larvae, and residual activity sufficient to suppress any migrating adults.
Climatic conditions such as intense rainfall or extended drought with high temperatures have been shown to reduce significantly larval and adult populations. In addition, several control tactics have been evaluated for their potential to reduce Mexican bean beetle populations in soybeans.
Cultural Control: Cultural control efforts may include destruction of overwintering locations and late planting of the soybean crop. The destruction of overwintering locations increases exposure to inclement weather conditions and can greatly reduce adult numbers the following spring. Under certain conditions, a combination of a trap crop with delayed planting of a portion of the field might be used to advantage. Since overwintering beetles actively forage upon emergence in the spring, beans planted early will attract a disproportionate number of beetles feeding during their preoviposition period.
Biological Control: Natural control organisms include at least 17 species of predators. They feed on bean beetle eggs, larvae and pupae. The beetles are protected by hard wing covers and by an offensive, yellow liquid which is secreted in small drops from the leg joints when the insects are disturbed.
Figure 5. A Mexican bean beetle larva, Epilachna varivestis Mulsant, becomes a meal for the spined soldier bug instead. Photograph by USDA.
Ten species of parasitoids are prevalent in soybeans during the vegetative stages, but only the tachinid fly Paradexodes epilachnae and the eulophid wasp Pediobius foveolatus seem to be promising in reducing the number of bean beetles. Because P. epilachnae is not native, it is necessary to import it when the Mexican bean beetle is an important pest. P. foveolatus is a parasitoid of epilachnine beetles from India. It parasitizes Mexican bean beetle larvae during the growing season, but fails to overwinter for lack of diapause capability and/or available host material. The annual inoculative releases of this insect, if conducted early enough and in conjunction with establishment of nurse plot areas of snap beans in a widespread manner, are capable of suppression of the Mexican bean beetle on soybeans.
Resistant varieties: It has been reported that some varietal differences in leaf feeding damage from Mexican bean beetle exist among common beans. Lima beans are less preferred than snapbean. Among snapbeans, the group called wax beans tends to be especially preferred. Other types of beans such as mung beans, P. aureus, cowpea, Vigna sinensis, and soybeans, Glycine max, do not escape Mexican bean beetle damage; however, they are not preferred and are most damaged when they are grown in the vicinity of snap and lima beans, more preferred hosts.
Insecticides: For curative control in outbreak circumstances, several insecticides are currently available. Selection of an appropriate insecticide and timing of its application are very important. Studies of residual efficacy have dealt primarily with soil-applied insecticides for Mexican bean beetle control. Certain systemic insecticides can provide control for 70 days when applied at planting or as a side-dress treatment during midseason cultivation; these materials only provide control for half a season. The foliar insecticides suppress Mexican bean beetle larvae for up to two weeks posttreatment in garden beans and soybeans.
Application of systemic insecticides at planting has become a standard practice in many areas where Mexican bean beetle has become an economic pest. Some recommended systemic insecticides do not provide protection long enough to prevent economic damage from either first or second generation populations of the Mexican bean beetle. Most early season infestations of Mexican bean beetle can and should be controlled by a foliar spray as needed. If foliar sprays do not control early-season populations or adult Mexican bean beetles continue to immigrate from surroundings areas, then a lay-by treatment of a long-residual systemic material applied during cultivation may provide season-long protection against economic insect damage.
Insecticides recommended for treatment of Mexican bean beetle are found in the Florida Insect Management Guide:
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