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 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.
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.
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.
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.
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.
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.
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:
Florida Insect Management Guide for legumes
Florida Insect Management Guide for soybeans
Author: Hussein Sanchez-Arroyo, University of Florida
Photographs: James Castner and John Capinera, University of Florida; and USDA
Project Coordinator: Thomas R. Fasulo, University of Florida
Publication Number: EENY-15
Publication Date: November 1997. Latest revision: June 2009.
Copyright 1997-2009 University of Florida
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