Mexican bean beetle develops only on legumes. Other plants are occasionally reported injured. Vegetable crops eaten are cowpea, lima bean, and snap bean, particularly the latter two bean types. Related crops such as faba bean, lentil, and mung bean seem to be immune. Field crops that may be attacked include alfalfa, sweet clover, various dry beans, and soybean. Formerly the field crops other than dry beans were relatively unsuitable and rarely injured. However, starting in the 1970s the eastern, and then midwestern states, began experiencing considerable damage to soybean by Mexican bean beetle. The natural host appears to be tick trefoil, Desmodium spp.; however, in the United States, Mexican bean beetle is almost always found associated with cultivated legumes.

Numerous predators, parasitoids, and microbial disease agents of Mexican bean beetle have been identified, but few native natural enemies are considered to be important. Species native to the United States have not adapted to Mexican bean beetle as a host, whereas species imported from Central and South America have failed to establish permanently. A parasitoid from India and Japan, Pediobius foveolatus (Crawford) (Hymenoptera: Eulophidae) has been cultured and released in eastern states. Annual release is necessary because the parasitoid is unable to overwinter in the United States. Small gardens, such as those in urban and suburban communities, are most suitable for Pediobius release.

Microbial pathogens, especially Nosema epilachnae and N. varivestris (both Microsporida: Nosematidae), occur in bean beetles. These pathogens are deleterious to Mexican bean beetle; Nosema epilachnae, in particular, reduces longevity and fecundity in bean beetles. However, these pathogens also infect the parasitoid Pediobius foveolatus. Infection of the parasitoid occurs when the immature stage develops in its host, or when the adult ingests the pathogen.

Although natural enemies may affect bean beetle abundance, weather is also thought to play an important role in population dynamics. Hot, dry weather is thought to be detrimental to survival of all stages, but especially the egg stage. Temperatures above 35–37°C can be lethal.

Mexican bean beetle usually exhibits 1–3 generations annually. In the western United States there normally is one complete generation with some individuals reproducing and developing a small second generation. In the southeast, where three generations are more common, a few beetles deposit eggs that form a small fourth generation. Adults are the overwintering stage. A life cycle may be completed in 30–40 days during the summer months, but may require 60 days during cooler weather.

Eggs are deposited on end in clusters of 40–60 eggs, usually on the underside of leaves. They are elliptical in shape, and measure about 1.3 mm in length and 0.6 mm in width. The eggs generally are yellow in color, but turn orange-yellow prior to hatch. Eggs hatch in 5–14 days, averaging 5.7 days. All females from the first generation deposit eggs, but in South Carolina only 94% of the second and 60% of the third generation beetles reportedly produced eggs as more and more beetles entered reproductive diapause.

During the process of pupation the larval covering, which contains the spines, is pushed back toward the point of attachment to the substrate. Thus, the pupa appears to bear spines, but this is simply remnants of its earlier life, and not firmly attached. Rather, the yellow-orange pupa is quite free from projections. Duration of the pupal stage averages 8.1 days in South Carolina.

Larvae and adults feed principally on leaf tissue, but under high density conditions, and when faced with starvation, they also feed on blossoms, pods, and stems. Bean beetles feed on the lower surface of the foliage, removing small strips of tissue and usually leaving the upper epidermis and veins intact. The upper epidermis soon dies and becomes transparent, leaving characteristic injury consisting of a number of small transparent spots that is reminiscent of a stained-glass window. Entire leaves are quickly reduced to skeletal lace-like remains that have little photosynthetic value and usually dry and die quickly. The larvae are particularly damaging, and a mature larva may consume an entire leaf in a day.

Mexican bean beetle also is capable of plant disease transmission. Larval and adult bean beetles fed on plants infected with cowpea mosaic virus become capable of transmitting the virus to healthy plants for a 2–4 day period

Modern insecticides have relegated Mexican bean beetle to minor status in commercial bean production. They remain a serious problem, however, in home gardens and elsewhere when insecticides are not used. Cultural practices are of limited value. The beetles fly long distances for overwintering, so crop rotation and destruction of overwintering sites is generally not practical. It is a useful practice, however, to destroy bean plants as soon as they have been harvested, as this may disrupt development of many immature insects and inhibit development of additional generations. The principal means of biological suppression of Mexican bean beetle is release of Pediobius wasps. However, commercial bean producers rarely consider such an approach, tending to rely instead on chemical insecticides. Some degree of resistance occurs within commercially available cultivars of beans, but this is not completely adequate.

Vegetable Pests and their Management