Distribution. Mexican bean beetle is native to Mexico and Central America, and it likely has resided in Arizona and southern New Mexico since the introduction of cultivated beans by indigenous peoples several hundred years ago. By the late 1800s, it was damaging beans throughout the southwest, particularly Colorado. A major increase in damage followed the accidental transport of Mexican bean beetle to northern Alabama about 1918, apparently in shipments of alfalfa hay from Colorado and New Mexico. The beetles, once gaining access to eastern states, spread rapidly to the northeast. By 1922, Mexican bean beetle had invaded Georgia, North and South Carolina, Virginia, Tennessee, and Kentucky. It reached Ohio and Pennsylvania in 1925, Ontario in 1927, New Jersey in 1928, and Connecticut in 1929. It is now found all over the continental United States. In Canada, Mexican bean beetle is found in eastern provinces, from Ontario to New Brunswick, and also is reported from British Columbia, but it is a common pest only in Ontario.
The central Great Plains, from North Dakota to Texas, formerly provided a natural barrier in spreading of the beetles. Although this barrier was bridged through human intervention, there remains an area that is relatively inhospitable to bean beetle, so they are infrequent in this region. This insect is also infrequent in Pacific Coast states. Thus, there are two fairly discrete populations—a western population in the Rocky Mountain region including the western edge of the Great Plains, and an eastern population that inhabits most of the eastern United States west to Kansas.
Host Plants. Mexican bean beetle develops only on legumes. Other plants are occasionally reported injured, but these invariably are growing adjacent to defoliated legumes and will not support reproduction of beetles. Among 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, 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. (See color figure 5.)
The natural host appears to be tick trefoil, Desmo-dium spp.; however, in the United States, Mexican bean beetle is almost found associated with cultivated legumes. Lupine, Lupinus spp., were found to support adults in California, but no reproduction occurred.
Natural Enemies. Numerous predators, parasi-toids, and microbial disease agents of Mexican bean beetle have been identified, but few native natural enemies are considered to be important (Howard and Landis, 1936). Among predators the soldier bug Stiretrus anchorago (Fabricius) and the spined soldier bug, Podisus maculiventris (Say) (Hemiptera: Pentato-
midae), are often cited as the most effective. Lady beetle species such as convergent lady beetle, Hippodamia convergens Guerin-Meneville; transverse lady beetle, Coccinella transversoguttata Brown; and Coleomegilla maculata (De Geer) (all Coleoptera: Coccinellidae) sometimes prey on the eggs or young larvae of bean beetle, and on occasion have been considered important predators.
Parasitoids have not been very effective at suppressing bean beetle. 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. Special attention was given to a species from Mexico, Aplomyiopsis epilachnae (Aldrich) (Diptera: Tachinidae). It was released widely in the eastern states, but apparently is not adapted to cold winters (Landis and Howard, 1940). In recent years a parasitoid from India and Japan, Pediobius foveolatus (Crawford) (Hymenop-tera: Eulophidae) has been cultured and released in eastern states. Annual release is necessary because the parasitoid is unable to overwinter in the United States (Schaefer et al., 1983). In Maryland, nurse crops of snap beans have been planted early to attract bean beetles, with parasitoids released into these small plantings. As overwintering or reproducing beetles appear elsewhere, parasitoids naturally disperse from the nurse crops to attack the expanding bean beetle population. The nurse crop approach allows development of a large population of parasitoids with minimal effort (Stevens et al., 1975a). Small gardens, such as those in urban and suburban communities, also are suitable for Pediobius release (Barrows and Hooker, 1981).
Microbial pathogens, especially Nosema epilachnae and N. varivestris (both Microsporida: Nosematidae), occur in bean beetles (Brooks et al., 1985). These pathogens are deleterious to Mexican bean beetle; Nosema epilachnae, in particular, reduces longevity and fecundity in bean beetles (Brooks, 1986). 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 (Own and Brooks, 1986).
Weather. Although natural enemies may affect bean beetle abundance, weather is also thought to play an important role in population dynamics. Hot and 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. Mellors et al. (1984) are among the recent authors to give information on the effects of various temperatures for several time intervals, with important earlier studies presented by Miller (1930) and Sweetman and Fernald (1930).
Life Cycle and Description. Mexican bean beetle usually exhibits 1-3 generations annually. In the western United States there normally is one complete generation, with a small number of individuals reproducing and developing a small second generation. However, during cool summers the members of the second generation are unable to complete their development and perish. In the southeast, where three generations are more common, a few beetles deposit eggs that produce a small fourth generation. Throughout the nation, adults are the overwintering stage. Overwintered adults typically are most abundant in June, followed by first, second, and third generation (when present) beetles in July-August, August-September, and October, respectively. A life cycle may be completed in 30-40 days during the summer months, but may require 60 days during cooler weather. In recent years much of the research has concentrated on Mexican bean beetle as a soybean pest, but life history parameters, while similar, are not the same as on more suitable hosts. Larvae reared on soybean tend to have higher mortality and longer development times than on snap bean (Bernhardt and Shepard, 1978; Hammond, 1985).
when freshly emerged, to reddish-brown when old. The reddish color is especially evident among the overwintered individuals. The beetles normally measure about 6-8 mm long and 4-6 mm wide, but size varies considerably depending on availability of food. In behavior, the beetle is rather sluggish, and responds to significant disturbance by dropping from the plant and by excreting small drops of blood from the articulations of their legs. This excretion reportedly is a form of a defensive chemistry that reduces predation by other insects. The pre-oviposition period of adults averages 11.5 days. Females commonly produce about 500 eggs, sometimes depositing over 1200 eggs. Mean fecundity is lowest in overwintering females, and increases in later generations. Adults overwinter under leaves and other plant debris, and under logs and stones. Aggregations of several hundred overwintering beetles are not uncommon. In some cases, overwintering may occur adjacent to larval host plants, but adults are strong fliers and often disperse several kilometers to suitable overwintering quarters, often in wooded areas. (See color figure 129.)
The biology of Mexican bean beetle was provided by many authors, but among the most complete are List (1921), Howard and English (1924), Eddy and McAlister (1927), Friend and Turner (1931), and Auclair (1959). Rearing procedures for both Mexican bean beetle and the parasitoid Pediobius foveolatus were given by Stevens et al. (1975b).
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. Larvae are particularly damaging. As the larvae are not very mobile, they usually concentrate their feeding on a single leaf, inflicting complete defoliation before relocating to another leaf.
Bean plants are quite tolerant of defoliation. Bean plants can often withstand 50% leaf loss, especially if it occurs early in the season, leaving time for the plants to recover, or if the affected foliage is older and physiologically inferior. Plants are most sensitive to damage during the pod establishment and illing stages (Waddill et al., 1984). On average, defoliation in excess of 10-20% usually results in decreased yield. Each larva may consume over 30 sq cm of foliage, with about 70% of the consumption occurring during the inal larval stage (McAvoy and Smith, 1979). In studies of dry bean response to bean beetle defoliation, Michels and Burkhardt (1981) estimated that loss would occur with as few as 1.0-1.5 larvae per plant. In contrast, Capinera et al. (1987) determined that field beans could tolerate up to 19% defoliation without yield reduction, and that the number of beetles necessary to inflict this level of injury varied from 3-20 depending on the length of the adult feeding period.
Mexican bean beetle also is capable of plant disease transmission. When Jansen and Staples (1970a) allowed larval and adult bean beetles to feed on plants infected with cowpea mosaic virus, they became capable of transmitting the virus to healthy plants for a two-day period. Wang et al. (1994) reported slightly longer virus retention times, about four days, and suggested that beetles lost their ability to transmit virus in direct proportion to the amount of foliage consumed. Thus, Mexican bean beetles are not particularly effective vectors, but because adults fly freely they may be important in redistribution of such diseases.
Cultural Practices. Cultural practices are of limited value. Beetles fly long distances to overwinter, so crop rotation and destruction of overwintering sites generally are 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.
Early-planted crops may be useful to attract adult beetles, where they can be destroyed by disking, insecticide application, or release of parasitoids. However, though this trap-crop approach works to lure beetles from a relatively unpreferred crop such as soybean to a preferred crop such as lima bean, it is not effective in the protection of preferred crops (Rust 1977).
Biological Control. The principal means of biological suppression of Mexican bean beetle is release of Pediobius wasps. (This species is discussed in the section on natural enemies). Despite extensive research demonstrating the feasibility of such releases, commercial bean producers rarely consider such an approach, tending to rely instead on chemical insecticides. Insecticides are somewhat compatible with parasitoid releases because some insecticides dissipate to non-toxic levels in 1-3 days, and parasitoid pupae are relatively immune to field applications of insecticides (Flanders et al., 1984).
Host-Plant Resistance. Considerable effort has been directed toward identification of species and cul-tivars resistant to bean beetle oviposition or feeding. In Ohio, Wolfenbarger and Sleesman (1961) showed that Phaseolus spp., principally snap and to a lesser extent lima beans, were susceptible to injury. Cowpea, faba, and other beans generally were not attacked. Campbell and Brett (1966), working in North Carolina, were able to identify some commercial varieties of both snap and lima beans that displayed resistance. Resistance was reflected in reduced oviposition on resistant varieties, and decreased size and lower fecundity of adults reared on resistant plants. Insect development time was not affected. Also, some so-called vegetable-type soybean cultivars, suitable for such uses as sprouts and tofu, have been evaluated for bean beetle resistance and found to vary considerably in susceptibility to damage (Kraemer et al., 1994). Raina et al. (1978) reported that greenhouse screening could be used to identify resistance.
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