Distribution. Bean thrips apparently is a native species that has spread to many other parts of the world, including Asia, Europe, and South America. In North America it is known principally from the western United States, including Oregon, Idaho, and Wyoming south to California and Arizona. It is most damaging in the dry interior valleys of California, and arid regions of nearby states. However, its distribution also extends eastward throughout the southern states to South Carolina and Florida, and south into Mexico.
Host Plants. Bean thrips is reported from numerous plants, though some are incidental hosts, on which the adult can feed but not reproduce successfully. Vegetable crops that support bean thrips include asparagus, bean, beet, cabbage, cantaloupe, carrot, cauliflower, corn, fennel, garlic, kale, leek, lettuce, melon, onion, pea, pepper, potato, radish, Swiss chard, tomato, and turnip. Severe damage is generally limited to beans, cantaloupe, lettuce, and pea. Other hosts include such field crops as alfalfa, clover, cotton, and hops; fruit such as apple, avocado, fig, grape, orange, peach, pear, persimmon, plum, prune, and tangerine; and ornamentals such as canna, California poppy, geranium, gladiolus, hollyhock, iris, nasturtium, and sunflower. Many wild hosts are known, including trees, grasses, and weeds. Among common weeds known to support bean thrips are field bindweed, Convolvulus arvensis; milkweed, Asclepias spp.; mallow, Malva par-viflora; mullein, Verbascum virgatum; redroot pigweed, Amaranthus retroflexus; common sowthistle, Sonchus ole-raceus; and prickly lettuce, Lactuca scariola. In California, the abundance of prickly lettuce is one of the most important elements in the ecology of bean thrips. Bailey (1937) provided a more complete list of host plants.
Natural Enemies. Field studies suggest that only about 40% of thrips attain the adult stage. Much of the mortality results from the feeding habits of general predators such as minute pirate bug, Orius tristicolor (White) (Hemiptera: Anthocoridae), the larvae of the predatory thrips Aelothrips fasciatus (Linnaeus) and A. kuwanae Moulton (Thysanoptera: Thripidae), the lacewing Chrysopa californica Coquillette (Neuroptera: Chrysopidae), and the convergent lady, Hippodamia convergens Guerin-Meneville (Coleoptera: Coccinelli-dae). However, the most important natural enemy seems to be the internal parasitoid Ceranisus russelli (Crawford) (Hymenoptera: Eulophidae). Levels of parasitism by C. russelli observed in the field have been quite variable rates, but up to 70% have been noted. An unspecified nematode was found in bean thrips nymphs, but the importance of this observation is uncertain.
Weather. Bean thrips suffer direct mortality due to weather conditions. They are largely restricted to areas with mean winter temperature of about 20°C or greater. However, during the winter the adults can survive temperature as low as —9°C for short periods of time. Rainfall is also important. Heavy rains dislodge larvae from the plant, washing them to the soil where they perish. Summer rainfall in excess of about 10 cm is detrimental, but winter rainfall also is detrimental to overwintering adults. Thus, rainfall is likely the principal factor restricting bean thrips damage to arid regions of western states.
Weather also indirectly affects thrips populations. Bean thrips require an abundance of early-season and late-season weeds if they are to attain high densities. In the absence of spring or autumn rains, the thrips populations are checked by the shortage of suitable host plants. Thus, a critical balance of rainfall is required; enough early and late in the season to assure adequate food, but not so much in the summer and winter to cause direct mortality from being crushed and drowned.
Life Cycle and Description. This species has several overlapping generations annually. Six generations are estimated from California. Bean thrips overwinters in the adult stage. In California, overwintered adults produce the first generation, generally in March, on weed hosts. Overwintered adults usually perish by late April or May. Adults from the first and second generations may find the weeds suitable, or may move to alfalfa to feed. By late June, however, thrips begin dispersal to numerous cultivated crops, and weeds contained within the crops, where additional generations develop. As crops mature, thrips disperse again to late-planted crops, where an additional generation develops. The first generation in the spring requires about six weeks for completion, but as the weather warms the generations are completed in about three weeks.
Prepupa and Pupa. Two additional immature stages of development, the prepupal and pupal stages, are passed in the soil. In general appearance, they greatly resemble the larval and adult stages. They differ from both, however, in having partly developed wings, or wing pads, and by not feeding. The prepupa and pupa are orange, with crimson markings on the thorax and abdomen. They measure about 1.0 mm long during the prepupal stage, but shrink slightly to about 0.8 mm during the pupal stage. Also, they can be differentiated from each other in that the wing pads are much larger in the pupal stage. Duration of the prepupal stage is only 1-2 days, with 1.6 days the normal development period at 21 °C, and 0.9 days at 32°C. The pupal stage is slightly longer, requiring 9.3 and 2.4 days at 21° and 32°C, respectively.
Adult. The adult is minute, the female measuring only about 1.0 mm long and the male 0.9 mm. The body color is grayish black, and the front wings are banded with two alternating light and dark areas. The hind wings are entirely dark. Both sets of wings are fringed, as is normal for insects of this order. When the wings are folded against the body, which is the usual condition, the body appears to bear two whitish bands across the central region. The eight-segmented antennae and the legs also are banded with alternating light and dark areas. Adults begin to copulate and commence oviposition 2-4 days after emergence. A sex ratio of about two females per one male is normal for this species. The adult overwinters within curled leaves, on the underside of pubescent foliage, under the old coverings of scale insects, and in other sheltered locations. Plants that remain green throughout the winter are favorite overwintering locations. If disturbed, the adults slowly move, but little or no feeding occurs during this period.
The most complete treatment of bean thrips biology was supplied by Bailey (1933a). However, a later treatment by Bailey (1937) and an earlier report by Russell (1912) also contain useful information. This species is included in a key to common vegetable-infesting thrips in Appendix A.
Damage results from larvae and adults feeding on the underside of foliage. In the process of feeding, the thrips extrude their stylets, puncture cells, and drain the liquid contents. Apparently the larvae feed more extensively and inflict more damage than do adults. The insects feed gregariously, so there tends to be a great amount of tissue destruction localized on individual leaves, and these leaves turn whitish and often are shed by the plant. The underside of the
foliage becomes covered with black spots of fecal material. Injury tends to be concentrated on leaves of intermediate age, though adults tend to move to newer tissue as it becomes available. As the foliage is depleted, thrips move to bean pods and stem material to feed. The scar resulting from insertion of the eggs into foliage seems to cause little injury.
Cultural Practices. The most important cultural practice is weed management. Elimination of early season weeds in, or adjacent to, crops can reduce thrips populations. Irrigation ditches and other sites where weeds may survive during winter and periods of drought should be examined for thrips, and be treated with insecticides if necessary. Overhead irrigation can decrease bean thrips densities, and reduce the need for insecticides.
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