Distribution. The squash bug, Anasa tristis, is found throughout the United States, and is known as a pest wherever it occurs. In Canada, it frequently is a pest in southern Ontario and Quebec and in southwestern British Columbia. Squash bug also occurs in most of Central America. The horned squash bug, A. armigera, has a more restricted distribution, occurring throughout the eastern United States west to Iowa and Texas, and is also known from Trinidad. Two other crop-feeding species of Anasa, A. andresii (Guerin) and A. scorbutica (Fabricius), are found in the southern states from California to Florida (Baranowski and Slater, 1986), but only rarely cause damage.
Host Plants. Squash bug has been reported to attack nearly all cucurbits, but squash and pumpkin are preferred for oviposition and support a high reproductive rate and survival (Bonjour et al., 1993). There is considerable variation among species and cultivars of squash with respect to susceptibility to damage and ability to support growth of squash bugs. Beard (1935) noted that New World varieties were preferred. Bonjour and Fargo (1989) reported survival of squash bug to be 70, 49, 14, 0.3, and 0% when nymphs were reared to the adult stage on pumpkin, squash, watermelon, cucumber, and muskmelon (cantaloupe), respectively. Cook and Neal (1999) compared the development of nymphs on a preferred host (pumpkin) with a non-preferred host (cucumber) and found that the cucumber-fed nymphs perished, and that cucumber feeding prolonged nymphal survival only slightly long than ingesting water only. Vogt and Nechols (1993) reported that nymphal survival on green striped cushaw cultivar of Cucurbita maxima (winter squash), Waltham butternut cultivar of C. moschata (winter squash), and early prolific straight-neck cultivar of C. pepo (summer squash) was 22, 17, and 49%, respectively. Field observations by Novero et al. (1962) indicated that butternut and sweet cheese cultivars of C. moschata were resistant to attack. Royal acorn and black zucchini cultivars of C. pepo were resistant and susceptible, respectively. Green striped cushaw and pink banana cultivars of C. maxima were intermediate in susceptibility to injury. In California, squash bug also caused injury to watermelon (Michelbacher et al., 1955). Horned squash bug is reported to prefer wild cucurbits, but sometimes feeds on cultivated varieties (Slater and Baranowski, 1978); its host preferences among cultivated cucurbits is nearly identical to A. tristis (Chittenden, 1899). (See color figure 11.)
Natural Enemies. Several natural enemies of squash bug have been recorded, principally wasp egg parasitoids (Hymenoptera: Encyrtidae and Scelio-nidae). Up to 30% parasitism among eggs collected in Florida has been reported (Chittenden, 1899, 1908a; Tracy and Nechols, 1987, 1988; Nechols et al., 1989; Vogt and Nechols, 1991,1993). Chittenden (1908a) also mentioned a bacterial disease, but it must not be an important mortality factor because it was not mentioned by investigators in subsequent life-history studies. Cannibalism among nymphs is common, but this mortality factor has never been quantified.
The best known natural enemy is a common parasi-toid of several hemipterans, Trichopoda pennipes (Fabri-cius) (Diptera: Tachinidae). Beard (1940) provided a detailed study of this parasitic fly, which also attacks the other Anasa spp. and some other coreids and pen-tatomids. The brightly colored adult fly is easy to recognize, having a gold and black thorax and an orange abdomen, with a prominent fringe of featherlike hairs on the outer side of the hind tibia. Flies develop principally in the adult bug, initially castrating the female, and then killing her when the fly emerges. In Connecticut, about 20% on the squash bugs were parasitized in late summer. In recent years, considerable research has been done on the relationship of T. pennipes and southern green stink bug, Nezara viridula (Linnaeus) (Hemiptera: Pentatomidae). (See the section on southern green stink bug for additional literature.)
Life Cycle and Description. A complete squash bug life cycle commonly requires from six to eight weeks. Squash bugs have one generation per year in the north, and two-to-three generations per year in southern locations such as Oklahoma (Fargo et al., 1988; Palumbo et al., 1991). In intermediate latitudes the early-emerging adults from the first generation produce a second generation, whereas the late-emerging adults go into diapause. In Kansas, the threshold for diapause induction is late July; adults emerging after this date exhibit progressively higher levels of diapause (Nechols, 1987). Both sexes overwinter as adults. The preferred overwintering site seems to be in cucurbit fields under crop debris, clods of soil, or stones, but occasionally adults also are found in adjacent wood piles or buildings.
The biology of horned squash bug is inadequately known, but the observations that have been made (Chittenden, 1899) suggest a life cycle virtually identical to squash bug. Horned squash bug differs considerably in appearance, however. The nymphs of A. armigera are white until the fifth instar, when their body becomes variegated brown and yellow. Nymphs of A. tristis, in contrast, tend toward gray or brown. In the adult stage, A. armigera has a much broader prothorax, with sharp angles. The dorsal surface of the abdomen of A. armigera is orange, whereas in A. tristis, it is usually black (Chittenden, 1899). A. armigera also lacks the median-yellow stipe that is found on the head of A. tristis (Slater and Baranowski, 1978).
A good summary of squash bug biology was given by Beard (1940). Rearing techniques were provided by Woodson and Fargo (1991). A description and life history of A. andresii was provided by Jones (1916a).
This squash bug causes severe damage to cucurbits, because it secrets highly toxic saliva into the plant. The foliage is the primary site of feeding but the fruit is also fed upon. The foliage wilts, becomes blackened, and dies following feeding; this malady is sometimes called "anasa wilt." Often an entire plant or section of plant perishes while nearby plants remain healthy. The localized injury probably results from the gregarious behavior of the bugs. The amount of damage occurring on a plant is directly proportional to the density of squash bugs (Woodson and Fargo, 1991).
Insecticides. Squash bug adults are unusually difficult to kill with insecticides. Although adult control can be accomplished if the correct material is selected, and it is advisable to target the more susceptible
nymphs. Squash bugs are not often considered a severe pest of large-scale cucurbit production, probably owing to the absence of suitable overwintering sites in well-managed crop fields and because the bug's effects are diluted by the vast acreage. Small fields and home gardens are most commonly damaged. The exception seems to be in Oklahoma, where two-to-three generations occur each year, and an additional generation allows attainment of higher bug densities and subsequent damage to commercial plantings. In Oklahoma, suppression of squash bug early in the year and maintenance at low densities are recommended (Fargo et al., 1988).
Insecticides applied to the base of plants are especially effective, apparently because bugs tend to cluster there (W. Cranshaw, personal communication). Such applications may also minimize interference with pollinators. Pollinators, particularly honeybees, are very important in cucurbit production, and insecticide application can interfere with pollination by killing honeybees. If insecticides are to be applied when blossoms are present, it is advisable to use those with little residual activity, and to apply late in the day, when honeybee activity is minimal.
Cultural Practices. Adult squash bugs preferentially colonize larger, more mature plants. Thus, early-planted crops may be especially prone to attack. Numbers are also highest on plants during bloom and fruit set (Palumbo et al., 1991). Use of early-planted crops as a trap crop has been proposed, but due to the high value of early-season fruit most growers try to get their main crop to mature as early as possible. The use of squash or pumpkin as a trap crop to protect less preferred host plants such as melons and cucumbers is reported to be effective (Chittenden, 1899). Trap crops have some benefit for squash bug control. Pair (1997) reported that small plantings of an attractive squash crop that were treated with systemic insecticide could suppress early-season populations on nearby cantaloupe, squash, and watermelon.
Plastic mulches provide good harborage for squash bugs, and densities are reported to be higher in the presence of white, black, and aluminized mulch (Cart-wright et al., 1990a). The tendency of squash bugs to aggregate in sheltered locations can be used to advantage by home gardeners. Smith (1910) recommended the placement of boards, large cabbage leaves, or other shelter for squash bugs, because they tend to congregate there during the day and are easily found and crushed. He also suggested hand picking, but the bugs are secretive and shy; they quickly hide if an observer approaches.
Row covers delayed colonization of squash, but bugs quickly invaded protected plantings in Okla homa when covers were removed to allow pollination (Cartwright et al, 1990a).
Removal of crop debris in a timely manner is very important. Squash bugs often are found feeding on old fruit or in abandoned plantings, so clean cultivation is essential to reduce the overwintering population.
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