Western Flower Thrips
Description and life cycle
Western flower thrips, formally limited to western North America, has become virtually cosmopolitan since the 1970s. This species is a key pest in the greenhouse production of flowers and vegetables. Out-of-doors it is a pest of several field and vegetable crops and also tree fruits. Although oviposition by this species causes a condition called pansy spot on some apple varieties in the west, it has not been shown to injure apples in the east. In addition to direct feeding injury, it transmits tomato spotted wilt virus.
Adult females are slender, about 1∕16 inch long, yellow or brown, and hold their fringed wings over their backs. Males are about two-thirds the size of females and generally lighter in color. Larvae resemble adults but lack wings and are smaller. This species is difficult to distinguish from less injurious species of flower thrips.
Dark brown adult females overwinter in leaf litter and other organic matter and emerge in late April and May. Populations build up on crop and weed hosts throughout the season, peaking in the summer. Generation time in summer conditions is about two weeks. Apparently drought conditions and above-average temperatures, such as occurred in Adams County in 1991, can result in damaging populations of this thrips.
Western flower thrips causes two types of direct injury to nectarine and peach trees. The first occurs during the bloom period when adult and larval thrips feed in flower parts and on the developing fruitlet under the shuck. Early season feeding causes scars on the fruit surface that expand as the fruit grows. Scarring injury has not been commonly observed in Pennsylvania.
The second type of injury occurs primarily just prior to and during harvest when adults move from alternate weed or crop hosts to the fruit. Adults and larvae feed on the fruit surface in protected sites, such as in the stem end, the suture, under leaves and branches, and between fruit. Feeding on the surface of ripe fruit removes cell contents and results in silver stipling or patches. Silvering injury is particularly obvious on highly colored varieties. Silvering that covers more than a 1∕8-inch-square area can result in downgrading of the fruit. Injured fruit also may contain thrips eggs.
Monitoring and management
Monitoring should begin at bloom in areas where this species is common. Extension guidelines in California recommend sampling blooms from 10 to 12 trees at each of three to four sites per orchard. Blooms should be slapped against a light yellow surface to determine abundance of adults. The presence of larvae is determined by dissecting at least 50 blooms per orchard. The extension guidelines recommend treatment if adults infest more than 5 per 50 blooms or if larvae are present.
The second critical monitoring period is when the first fruit ripens. Count the number of adult thrips on 10 fruits at each of five sites per orchard. Sample fruit from the ends of branches in the lower third of the canopy. Five adult thrips per 50 fruits and the presence of silvering may indicate a damaging population.
Several cultural practices may reduce injury by western flower thrips. These include proper thinning to reduce the amount of protected feeding sites between fruit, reducing the amount of clover in rod middles, not mowing adjacent fields or weedy row middles during bloom or harvest, and avoiding the use of insecticides, such as carbaryl, that are not effective against these thrips and may actually increase the amount of injury occurring during harvest by killing natural enemies.