Distribution. This is the most common and widespread of the thrips species found on corn. It occurs throughout the United States and southern Canada.
Host Plants. Among vegetable crops, only corn is affected. Other crops infested are barley, oat, timothy, and wheat. Numerous forage and weed grasses known to be suitable hosts include Agrostis spp., Arrhe-natherusm sp., Avena sp., Bromus spp., Elymus spp., Festuca spp., Lolium sp., Panicum spp., and Poa spp.
Natural Enemies. The natural enemies of grass thrips are poorly known because this minor pest has been studied infrequently. See the sections on onion thrips, Thrips tabaci Lindeman, or western flower thrips, Thrips occidentalis Pergande, for discussion of natural mortality factors.
Life Cycle and Description. Adult females overwinter at the base of grass stems just above the soil. In the spring, as grass begins to grow, egg deposition begins. The number of annual generations is estimated at eight or nine in Massachusetts, with a generation requiring 2-4 weeks.
often at the base of a leaf sheath. The pupal stages are sluggish and do not feed. The antennae are folded back over the head. The thorax bears wing cases which are long for pupae destined to produce winged adults, or short if giving rise to wingless individuals.
Adult. Adults feed more openly than do larvae, often feeding on the leaves rather than within the leaf sheaths. Adults have both winged and wingless forms. Most overwintering females are wingless, but the winged forms soon predominate, accounting for about 90% of the thrips during the spring months. The proportions shift over the course of the summer, with wingless forms common late in the season, accounting for about 98% of the thrips. The winged individuals are larger, about 1.5 mm long, bear two pairs of fringed wings, and are brown. The wingless form is shorter, measuring about 1.0 mm long, and may lack signs of wings or may bear short protuberances (wing buds). The wingless forms are pink. The antennae of adults protrude forward, as in the larvae, but the head is relatively wide, nearly as wide as the thorax. Males are infrequently found, the females generally reproducing parthenogenetically. The males are not as slender as the females, the eyes are located more dorsally, and the tip of the abdomen bears paired copulatory structures that are absent from the females. Females oviposit for a period of 4-6 weeks.
The biology of grass thrips was given by Fernald and Hinds (1900) and Cary (1902), but important observations were made by Kamm (1972). This species is included in a key to common vegetable-infesting thrips in Appendix A.
Larvae and adults puncture individual cells and remove the sap. This kills the cells, and imparts a silv
ery appearance to the tissue. The growing point or top of the plant is most often affected, usually while still immature, resulting in a condition called "silver-top." This thrips is said to be highly mobile and very destructive to grasslands in Oregon (Kamm, 1971, 1972). However, grass thrips typically do not persist in corn, and cause little damage. Usually thrips disperse into young corn in June and within a generation they again disperse. Thrips injury to corn usually occurs only if the corn is also under moisture stress.
Grass thrips tend to be a transient problem, and rarely require action. Presence of grasses, including small grains, may predispose a corn crop to infestation. Destruction of grasses in the autumn or winter may eliminate the overwintering stage. If corn is rapidly growing, recovery from thrips feeding is likely. Foliar insecticides are effective for suppression.
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