Thrips are consistent and predictable insect pests of seedling cotton in Georgia. Preventive insecticides are recommended at planting to reduce thrips infestations and seedling injury. Supplemental foliar insecticides are needed in some environments and applications should be based on scouting and thresholds. The need for supplemental foliar insecticide is dependent upon the severity of thrips infestations, the at-plant insecticide used, and the rate of seedling growth.
At-plant insecticide options include in-furrow granule applications of aldicarb, in-furrow liquid applications of imidacloprid or acephate, and commercial seed treatments of imidacloprid, thiamethoxam, and acephate. Imidacloprid seed treatment is the most common at-plant insecticide used. Thrips control and residual activity is greater for in-furrow granules and liquids compared with the seed treatments. We would expect 3-4 weeks activity with in-furrow options and 2+ weeks of activity with the imidacloprid seed treatment.
The Thrips Infestation Predictor for Cotton (TIPs) Tool uses planting date, temperature, precipitation, and knowledge of when and how intense thrips infestations will be to predict risk of thrips injury to cotton for specific geographic locations. For many areas, the tool is predicting thrips infestations to be similar to those observed during 2017 and 2018 which were both relatively low pressure years. The TIPs Tool can be found here.
Vigor or the rate of seedling growth influences seedling injury from thrips. Thrips initially feed on the underside of cotyledons; damaged cotyledons will appear silvery on the lower surface of cotyledons. The majority of thrips eggs are laid on the cotyledons and it takes about 5-6 days for an egg to hatch.
Once a terminal is present thrips will move to and feed on unfurled leaves in the terminal. As the leaves unfurl and expand the characteristic crinkling and malformations become obvious. A rapidly growing seedling may unfurl a true leaf every 3 days where as a seedling which is stressed may take 4-5 days to unfurl. Again, thrips are feeding on the unfurled leaves so thrips feed for a more extended time on the same unfurled leaf of a stressed plant than a rapidly growing plant. The same infestation of thrips will create more damage on a slow growing plant.
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The decision to use a foliar insecticide to supplement at-plant insecticides for thrips control should be based on scouting. Scout thrips by randomly pulling a seedling and “slapping” the seedling against a piece of paper or box to dislodge the thrips. Do this on several plants and determine the average number of thrips per plant. Be observant for immature thrips when making counts. Immature thrips are wingless and crème colored. Adult thrips are usually brownish or almost black in appearance and have wings (depends on species, tobacco thrips is the most common thrips species infesting cotton and adults will be dark brown or black).
The threshold for thrips is 2-3 thrips per plant with immatures present. The presence of numerous immature thrips suggests that the at-plant insecticide is no longer providing acceptable control (i.e. thrips eggs laid on the plant, egg hatched, and immature thrips is surviving). Foliar insecticide options include the systemic insecticides Orthene, Bidrin, and dimethoate. Note that these products are systemic.
Pyrethroids will not provide acceptable control thrips in cotton.
Economic damage from thrips rarely occurs once seedling reach the 4-leaf stage and are growing rapidly. It is important that we make thrips decisions early in the plants development. Seedlings become more tolerant to thrips feeding in terms of yield potential with every true leaf it puts on. 1-leaf cotton is much more susceptible to yield loss than 3-leaf cotton.