Flower thrips, Frankliniella tritici
Onion thrips, Thrips tabaci
Tobacco thrips, Frankliniella fucas
Western flower thrips, Frankliniella occidentalis
The most common species of plant-feeding thrips in Texas cotton include flower thrips, onion thrips, and western flower thrips.
Thrips are slender, straw-colored insects about 1/15 inch long, with piercing-sucking mouthparts (Fig. 6). Adults are winged and capable of drifting long distances in the wind. They attack leaves, leaf buds, and very small squares, causing a silvering of the lower leaf surface, deformed or blackened leaves, and loss of the plant terminal (Fig. 7). Under some conditions, heavy infestations may reduce stands, stunt plants, and delay fruiting and maturity. Thrips damage is most evident during cool, wet periods when seedling cotton plants are growing slowly. Rain, blowing sand, wind, residual herbicide damage, and seedling diseases can worsen thrips damage.
Under favorable growing conditions, cotton can sometimes recover completely from early thrips damage. In areas in which seedling emergence typically occurs under warm conditions, thrips are usually of minor concern.
Management and Decision Making
Insecticide seed treatments have become an industry standard.
Seed treatments usually provide thrips control until the two-to-three- true-leaf stage. In areas with a history of frequent, heavy thrips infes- tations, consider using systemic insecticides in addition to treated seed. Foliar sprays are often applied too late to prevent damage, and research shows that applying foliar sprays after significant thrips damage occurs does not increase yields.
Growers who may need to use post-emergence sprays should:
- Scout fields twice a week as the cotton
- Begin inspections once the cotton reaches about 50 percent stand
- Randomly select 25 plants from four regions of the field and inspect them, looking for adult and immature
- Look carefully through the terminal growth, picking it apart with a pencil lead, toothpick, or another pointed object, uncurl- ing all of the leaves (Fig. 8). Thrips often hide in tight locations, especially during rainy, windy
- Look at the tops and undersides of each leaf, paying particular attention to the area where the leaf veins intersect the central leaf vein.
Chemical Control and Action Thresholds
Preventive in-furrow or seed treatments usually provide adequate thrips control until the second true-leaf stage. However, under adverse growing conditions, a foliar treatment may still be necessary. Base the decision to apply an insecticide on the number of thrips present and the stage of plant development. As plants add more leaves, the number of thrips per plant needed to justify an insecticide application increases. Treat fields from cotyledon to first true-leaf stage when one or more thrips per plant are present. Resistance to neonicotinoids has been confirmed in thrips species in other parts of the Cotton Belt, but, so far, not in Texas.
Table 3. Thrips action threshold
|1 true leaf||1 thrips per plant|
|2 true leaves||2 thrips per plant|
|3 true leaves||3 thrips per plant|
|4 true leaves||4 thrips per plant|
|5–7 leaves or squaring initiation||Treatment is rarely justified.|