Thrips are the most predictable insect pest of Georgia cotton and are the only insect pest of cotton that a preventive or automatic insecticide is recommended for control. Thrips infest near 100 percent of Georgia cotton each year and a preventive systemic insecticide should be used at planting. Research at the University of Georgia demonstrates a consistent yield response when using a preventive insecticide at planting for thrips control.
Thrips infest cotton at emergence and immediately begin feeding on the underside of cotyledons and depositing eggs. Excessive feeding on the cotyledons results in the underside of cotyledons having a “silvery” appearance. Thrips eggs hatch in about 5-6 days. Immature thrips are cream colored and lack wings whereas adults are brownish to black with two pairs of wings. Once the terminal is formed thrips feed primarily on unfurled leaves in the plant terminal. This feeding results in crinkled malformed leaves once they expand. Excessive thrips injury results in stunted seedlings, delayed maturity, reduced yield potential, and in severe cases stand loss.
Seed applied or infurrow applications of systemic insecticides are taken up by the plant as it germinates and develops providing protection during early growth stages (thrips begin infesting cotton and depositing eggs the day it emerges). Commonly used at plant insecticides include seed treatments (imidacloprid, acephate, thiamethoxam), infurrow applications of aldicarb granules, and liquid infurrow applications of imidacloprid or acephate. Aldicarb and liquid infurrow applications typically provide increased and longer residual control compared with seed treatments. Supplemental foliar applications of insecticide may be needed if environmental conditions are not conducive for uptake of the at-plant systemic insecticide or if heavy thrips infestations occur. Systemic foliar insecticides (acephate, dicrotophos, or dimethoate) should be applied when 2-3 thrips per plant are observed with immatures present. The presence of immatures suggests that the at-plant insecticide is not providing control (i.e. thrips eggs laid on the plant, hatched, and now immatures are surviving). It is near impossible to achieve acceptable thrips control if an at-plant systemic insecticide is not used at planting. Although supply will be limited this year, ThryvOn, which is a new transgenic trait, provides excellent control of thrips.
When formulating your thrips management program, planting date and tillage practice should be considered. In the coastal plain of Georgia thrips infestations and injury potential are typically greatest on early planted cotton. Historically, cotton planted prior to May 10 is at greatest risk for thrips injury but this date can vary from year to year and by location. The Thrips Infestation Predictor for Cotton (found at: https://products.climate.ncsu.edu/ag/cottontip/) is an online tool which calculates thrips risk based on your location and planting date. Risk of thrips injury is less in reduced tillage compared with conventional tillage; the more residue on the soil surface the greater the reduction in risk of thrips injury. Understanding your risk will allow you to make more informed decisions concerning which at-plant insecticide you use and the potential that a supplemental foliar insecticide may be needed. For example, if thrips risk is high consider using aldicarb or an infurrow liquid application at planting since they provide increased control and longer residual activity or be proactive and plan a foliar spray at the 1-leaf stage if a seed treatment is used. If risk is low, seed treatments will likely provide acceptable control. Seedling injury and potential yield impacts from thrips feeding are significantly increased by slow seedling growth due to cool temperatures or other plant stresses such as herbicide injury. If seedlings are growing slowly, be more aggressive with thrips management. Seedlings are most vulnerable during early growth stages; small seedlings less than 2 true leaves are more sensitive to yield loss than 3-4 leaf seedlings. Seedlings are susceptible to thrips until they reach the 4 true leaf stage and are growing rapidly; growing rapidly is an important.
Professor and Extension Entomologist
University of Georgia