Article by Jack Bacheler.
Cotton Insects and Weather
As always, our unfolding ‘cotton insect pest year’ will be influenced by present and upcoming weather patterns. Although most areas of the state presently have adequate moisture, our present and predicted upcoming extremely hot and dry conditions can significantly impact on insects and their potential damage, both positive and negative. In most years, insects on the front burner will be stink bugs, bollworms, plant bugs, spider mites *(we like to claim these arthropods as insects) and cotton aphids.
Plant Bugs on Cotton
We have had some fields treated for a combination of square retention counts approaching or less than 80% along with adult plant bugs, probably recently assisted by the accelerated movement adult plant bugs from rapidly drying down non-cotton hosts to cotton. Producers should be advised to withhold treatment until both square retention drops to the range of 80% or less and 8 or more plant bugs per 100 sweeps are found. At this time of year in NC, a high proportion of plant bugs found on cotton should be adults. Once cotton is approximately a week into blooming, the use of a black or dark-colored ground cloth is advised because the small bright green plant bug nymphs may become more prevalent and are more easily seen on a dark background. Black or dark ground cloths can be 1) constructed (either 2.5 to 3 feet in length and unrolled to the appropriated row width between two stakes, such as tobacco “sticks”, 2) made by spraying readily available white ground cloths, or 3) purchased commercially from some vendors such as Great Lakes IPM in both 2.5 and 3-foot lengths.
If treatment is needed for plant bugs, the use of chloronicotinoids such as Centric, Admire Pro and Belay should be restricted to cases of exceeding the threshold by perhaps 1.5-fold or less. This class of insecticides may help lessen the need for subsequent sprays for cotton aphids and spider mites. The bigger guns such as Bidrin, Brigadier, acephate, Transform and Endigo should be deployed in cases of higher plant bug levels and/or for subsequent sprays. The use of these latter materials has a greater negative impact on several beneficial insects, however, increasing the odds of having to treat later for aphids and spider mites.
Spider Mites on Cotton
Given that we have already received several reports building mite populations this past week and because this species often thrives in hot dry weather, cotton fields should be monitored for this pest. Although most immature and adults mites and their eggs will be on the undersides of leaves, initial mite infestations appear as light stippling from above. Because spider mites are tiny arthropods, their eggs and immature forms are best viewed under a hand lens. Worsening infestations are characterized by reddening or “bronzing”, then yellowing and finally by defoliation of primarily lower leaves. Our suggested threshold is “general leaf discoloration (chlorosis, bronzing, or both), plus live mites over most of the field and defoliation from mites in 25 percent or more of the field.” If rain is imminent, we advise producers to delay treatment and reevaluate 3 to 4 days after the rain due to 1) the potential development of a mite fungal pathogen and 2) rainfall relieves cotton plants of much of the stress from mites under dry conditions. High volume (a minimum of 10 or more gallons of finished product per acre) and pressure (50-70 psi) are recommended for mite control.
Stink Bugs on Cotton
Some cotton is now beginning to bloom. Producers should be encouraged to assess internal damage to quarter-size bolls by approximately the end of the first week or early in the second week of bloom. Remember that weeks 3-5 of the bloom period constitute the interval when cotton is most susceptible to yield-reducing boll damage. The more protective recommended 10% internal boll damage threshold recommended during this time period is also when scouting really pays for itself – and then some. Stink bug levels appear to be on the high side so far this spring and summer, although their potential damage to this year’s cotton crop will be determined over the next 5 weeks, primarily based on this crop’s attractiveness and susceptibility to brown and green stink bugs. Generally, the wetter the weather, the greater the potential damage from stink bugs. If we receive extended hot dry weather in the coming weeks, stink bug damage to cotton will likely be less due the increasingly unattractive plants. Most of us would opt for higher potential stink bug damage if it were associated with more generous rainfall patterns and higher potential yields.
Bollworms on Cotton
Based on the timing of heavy silking and ear development over most of the state’s field corn, it would appear at this time that the bollworm moth flight should be about on schedule. The intensity of this flight will in part be determined by the percent of corn ears infested with earworms. As contrasted to conventional non-Bt cotton, if a bollworm threshold is met, our 2-gene WideStrike and Bollgard II varieties often reach the treatment threshold in the range of 7 to 10 days or so later than conventional varieties.