|Sampling Whitefly Nymphs|
Ellsworth, P.C., J.Diehl and S. Naranjo. Rev. 9/00. Sampling Sweetpotato Whitefly Nymphs in Cotton. IPM Series Number 6. Publication number 196006. Cooperative Extension,
Sampling is necessary for timing control of whiteflies. Sampling nymphs is particularly important to gain the most benefit from the application of insect growth regulators (IGRs). Unlike most of the insecticides used against whiteflies in
Start sampling weekly for both nymphs and adults once adults have been found in sweep samples. Procedures for designating a management unit, choosing a plant to sample, and choosing a leaf are the same whether sampling nymphs or adults (see IPM Series No. 2, "Sampling Sweetpotato Whiteflies in Cotton"). Continue sampling at least weekly for both nymphs and adults as long as IGRs are being considered for use. Continued sampling of both nymphs and adults after IGRs have been applied will enable evaluation of control efficacy. Sampling for nymphs and thresholds for first IGR use are based on our best information, including research conducted in
3rd (left) & 4th (right) instar nymphs (Tong-Xian Liu)
The first use of an IGR requires measurement of both nymphs and adults in the field. The sampling and threshold system performs best when no other insecticides have been applied for the control of whiteflies. Applying conventional insecticides before first IGR use can alter the relationship between nymph and adult densities and reduce natural enemy levels. An average of 1 large nymph per disk with at least 3 - 5 adults per leaf is the threshold for using IGRs. Second IGR use depends on labelled intervals, prior insecticides used, and general levels of whitefly increase.
Making the decision to spray an IGR, and then which one, requires measurement of current and past whitefly levels in your field and an understanding of the two types of IGRs available. If both whitefly nymphs and adults are below the levels identified by the threshold (see figure), then the grower should not spray an IGR. If both whitefly life stages are above the threshold level, then the grower should spray with the IGR of choice. Our current data and experience would suggest that the majority of whitefly population levels would fall in one of these two definitive decision zones. If nymphs exceed, but adults do not exceed the threshold level, the grower can wait and re-sample in 3 days or apply Applaud? which is effective against the nymphs. If the adults exceed, but the nymphs do not exceed the threshold level, the grower can wait and re-sample in 3 days, or use a conventional insecticide to lower adult counts, or apply Knack? which can sterilize adults and developing eggs. In these last two exceptional cases, the grower should examine the growth trend from the previous sample periods to help decide whether a treatment is necessary (i.e. sharp increase, use an IGR).
fewer than 3/leaf
at least 3-5/leaf
fewer than 1
Wait and re-sample in 3-7 days.
Spray with either IGR.
Related and updated sampling guidlines can be found in Whiteflies in Arizona Series No.11: "Binomial Sampling of Nymphs" (rev. 6/97), 49KB pdf file.