Sampling Whitefly Nymphs

Sampling and Action Thresholds for Sweetpotato Whitefly Nymphs in Cotton

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, College of Agriculture and Life Sciences, University of Arizona, Tucson, Arizona.

 

Why Sample Nymphs?

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 Arizona in the past, IGRs affect immature whiteflies instead of adults. Timing the first use of an IGR requires sampling both whitefly nymphs and adults. The objective is to time the first use of an IGR during the first period of accelerated population growth. IGRs should not be used late in the whitefly infestation. Therefore, IGRs will probably be among the first insecticides used. Adult sampling is sufficient for timing use of insecticides other than IGRs. Use of IGRs within an integrated resistance management program is explained in IPM Series No. 3 & 4.

Plan Your Sampling Program

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 Arizona by the USDA-ARS and The University of Arizona, and experience with the use of these compounds in Israel. This sampling and threshold program will be evaluated for the first time in 1996.

How to Sample

1. Designating a management unit and choosing a leaf to sample is the same as for adults (seeUniv. of Ariz. IPM Series No. 2, "Sampling Sweetpotato Whiteflies in Cotton").
2. Only the 5th main stem leaf below the terminal should be used for all nymph and adult counts.
3. Sample for nymphs within a "quarter-sized" disk wedged between the main veins. This area of the leaf can be located by placing a quarter between the two major veins of the cotton leaf (see figure below).

 

4. Only count large nymphs, those that are visible to the naked eye. These 3rd & 4th instar nymphs appear as flattened, egg-shaped disks or "scales."
5. Although the 3rd & 4th instar nymphs should be visible to the naked eye, some individuals may blend in with the leaf surface. Look for the two yellow spots on the 3rd and young 4th instars and the developing red "eye" spots on the largest 4ths (a.k.a. pupae).

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7. This sampling technique should be repeated on at least 30 leaves per field as outlined in IPM No. 2. Calculate the average number of nymphs per leaf from the 30-leaf sample. [For a streamlined, more accurate binomial method of counting, See Whiteflies in Arizona Series No.11: "Binomial Sampling of Nymphs" (rev. 6/97), 49KB pdf file.]
8. Adult whiteflies should be monitored on this leaf position, too. Both the adult and nymph levels are used together to form a threshold for taking action with IGRs. Five adults are present in this example.

Threshold for First Use of IGRs

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

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).

 

 

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.

Read about the research behind this sample plan (PDF1, PDF2, PDF3, PDF4)