Peter Ellsworth, Lydia Brown, and Gilberto Castro of the University of Arizona, and Steven Naranjo of USDA-ARS
In seven minutes or less and just seven steps, you can determine the size and structure of whitefly, Bemisia tabaci, populations in a cotton field. With this information, more precise and appropriate control decisions can be made.
The whitefly sampling system requires the examination of 30 leaves and 30 leaf disks for the presence of adults and live, large nymphs.
Whitefly populations are best estimated from the fifth main stem leaf below the terminal. Main stem leaves are attached directly to the main stem by the petioles.
The first main stem leaf is defined as the first unfolded leaf in the terminal and is one inch in diameter or larger. Adults are counted from the fifth leaf position. Large nymphs are counted on a disk-sized area in a specific location on the underside of this leaf.
Steps to efficient whitefly sampling include:
1 - Familiarize yourself with the variety, crop growth stage, and general location of the fifth main stem leaf in the field.
2 - Enter a field for at least 25 paces and select plants at random and five to 10 steps apart. Be careful to keep your shadow from passing over plants to be sampled.
3 - Turn the fifth leaf over slowly by its tip or petiole and count the leaf as infested with adults if it has three or more adults on it. Include in the counts any whiteflies that fly up from the leaf as it is turned over.
4 - Detach the leaf by the petiole from the main stem. If it fails to yield easily and snap off, then you have selected a leaf that is too high on the plant. Re-check the leaf position to make sure you are sampling from the fifth leaf. It is okay to count adults from main stem leaves. However, the nymph counts must come from the fifth leaf.
5 - Using a specially designed 8x loupe, locate your disk-sized area between the central main and left lateral veins of the leaf. Examine this area for the presence of live, large nymphs (instars three or four). Count the leaf disk as infested if it has at least one live large nymph.
6 - Adult and nymph sampling should be conducted in at least two different areas of a field - 15 leaves and 15 leaf disks from each area (for a total of 30 per field or management unit).
7 - Decisions are then based on the percentage of leaves infested with adults plus the percentage of leaf disks infested with live, large nymphs.