Green peach aphid, Myzus persice
Cowpea aphid, Aphis craccivora
Cotton aphid, Aphis gossypii
In Texas, three species of aphids feed on cotton plants as secondary pests: the cotton aphid, the cowpea aphid, and the green peach aphid (Figs. 29, 30, and 31). Cotton aphids are the primary aphid species of concern in cotton. Their color varies from light yellow to dark green
or almost black. They are not shiny and can occur anytime during the growing season. The cotton aphid and the cowpea aphid are the only species that establish sustainable, reproductive colonies during
most of the growing season. Common on seedling plants, the cowpea aphid is shiny black with white patches on its legs. The nymphs of the cowpea aphid are ash-gray. Green peach aphids are light green or pink and can occur on cotton seedlings early in the growing season.
Aphids have piercing-sucking mouthparts and those that infest cotton have two protrusions (cornicles) at the back of the abdomen. Aphid adults can be winged or wingless. The immatures or nymphs look like the adults but are smaller. They are usually found on the undersides of leaves, on stems, in terminals, and sometimes on fruit (Fig. 32). They suck phloem sap and its sugars from the plant, robbing it of energy otherwise used for growth or fruit production.
Heavy and prolonged infestations can cause leaves to curl down- ward (Fig. 33), older leaves to turn yellow and shed, squares and small bolls to drop off, and smaller bolls to develop, resulting in incomplete fiber development.
Late in the season, honeydew excreted by aphids can drop on fibers in open bolls. A black, sooty fungus sometimes develops on honeydew deposits during wet periods. The honeydew-contaminated lint from such bolls is stained, sticky, and of lower quality, making it difficult to harvest, gin, and spin the fiber (Fig. 34).
Natural controls such as parasites, pathogens, predators, and unfa- vorable weather can keep aphid populations below damaging levels. But, cotton aphid populations can increase when treatments with
Scouting and Decision Making
Scout fields infested with cotton aphids twice a week since aphid numbers can increase rapidly (Fig. 35). From plants across the field, sample 60 leaves divided among the top, middle, and lower portion of a plant to determine actual infestation levels.
Chemical Control and Action Thresholds
Table 6. Aphid action threshold
|Cotton stage||Action threshold|
|Prior to first cracked boll||40–70 aphids per leaf*|
|After first cracked boll||10 aphids per leaf**|
|*Higher the yield potential (>1000 lbs lint/acre), lower the threshold
**Where rainfall is not likely to wash honeydew from the lint