Cotton Aphid

Cotton aphid is the most common aphid on cotton in California and it can be present at any time during the growing season. Cotton aphid is highly variable in body size and color, and adults may be winged or wingless. Nymphs and adults of wingless cotton aphids vary in color from yellow to green to nearly black. The darker forms tend to be substantially larger. Nymphs that are developing into winged adults look very different from the nymphs developing into wingless adults: they bear small welts or protuberances on their bodies and may be covered with a coat of dusty-appearing whitish wax. Their body color is often greenish blue, or amber and blue.

The different forms of the cotton aphid differ in their ability to cause population outbreaks and plant damage, therefore it is important to be aware not only of the number of aphids present, but also of their color form. The small yellow aphids develop slowly from newborn nymph to adult and do not produce many offspring; thus, their populations rarely increase rapidly. The larger, darker aphids (green and black) are quite different; they develop more rapidly, produce many more offspring in a rapid burst, and can generate rapid population growth rates.

Additionally, damage caused by cotton aphid varies seasonally with the growth stage of the plant.

Presquaring (Early Season)

Heavy populations on seedling cotton can cause crinkling and cupping of leaves, failing to expand, defoliation, and a severe stunting of seedling growth. In addition, honeydew contamination on leaves may make the leaves appear wet and shiny. Cotton appears to be able to compensate fully for early season damage as long as the aphid feeding ceases.

Squaring and Boll Production (Mid-season)

Low aphid numbers (<25/leaf) on mid-season cotton often do not generate any obvious damage symptoms. High aphid numbers (>50/leaf) create the same symptoms as observed on seedling cotton (cupped, crinkled leaves, honeydew accumulations, sooty mold, and in extreme cases, limited defoliation). High aphid numbers at this time can decrease the size of bolls, stunt plant growth, and may increase square and boll shedding.

From the Opening of the First Boll until Harvest (Late Season)

The cotton crop is most sensitive to cotton aphid damage at this time because honeydew can contaminate the exposed cotton lint, creating "sticky cotton". Aphid populations as low as 5/leaf can result in honeydew deposition on lint.

Generally, cotton aphid populations on seedling cotton plants (presquare) in most regions of the western United States are not considered a pest problem. However, some areas have consistently severe and prolonged problems with early season aphids. Growers in these areas may need to adopt a more aggressive approach to monitoring and controlling these pests, especially when their fields have a history of early season aphids persisting into the period when squares are produced and yield losses can occur.

During the pre-squaring period of the crop, natural control of aphids is generally strong. The parasitic wasp Lysiphlebus testaceipes and a group of aphid predators (including the lady beetles Hippodamia convergens and Coccinella novemnotata franciscana and the predatory larvae of syrphid flies) are important natural enemies.

During the period of square and boll production and continuing until harvest, parasitic wasps and coccinelid beetles may still be present, especially if aphids reach extremely high densities, but in most fields they are rare. The most common aphid natural enemies at this time are minute pirate bugs (Orius tristicolor), bigeyed bugs (Geocoris spp.), damsel bugs (Nabis spp.), a complex of green lacewings (Chrysoperla and Chrysopa spp.), and a fungus (Entomophthora sp.). Although these natural enemies do provide some control, they generally are not able to strongly suppress aphid populations, or cause strong suppression only after severe damage has occurred to the plant.

Augmentative releases of predatory green lacewings generally are not effective. Natural densities of lacewing eggs are often quite high, making it prohibitively expensive to achieve meaningful increases in egg densities through releases. Furthermore, lacewing larvae are generalist predators and will feed on (cannibalize) other green lacewings, reducing the effectiveness of augmentive releases to suppress aphids.

Higher cotton aphid numbers consistently develop on late-planted cotton (late April to early May) when compared to early-planted cotton (early April). Additionally, aphids prefer cotton plants that are well watered and highly fertilized. Thus avoid excessive or poorly scheduled nitrogen applications that stimulate growth later in the cropping season.

Cultivar selection also appears to influence aphid population growth. Pima cultivars appear to be more susceptible to aphid infestations and associated damage. Within the Acala cotton cultivars, hairy-leaf varieties, which comprise the majority of the market, are more susceptible to aphids than are smooth-leaf varieties.

Cultural and biological controls and sprays of insecticidal soap, oils, and azadirachtin are acceptable for use on organically grown cotton.

Chemical management of cotton aphid can be extremely erratic and unpredictable. Part of the problem is that cotton aphid has developed resistance to many chemical classes, including organochlorine, organophosphate, carbamate, and pyrethroid insecticides. In addition, these broad-spectrum pesticides kill the natural enemies of the cotton aphid. Another resistance concern is with the neo-nicotinoid insecticides. Repeated applications of any neonicotinoids can result in resistance to all neo-nicotinoids.

To manage resistance, follow the basic principles of IPM: (1) spray only when pests reach economic thresholds; (2) start with the most selective pesticides and avoid pyrethroids early in the season in order to preserve natural enemies; (3) save the broad-spectrum pesticides for mid- to late-season aphid outbreaks; and (4) rotate insecticides that have a different mode of action group number if you have to spray more than once. The following table summarizes these insecticide resistance guidelines.

In some areas along the eastern edge of the southern San Joaquin Valley (especially eastern Tulare County) early season aphids can be a persistent problem. If early season populations have a history of being severe, a seed treatment may be warranted; otherwise, seed treatments are not usually applied for cotton aphid control.

Because research has shown that cotton aphids can be stimulated by pyrethroids and increase in numbers, be sure to carefully monitor for aphids following application of pyrethroid insecticides.

The critical time for monitoring aphids is from crop emergence through preharvest. To improve the efficiency of your monitoring program, combine sampling of aphids with monitoring for other pests. From crop emergence to seedling growth, sample aphids, mites, and thrips together as described in MONITORING SPIDER MITES, APHIDS, AND THRIPS. From early squaring through boll development, combine sampling for aphids with monitoring for mites and whitefly as described in MONITORING SPIDER MITES, APHIDS, AND WHITEFLY. From first open boll to preharvest, combine sampling for aphids with whitefly monitoring as described in MONITORING APHIDS AND WHITEFLY.

Make insecticide applications only when the cotton aphid population exceeds the economic threshold. Terminate the crop as early as feasible, using the nodes above cracked boll (NACB) method described in MONITORING PLANT GROWTH.