Green peach aphid, Myzus persice
Cowpea aphid, Aphis craccivora
Cotton aphid, Aphis gossypii
Description. Three species of aphids, or plant lice, commonly feed on cotton plants in Texas: the cotton aphid, the cowpea aphid and the green peach aphid. Green peach aphids are light green or pink in color and may occur on cotton seedling. However, the cotton aphid and the cowpea aphid are the only species that tend to establish sustainable, reproductive colonies. The cowpea aphid is shiny black with white patches on the legs and can be common on seedling plants. The nymphs of the cowpea aphid are ash-gray in color. Cotton aphids are highly variable in size and color, varying from light yellow to dark green or almost black. However, unlike cowpea aphids, cotton aphids do not have a shiny appearance and may occur anytime during the season. Although size can vary based on environmental conditions, adult aphids tend to be about 1/16th inch in length, and are soft bodied and pear shaped. Aphids have piercing-sucking mouthparts and those infesting cotton will have two protrusion on their rear tips called cornicles. Aphids adults can be winged (alate) or wingless (apterous). The formation of winged types is usually in response to overcrowding or poor host quality. The immatures or nymphs of the aphid are similar in appearance to the adult but smaller.
Aphids encountered in cotton reproduce asexually, giving birth to live young without mating. In Texas cotton, there are no male aphids. Aphids have a tremendous reproductive capacity and nymphs are born with developing embryos already present; essentially aphids are born pregnant. One female may produce as many as 80 young females that mature within 8 to 10 days. Thus, it is possible for aphids to have as many as 50 generations per year. These generations also occur as frequently as every 5 to 7 days under optimum conditions.
Aphids usually are found on the undersides of leaves, on stems, in terminals, on blooms and sometimes on fruit.
Damage. Aphids suck phloem sap from the plant robbing the plant of energy that would otherwise be utilized for growth or fruit production. Heavy and prolonged infestations can cause leaves to curl downward, older leaves to turn yellow and shed, squares and small bolls to shed, and bolls to be reduced in size, resulting in incomplete fiber development.
A few aphids in a cotton field may actually be a good thing. Aphids are favored prey by many insect predators and attracting predators into a field may help manage other pests such as bollworms and budworms.
Because the food source aphids are feeding on is rich in sugars yet relatively poor in amino acids, the aphids have to ingests more sugary sap than needed in order to filter enough amino acids for optimal nutrition. To rid itself of excess sugar, the aphids excrete copious amounts of a sugary substance called honeydew. Honeydew will accumulate on the leaves giving them a shiny, sticky appearance. A black sooty mold will often grow on the honeydew covering the leaf which may partially inhibit photosynthesis. More importantly, the honeydew may accumulate on the lint of open bolls. Low infestation levels of cotton aphids can excrete sufficient honeydew to contaminate the lint of open bolls and create significant and costly problems at the textile mill. Mills are reluctant to buy and may even reject contaminated lint.
Management and decision making.
Cultural management. Aphids tend to develop higher populations on hairy cotton varieties opposed to those that are smooth-leaf varieties. Aphids populations sometimes tend to be higher in clean-till or conventional-till production systems relative to those planted into small grains or sorghum residue. Planting date can greatly influence the risk of developing abundant aphids. In general, late-planted cotton tends to have higher aphid numbers develop than early plantings. A uniform stand can also play a role. Aphids tend to be more prevalent in skippy stands or cotton planted in a skip-row pattern. Avoid excessive nitrogen. Nitrogenous compounds are the staple of aphid nutrition and excessive nitrogen may make the cotton a more nutritionally suitable host, resulting in greater numbers.
Biological control. Predators, parasitoids and aphid killing fungi are often the most effective means of managing an aphid population. These beneficial organisms can effectively prevent aphids from reaching the action threshold, or can quickly reduce the aphid population to sub-threshold levels given the chance. Thus, it is important that when possible, care be taken to avoid killing these natural enemies when treating for other pests such as cotton fleahoppers, bollworms or lygus.
Lady beetles are usually the most notable aphid predator found in cotton.
Research conducted by the University of Arkansas has demonstrated that once a lady beetle population has reach 0.3 adults or 0.2 larvae per 1 ft-row, the aphid populations will usually decline within a few days. The parasitoid Lysephilebus testaceipes is a small wasp, about 1/16th inch in length, that lays it’s egg in individual aphids. These eggs hatch and the wasp larva feeds on the internal structures of the aphid. Parasitized aphids become swollen and are termed, mummies. Each mummy will produce a single parasitoid. Another important natural control agent is a fungus, Neozygites fresenii, that infects and kill aphids. This fungus is most prevalent under humid conditions but can be found in drier areas under dense, shady plant canopies. This fungus can quickly reduce an aphid population to very low numbers. The parasitoid and the fungus tend to need relatively high aphid populations to develop a high enough epizootic to control an aphid population. Thus, these agents often come in when the aphid population is near or has exceeded the action threshold. However, if the aphid population contains 20% mummies or fungal infected aphids, the population will usually sharply decline within a week.
Chemical control and Action thresholds.
Early Season. Aphids in pre-blooming cotton, from emergence to first bloom, rarely develop to economically damaging levels. These aphids are an important food source for natural enemies such as lady beetles and parasitoids. Thus, insecticide treatments for aphids in pre-blooming cotton should be avoided if possible. The choice of insecticides used for early season pests (thrips, cotton fleahoppers) can influence both aphid and natural enemy populations. Select an insecticide that controls the pest you are targeting but has the least detrimental impact on natural enemies.
|Aphid Action Threshold|
|Cotton stage||Action threshold|
|Prior to first cracked boll||50 aphids per leaf|
|After first cracked boll||10 aphids per leaf*|
|*where rainfall is not likely to wash honeydew from the lint|
Mid-Season. Use the action threshold and natural enemy considerations when determining the need for an insecticide application for aphids. Adherence to the threshold provides an opportunity for parasitoids and predaceous insects, such as lady beetles, to control the aphid populations without the use of insecticides. When maturing grain sorghum or corn fields are nearby, natural enemies often move into cotton where aphid populations are increasing. If the number of mummies or fungus killed aphids is 20% of the total aphid population (live and dead aphids), or if the lady beetle population has reach 0.3 adults or 0.2 larvae per 1 ft-row, then an insecticide application may be averted.
Consider alternatives to the pyrethroids for managing pests, such as bollworms and Lygus, when low numbers of aphids are present. Pyrethroids are notorious for flaring aphids. Avoid using excessively low rates of aphicides, especially in cases where coverage may not be ideal. Maximize insecticide coverage. Cotton aphid infestations develop on the undersides of leaves throughout the plant canopy. Thorough top-to-bottom coverage through increased spray volume and nozzle selection is very important. The use of drops on ground application equipment is recommended. A minimum of 10 gallons total spray volume/acre for ground equipment and 5 gallons/acre by air is suggested. Some aphicides perform better when applied with crop oil concentrate while others may perform worse (see label suggestions).
Repeated use of the same insecticide chemistry can reduce aphid response to similar insecticides used later in the season. Cotton aphids are notorious for developing resistance to insecticides. Avoid exposing an aphid population to multiple applications of the same class of insecticide regardless of the initial target pest, e.g. if you treated for cotton fleahopper with an organophoshate insecticide and aphid were present in the field, and then 2 week later needed to treat aphids, avoid using an organophosphate; choose an alternative chemistry.
Late Season. Avoid sticky cotton once bolls open. Low infestation levels of cotton aphids can excrete sufficient honeydew to contaminate the lint of open bolls and create significant and costly problems at the textile mill. Mills are reluctant to buy and may even reject contaminated lint. Factors that increase late season aphid populations include late irrigations or rainfall events, plant regrowth, insecticide applications (especially pyrethroids) and cotton treated with the boll openers containing the active ingredient, Ethephon. Timely rainfall of at least 1/4-inch or more can reduce honeydew deposits. Center pivot irrigation systems, configured for above canopy application or for use in a LEPA system with drops configured to give upward penetration of water into the plant canopy, can also be used to reduce honeydew deposits. Three irrigation applications of at least 1/4-inch can remove 75-90% of the honeydew. Whether rainfall or irrigation treatments reduce honeydew contamination below troublesome levels will depend upon the initial level of contamination and multiple water applications may be necessary. Also, if aphids remain on plants during or after a rain or irrigation, the lint remains susceptible to further contamination.
|Suggested Insecticides and Rates for Managing Aphids in Cotton|
|Lb active ingredient per acre||Amount of formulated
per gal or lbs
|Mode of Action Group (IRAC)|
(Bidrin 8 )
|0.5||8 fl oz||16||1B|
1not recommended for populations exceeding 100 aphids per leaf.