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Aphids
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Aphids |
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Identifying and Controlling Aphids - a common garden insect pest
General information plus a brief review of the three most destructive aphidsNote: The following information has been compiled from 40 years of professional experience, agronomy textbooks, entomology literature and university extension service websites. Aphids are probably the most commonly known garden pest but generally it takes substantial numbers to cause any real damage. Aphids are not considered a serious threat to most crops and the damage done by a few is practically nil. However, because aphids can reproduce so rapidly (fivefold weekly if conditions are favorable), they can quickly sap a plant of strength. While the damage is often limited to wilting, curled or yellow leaves, and perhaps stunted growth, heavy infestations can kill seedlings. Prolonged infestations in potatoes can cause sizeable reductions in yield, especially early infestations. So, for a small garden, it would take more than a few dozen to warrant control measures. There are exceptions however, some plants are sensitive to the saliva of certain aphid species... just a few can cause severe distortion of the leaves. While Aphids can also cause malformed fruit, they do not cause any permanent injury to the fruit trees, ornamental trees or bushes and can easily be washed off with water. Aphids prefer actively growing plants and the tenderness of the most recent plant tissue. Another danger is that it only takes one aphid (a known vector) to spread a virus. There are several aphid-transmitted viruses associated with beans, beets, bok choy, chards, cucumbers, lettuce, melons, potatoes, pumpkins and squash. Aphids are especially well known for spreading maize dwarf mosaic virus in corn (and maize) and yellow dwarf virus in barley... often a farmer's sole justification for their control. There are 30 some species of these soft-bodied aphids recognized as crop pests. Three of the more common are:
Even though aphids are hard to tell apart, this should not pose a problem for biological control measures because, in the opinion of aphid predators, aphids are aphids. Aphids are easy to spot because they usually mass together in small colonies on leaves and stems. They are stationary for long periods and their movements are slow. For a rough idea how many aphids would be considered 'too many', 50 aphids per corn stalk is considered a light infestation, 400 is considered moderate. No recommendations of pesticides are made here however due to the unknown variables and legal ramifications… consult with your local pesticide supplier, university extension agent or nursery. A.O. Kime - former Arizona and California agricultural Pest Control Advisor (1970-1992) and family farmer (1973-1998) Aphids in general
Cornicles are an exclusive feature of aphids, no other insect family has them. From their cornicles aphids excrete a sticky, glossy substance known as 'honeydew'. This 'sweet and sticky' substance is very noticeable on plants and a delicacy of ants, moths and adult lacewing. Honeydew is the first evidence of the presence of aphids because aphids cause no immediate visible damage. Since aphids do not 'chew', and have only piercing/sucking mouthparts to draw out plant fluids, curled and yellow leaves are not evident for days and stunting is not noticeable for weeks. As another sign, a fungus called 'sooty mold' often grows on these honeydew deposits which will cause ugly black splotches on the leaves, stems and branches. Sooty mold reduces photosynthesis. Aphids are often oval shaped, plumpish, generally green and very-very small, ranging for 1/16" to 1/8". Aphids may also be bluish green, yellow, red, brown or black depending on the species and these colors often look 'waxy' and have a grayish hue. There are both winged and wingless forms of aphids but the wingless of a species comprise the vast majority... of these most are female. Winged aphids explains how they can rapidly spread from plant to plant and field to field, but the alleged 'factor' which seems to create adults with wings, those capable of flight, is quite odd. While it is generally agreed there are never more than a few winged aphids which develop in a colony, strangely, it is also the consensus that those few only appear when conditions get crowded (say, several hundred aphids per plant). Magic? An ancient mythological explanation? Well, it obviously remains a mystery although perhaps it's just a case of an occasional rarity, like albinos occasionally occur. Maybe the odds are predetermined to be, say, one in every so-many thousand. Yet, if it is a case of those few only appear when conditions get crowded, then it truly is a metaphysical mystery. Although, we know, life itself is a greater mystery. Surprisingly, here are some academic explanations:
Lifecycle: Over-wintering eggs will hatch in the spring as wingless females, Upon becoming adults, these wingless females are capable of holding eggs in their bodies and giving live birth* to 2-3 offspring daily and reportedly as high as 10-12. They can do so without fertilization and therefore are considered parthenogenetic. Aphids can produce many generations in a season, a few of which develop wings (for the purpose of migrating). Depending on the climate, from 8 to 20 generations in a season have been recorded and populations can increase fivefold weekly under favorable conditions. The young are called nymphs which molt their skin about four times before becoming adults. In warm weather, a newborn nymph will become an adult in 7-8 days and produce 30-80 offspring during their reported 23-41 days of their longevity. In the fall, the wingless male and females mate and the resulting fertilized eggs (yes, oviparous this time) are laid for over-wintering... often in perennial plants and trees. This would include winter-hardy weeds such as mustard but exactly which type of trees depend on the aphid species. In climates without cold winters however, the living young can be produced continually. * Some species such as the Lettuce root aphid will produce eggs instead of live births (an oviparous species). Natural enemies: The natural enemies of aphids are the green lacewing (larvae), lady beetles (both adults and larvae), syrphid flies (larvae), bigeyed bugs, ground beetles, and the parasite lysiphlebus testaceipes. Notes on using insect predators and parasites:
Natural weaknesses:
Cultural tips:
Corn leaf aphid (rhopalosiphum maidis)
Corn leaf aphids are bluish-green to gray with black cornicles and legs, 1/8" or less. Unlike some aphids, they do not inject toxins. They prefer the tenderness of the corn whorl and the developing tassel. However, exactly when they begin to appear in corn is a factor as to whether yields are affected. Yields may only suffer if the aphids are present in large numbers 2-3 weeks prior to tassel emergence but usually they arrive too late or their numbers aren't sufficient to affect yields. The closer it is to tassel emergence, the number of aphids which can be tolerated increases. After tassel emergence, the aphid population is no longer a threat and will move on to either younger corn or other host crops. In the northern states, infestations occur more commonly in late-planted corn. Host crops: corn and small grains - specifically barley and sorghum (milo/maize) Host weeds: johnsongrass, crabgrass Natural enemies: Green lacewing (larvae), lady beetles (both adults and larvae), syrphid flies (larvae), bigeyed bugs, ground beetles and the parasite lysiphlebus testaceipes.
Greenbug aphid (schizaphis graminum)
Greenbug (aphids) nymphs are pale green with a dark-green stripe down the middle of their back plus blackish lobes atop the thorax. Being only about 1/16" long, Greenbug aphids are smaller than most other aphids and often confused with the Rose-grain aphid. Unlike other aphid species, they also appear capable of over-wintering (in the egg stage) in northern states like Ohio. There have been reports that several bio-types have evolved to fit local conditions such as temperatures and available crops. In substantial numbers, the Greenbug aphid is considered one of the most destructive insect pest of small grains. Aside from being a vector of barley yellow dwarf virus, greenbugs also inject toxins that can kill seedlings and damage mature plants. Damage from the feeding action of greenbugs is more significant if it occurs before tillering. However, very heavy infestations are known to cause wheat plants to turn yellow and die. Host crops: corn, wheat, oats, sorghum (milo/maize) and other same grains and grasses including bluegrass Host weeds: Since their host crops also include winter and spring grains, they don't need a host weed in order to 'over-winter'. In absence of these crops, they over-winter in various weeds, volunteer wheat, oats and grains. Natural enemies: Green lacewing (larvae), lady beetles (both adults and larvae), flower fly (larvae), ground beetles, and the parasite lysiphlebus testaceipes.
Green peach aphid (myzus persicae)
Green peach aphid nymphs are greenish initially, then become yellowish with lateral stripes (three) down their back. They are about 1/12 inch long and their cornicles match their body color but all other appendages are pale. They over-winter in peach, plum and apricot trees (Prunus spp) but before leaving, several generations are produced which can cause damage to the fruit (buds/flowers). They are considered a highly effective vector of plant viruses, reportedly responsible for transmitting over 100 viruses. Generally 4 molting (instars) of the nymphs will occur, each about every two days. Their shed skin is left on the leaves. In one observation, the daily reproduction rate averaged 1.6 nymphs per female. This is low compared to other claims... although reproduction rates have some correlation to temperatures. For clarification purposes, the term 'strain' is sometimes used to distinguish between (1) sexual generations (yellow strain) and (2) non-sexual generations (green strain) in locations where they co-exist. It is helpful as the following illustrates...
Host crops: Vegetables: Artichokes, asparagus, beans, beets, broccoli, brussels sprouts, cabbage, carrots, cauliflower, cantaloupe, celery, corn, cucumbers, fennel, kale, kohlrabi, turnips, eggplant, lettuce, mustard, okra, parsley, parsnip, peas, peppers, potatoes, radishes, spinach, squash, tomatoes, turnips, watercress and watermelons. Those grown in greenhouses are not immune (avoid using infested plants).Field crops: Tobacco, sugar beets and sunflowers. Ornamentals: Numerous but unspecified flower and ornamental plants including those grown in greenhouses (avoid using infested plants). Host weeds: Nearby broadleaf weeds such as field bindweed, lambsquarters and pigweed can intensify an aphid problem because they are known to harbor them. Weeds can also harbor the Beet western yellow virus. Natural enemies: Green lacewing (larvae), lady beetles (both adults and larvae), syrphid flies (larvae), bigeyed bugs, ground beetles, and the parasite lysiphlebus testaceipes.
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