Aphids Homoptera, Aphididae
Aphids feed on plant sap by inserting their mouthparts into plant tissue. They excrete a sugary liquid, or honeydew. The honeydew not only clogs the pores of the leaves, but also encourages the growth of black, sooty mold, which can prevent light from reaching the photosynthetic tissue of the plant. Aphids weaken the plant by draining it’s fluids, may cause severe distortion of growth, and are common means of transmitting plant viruses.
The body structure of aphids is simplified to perform only the functions of feeding and reproduction, while retaining the ability to walk. Even wings and flight muscles are dispensed with, except when these are needed to escape to a new food source. Thus all nutrition is directed to the needs of reproduction.
During favorable climatic conditions, all aphid young are born female and therefore all will contribute to population growth. The young are born fully formed and able to feed immediately. They grow rapidly, molting (shedding their skin) 4 times before they mature, often reaching maturity within a week. Because fertilization is not required, ova can start developing within an aphid as soon as, or even before, it is born. Young are then produced at a rate of 3 to 6 per day.
Within a greenhouse, environmental conditions are favorable, host plants are plentiful and the natural enemies of aphids are usually absent. An aphid population is therefore able to grow geometrically for a considerable period of time. (See the following sections for recommended release rates of bio-control items and pricing information.)
|Green Peach Aphid (Myzus persicae)The green peach aphid, Myzus persicae (Sulzer), is found throughout the world, including all areas of North America, where it is viewed as a pest principally due to its ability to transmit plant viruses. In addition to attacking plants in the field, green peach aphid readily infests vegetables and ornamental plants grown in greenhouses. This allows high levels of survival in areas with inclement weather, and favors ready transport on plant material. When young plants are infested in the greenhouse|
|and then transplanted into the field, fields will not only be inoculated with aphids but insecticide resistance may be introduced. These aphids also can be transported long distances by wind and storms.Adults – Up to 8 generations may occur in the spring, but as aphid densities increase winged forms are produced, which then disperse to summer hosts. Winged (alate) aphids have a black head and thorax, and a yellowish green abdomen with a large dark patch dorsally. They measure 1.8 to 2.1 mm in length. Winged green peach aphids seemingly attempt to colonize nearly all plants available. They often deposit a few young and then again take flight. This highly dispersive nature contributes significantly to their effectiveness as vectors of plant viruses
The offspring of the dispersants from the overwintering hosts are wingless, and each produce 30 to 80 young. The wingless (apterous) aphids are yellowish or greenish in color. They measure about 1.7 to 2.0 mm in length. A medial and lateral green stripes may be present. The cornicles are moderately long, unevenly swollen along their length, and match the body in color. The appendages are pale. The rate of reproduction is positively correlated with temperature, with the developmental threshold estimated to be about 4.3º C. As aphid densities increase or plant condition deteriorates, winged forms are again produced to aid dispersal. The nymphs that give rise to winged females (alatae) may be pinkish. The dispersants typically produce about 20 offspring, which are always wingless. This cycle is repeated throughout the period of favorable weather.
Damage – Green peach aphids can attain very high densities on young plant tissue, causing water stress, wilting, and reduced growth rate of the plant. Prolonged aphid infestation can cause appreciable reduction in yield of root crops and foliage crops. Contamination of harvestable plant material with aphids, or with aphid honeydew, also causes loss. However, green peach aphid does not seem to produce the high volume of honeydew observed with some other species of aphids. Blemishes to the plant tissue, usually in the form of yellow spots, may result from aphid feeding. Leaf distortions are not common except on the primary host.
|Melon Aphid – (Aphis gossypii)Melon aphid occurs in tropical and temperate regions throughout the world except northernmost areas. In the United States, it is regularly a pest in the southeast and southwest, but is occasionally damaging everywhere. Because melon aphid sometimes overwinters in greenhouses, and may be introduced into the field with transplants in the spring, it has potential to be damaging almost anywhere.|
|Egg: When first deposited, the eggs are yellow, but they soon become shiny black in color.Nymph: The nymphs vary in color from tan to gray or green, and often are marked with dark head, thorax and wing pads, and with the distal protion of the abdomen dark green. The body is dull in color because it is dusted with wax secretions. The nymphal period averages about seven days.
Adult: The wingless (apterous) parthenogenetic females are 1 to 2 mm in length. The body is quite variable in color: light green mottled with dark green is most common, but also occurring are whitish, yellow, pale green, and dark green forms. The legs are pale with the tips of the tibiae and tarsi black. The cornicles also are black. Small yellow forms apparently are produced in response to crowding or plant stress. Winged (alate) parthenogenetic females measure 1.1 to 1.7 mm in length. The head and thorax are black, and the abdomen yellowish green except for the tip of the abdomen, which is darker. The wing veins are brown. The egg-laying (oviparous) female is dark purplish green; the male is similar. The duration of the adult’s reproductive period is about 15 days, and the post-reproductive period five days. These values vary considerably, mostly as a function of temperature. The optimal temperature for reproduction is reported to be about 21 to 27 degrees C. Viviparous females produce a total of about 70 to 80 offspring at a rate of 4.3 per day.
Damage: Melon aphids feed on the underside of leaves, or on growing tip of vines, sucking nutrients from the plant. The foliage may become chlorotic and die prematurely. Their feeding also causes a great deal of distortion and leaf curling, hindering photosynthetic capacity of the plant. In addition, they secrete a great deal of honeydew which provides a substrate for growth of sooty mold, so the quality of fruit may be impaired and the photosynthetic capacity of foliage further hindered.
Melon aphid effectively transmits potyviruses, although it is only one of dozens of species implicated in the spread of plant viruses in cucurbits. Cucumber mosaic virus, watermelon mosaic virus 2, and zucchini yellow mosaic virus are transmitted despite applications of insecticide and oil sprays, probably because the viruses can be transmitted within 15 seconds.