APHIDS
General Description
Aphids or "plant lice" are soft bodied pear-shaped insects generally less than 1/8 inch long. Their color varies according to species, however the common ones are usually green, yellow or black. The most distinguishing feature in the identification of aphids is the two short cornicles or tubes which extend from the end of their body. These structures are partly responsible for secretions of a fluid thought to be useful as a defense mechanism.
Important Species
At the present time, there are approximately 1,375 species of aphids identified in the U.S. and Canada. However, the most common species encountered in north Florida on woody plants are:
The woolly hackberry aphid is new to the Western Hemisphere and is a serious nuisance in some areas.
Biology
Most aphids, even adults, are wingless but when colonies become overcrowded or the host plant becomes undesirable, winged forms are produced and these establish new colonies. An exception to this are adult crapemyrtle aphids which are all winged. Temperature and photoperiod also affect the production of winged and wingless forms.
Aphids are unlike most insects in two ways:
- in Florida, almost all are females which reproduce without mating; and
- they seldom lay eggs, but give birth to living young.
Aphids have the ability to reproduce rapidly and there are many generations per year. Each female aphid produces 50 to 100 daughters during her life span, and each daughter can begin reproducing in six to eight days.
Most species of aphids are specialists, feeding on one or a few closely related plants. Examples are crape myrtle, oleander, podocarpus and the rose aphid. A few aphids, such as the green peach and melon aphid, infest a wide variety of plants. Some of the more frequently infested plants include camellia, crape myrtle, gardenia, oleander, photinia, podocarpus, rose, viburnum, and hibiscus.
Seasonal Abundance
Aphid populations are largest during the spring, on the flush of new growth. Reproduction is augmented by nitrogen levels and this element is particularly high in those portions of the plant that grow most rapidly. Also during the spring months, predators and parasitoids have not usually reached sufficient numbers to suppress the aphids. Some aphid species are present all year, especially the green peach aphid which also feeds on older leaves.
Damage
Aphids have piercing-sucking mouthparts and cause damage by sucking the plant juices. They are commonly found on the stems, undersides of leaves and on flower buds in colonies of individuals. However, their ability to transmit plant virus diseases may be more harmful than any direct feeding damage.
Aphids seem to be especially troublesome on plants that are in shaded areas. Their feeding causes the leaves to curl or crinkle and flower buds may become hardened causing the flowers to be distorted.
Aphids excrete large amounts of honeydew which is a sugary liquid composed of unused plant sap and waste products. This provides an excellent medium for the growth of a black fungus called "sooty mold". Besides being unattractive, sooty mold interferes with photosynthesis and somewhat retards the growth of the plant. Sooty mold usually weathers away following control of the insect infestation.
Sampling
Examine the terminal stems and the undersides of leaves (especially the new growth) one to two times per week during the growing season. Finding tiny white cast skins deposited on the upper leaf surface is frequently the first indication of an infestation. Molting aphids, inhabiting the undersides of leaves immediately above such deposits, are the source of these cast skins. Ants feed on honeydew, therefore when ants are observed, plants should be examined closely for aphids.
Aphids are attacked by a very large number of predators, parasites and pathogens. Some of the common predators include several species of lady beetles, lacewing larvae and syrphid fly larvae. Many small hymenoptera parasites infest aphids, most being species specific. Parasitized aphids are easy to detect, their bodies turn a tan or cream color, become hard and shiny and are commonly referred to as mummies. When the parasitic wasp emerges from the aphid, it leaves a small round hole in the body.
Pathogenic fungi sometimes attack aphids if weather conditions are favorable. An infected insect becomes mummified and covered with white, gray, tan, pink or green powdery-looking mold. As the disease progresses, the insect disappears as mycelium expands over the body. Carefully evaluate the presence of beneficial organisms before applying a pesticide.
Cultural Control
Use slow release nitrogen fertilizer sources such as ammonium or urea based formulations to avoid rapid and excessive flushes of new growth.
Chemical Control
Insecticidal soaps and oils should be considered when a pesticide is required. They are effective against aphids and the least toxic to people, other non-target organisms and the environment.
For the most current insecticide recommendations to control these pests, please contact the local county office of the Cooperative Extension Service. If you have access to the World Wide Web you can consult the University of Florida's Insect Management Guide at:
http://edis.ifas.ufl.edu/TOPIC_GUIDE_Insect_Management_Guide
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