Aphids

The most common aphid on citrus in the coastal and intermediate districts of Southern California is the spirea aphid. In the San Joaquin Valley, the cotton or melon aphid is the most commonly found aphid on citrus. The three most common aphid species can be distinguished by color: spirea aphids are always bright green, black citrus aphids are shiny black, and the cotton aphid can range in color from yellow, to green, to dull black. A colony of cotton aphids is usually composed of several different color forms.

Most aphid species overwinter in the egg stage, and these eggs hatch in the spring into males and females that mate and produce live young. Subsequent generations do not require mating and produce live young that can result in a large increase in aphid numbers in a relatively short time.

Aphids feed on flower and leaf buds and on the underside of leaves (mainly feather growth), causing leaves to curl toward the stem. Spirea aphid, black aphid and cotton aphid can all transmit citrus tristeza virus. Aphids also secrete honeydew, which mainly comes from the excess sap ingested by the insect. Honeydew may be produced in sufficant quantities to cause objects to become sticky. This can eventually attract other pests such as ants or prevent leaves from acquiring proper sunlight for photosynthesis.

Aphids are generally not a problem on citrus except on young trees or continually flushing varieties (varieties that continually have new leaf growth), because their numbers decline when the foliage hardens off and the natural enemies are very effective in controlling them. Natural enemies normally fully control aphids within 6 weeks and an insecticide application is rarely warranted. Treatment of aphids to prevent transmission of tristeza virus has been shown to be effective only if conducted over a large area (several miles in each direction) because aphids can fly long distances.

The principle parasites of aphids are braconids and chalcidoids (two families of parasitoid wasps). Lacewings, a number of coccinellid (e.g. lady beetles) and syrphid predators, parasites, and fungal diseases usually keep aphid numbers below damaging levels. A moderate number of aphids (about 40% of growth flushes infested) can be considered beneficial on mature trees because aphids and their honeydew provide a good food source for natural enemies of other pests early in the season when other hosts are not available.

Use biological control on organically managed citrus.

Populations of cotton aphids in the San Joaquin Valley have been shown to have resistance to organophosphate, carbamate, and pyrethroid insecticides.

On newly established trees and on new growth flushes on mature trees, it is not uncommon for aphids to cause curling of leaves and produce honeydew. Treatment is usually not warranted because citrus can tolerate extensive leaf curling without yield effects. Pesticide applications are reserved for special situations such as areawide treatment programs for managing citrus tristeza virus.