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Biological Control of Aphids

Aphid Midge--Aphidoletes aphidimyza (Cecidomyiidae)
The aphid midge, Aphidoletes aphidimyza, often contributes to biological control of spirea and green aphids in pome fruits. They feed on many species of aphids on many type of crops, but are not generally found in stone fruits because of their susceptibility to pyrethroids. Generally tolerant to organophosphate insecticides as immatures and slightly less so as adults, all stages are susceptible to carbamates, pyrethroids, neonicotinoid, and certain miticides. This species can be reared and is sold from biological control companies for mass releases in many crops, but especially for aphid control in greenhouses.

Description and life cycle
Adults are tiny, delicate flies (1/16 inch) similar to mosquitoes and feed on honeydew. Each female may lay up to 70 reddish-orange eggs that are laid singly or in groups, in numbers that are proportional to aphid density. The tiny larvae are bright orange to red maggots that are about 1/10 inch long with the head at the pointed end. A single larva may eat from 7 to 80 aphids to complete development, with early instars also feeding on mites. The life cycle lasts from 3 to 6 weeks with 3 to 6 generations/year, depending on the temperature and host species. Larval development lasts from 12 to 17 days with 15 to 32 days spent as pupae in the soil beneath the trees.

Monitoring and management
A ratio of one midge egg or larvae per five aphids may result in complete aphid control within a few days, but ratios of one egg per larvae to 15 aphids may still be adequate for control over a longer period of time. Insecticides used during the period that aphid colonies are building should be chosen with regard to the level of toxicity to predators (see Table 4-5).

Ladybird Beetles (Coccinellidae)
Adults from these easily recognized beetles are oval, often brightly colored and spotted, and vary in size from 1.5 mm to 6 mm. Approximately a dozen of the 450 species found in North America are found in fruit with most feeding primarily on aphids, but some like Stethorus specialize on mites while others specialize on scales and mealybugs. A number of species require pollen as adults to reproduce and some can be important predators of moth eggs.

Multicolored Asian Ladybird Beetle--Harmonia axyridis
The multicolored Asian ladybird beetle has recently become the most common and most effective aphid predator in Pennsylvania orchards, replacing Coccinella septumpunctata and several native species. H. axyridis is native to Asia, but was released in Pennsylvania in 1978 and 1981. However, overwintering individuals were not recorded until 1993, and the populations that have become established may have resulted from an accidental introduction by an Asian freighter in New Orleans.

Description and life cycle
Adults are about 9/32 inch long and 7/32 inch wide. They are oval or convex in shape and range from yellow to orange above. The segment behind the head hides the head from view and is cream to yellow in color, with a black “M” design in the center. Underneath, the adults are black with an orange border around the abdomen. The wing covers have from zero to 19 black spots. Eggs are laid in upright clusters of 15 to 20 and are oval and yellow. Larvae are elongate, covered with spines, and are black and orange.

Overwintering occurs as adults, often in houses and other buildings. Eggs are laid on the undersides of leaves of various plants. The life cycle from egg to adult takes about 30 to 36 days with larvae eating 600 to 1,200 aphids during development and adults eating 90 to 270 aphids per day. Adults may live for over a year.

Monitoring and management
Highly mobile fliers and voracious as both adults and larvae, ladybird beetles are often the most important aphid predators in apple orchards and can quickly control extremely high pest populations. These beetles have a slight tolerance to organophosphate insecticides, but should be conserved by selective pesticide use (see Table 4-5).

Green and Brown Lacewings (Chrysopidae and Haemorobiidae)
Green lacewing adults are 6/10 to 9/10 inch in length, green with transparent wings with an interconnecting network of fine veins. The many different species are difficult to distinguish, but the adult of the most common green lacewing species has golden eyes. The adults feed on nectar, honeydew, and pollen with females producing 400 to 500 eggs each over a relatively long life of up to 3 months. Green lacewing eggs are laid on the tips of long, white, hair-like stalks to prevent cannibalism. The larvae (called aphid lions) are generalist predators of mites, thrips, soft scales, and almost any other soft-bodied prey. They are voracious aphid predators, eating 100 to 600 aphids during a 1 to 2 week development period and can be important predators of moth eggs and larvae as well. Prey are seized in hollow, sickle-like jaws protruding from the head and sucked dry. The larvae make a small, round, and white pupal case, often on the stem or calyx end of the fruit where they overwinter or, in the case of one species, overwinter as adults in bark crevices and other protected places.

Brown lacewings are smaller (1/5 to 6/10 inch long) and are predatory, both as adults and larvae. They are much more tolerant of colder weather than the green lacewings and are more useful predators early in the season. Females lay 100 to 460 eggs, but not on stalks like the green lacewings. Larvae may consume more than 20 aphids per day or 30 to 40 mites per day. Developmental times are slower with most species only having two generations per season. Both types of lacewings have some tolerance to organophosphate insecticides, but should be conserved by selective pesticide use (see Table 4-5).

Minute Pirate Bug--Orius insidiosus
Generalist predators of aphids and mites, these are very small 1/10 inch, black, somewhat oval-shaped bugs that look like miniature, dark, tarnished plant bugs. They are most easily recognized by white, shiny wing patches on the adults. Able to feed on a wide variety of small prey, including thrips, leafhoppers, moth eggs, and young larvae, they are able to subsist on pollen or plant juices when prey are not available. This habit of feeding on plant juices may make them more susceptible to plant systemic products like some neonicotinoid insecticides. They are efficient at searching out high-prey densities and will aggregate where there is an abundance of prey. When handled, pirate bugs are capable of causing a mild sting with their beak. Orius has several generations/year and take about 20 days to develop from egg to adult. The adults live about 35 days with each female inserting about 130 eggs into plant tissues. Immature stages and adults can eat about 30 mites/aphids per day. Adults appear in late April, continue to feed all season until early fall, and then overwinter in the leaf litter both inside and outside orchards. They have some tolerance to organophosphate insecticides, but should be conserved by selective pesticide use (see Table 4-5).

Syrphid Flies
Several species of syrphid flies are among the most voracious of aphid predators in Pennsylvania orchards.

Description and life cycle
Adults are known as hover flies and resemble bees except that they have only one pair of wings. They are generally brown to black with yellowish areas. Their food source is pollen, nectar, and aphid honeydew, which is necessary for proper development of the eggs. Eggs are white, elliptical, and less than 4/100 inch long. The larvae, or maggots, are elongate, tapering gradually toward the head end and may be cream, yellow, gray, or a combination of these colors.

Adults lay eggs in the midst of aphid colonies. Larvae cast their head side to side to locate aphids, which they pierce and consume. A single larva may destroy hundreds of aphids as it completes its three development stages in about 3 weeks. There may be five to seven generations per year with most species overwintering as adults or last instar larvae.

Monitoring and management
Check for the presence of eggs and larvae in aphid colonies. Control of green aphids may result if 20 percent of the aphid colonies have syrphid larvae present.

Lepidopteran Predators--Ground Beetles (Carabidae) and Rove Beetles (Staphylinidae)
These are two of the largest families of beetles with 1,500 ground beetle and 3,000 rove beetle species in North America. Many are generalist predators that are effective in controlling pests that pupate or overwinter in the ground cover or on the trunks (e.g., codling moth, Oriental fruit moth, apple maggot, plum curculio, European apple sawfly, leafrollers). Many live in the groundcover away from pesticide applications made to tree foliage, but some may climb trunks. All are very pesticide susceptible and are often used as indicators of environmental quality.

Wooly Apple Aphid Parasitoid--Aphelinus mali
Adults of these wasps are very small and their eggs are inserted singly into the body of aphids where they will develop internally to kill the host. There are six to seven generations each year with about 20 to 25 days per generation to develop. Larvae or pupae overwinter within the mummified body of the aphid. Rootstocks with resistance (e.g., M.106) to wooly apple aphid and A. mali provide adequate control of both root and aerial colonies of this pest during most seasons unless disrupted with toxic pesticides. They have some tolerance to organophosphate insecticides, but should be conserved by selective pesticide use (see Table 4-5).

Lepidopteran Parasitoids
Tachinid Flies--important parasitoids of leafrollers in the spring. One species, Actia interrupta, is currently the most important parasitoid of the obliquebanded leafroller. Eggs are laid on the skin of larvae to hatch and develop externally on the larvae to eventually leave just an empty husk of skin. Pupae are generally found near the host remains and resemble a grain of wheat in size and shape. All species appear to be very susceptible to pesticides and are important only in pheromone disruption or orchards with minimal pesticide sprays.

Braconid and Ichneumon Wasps--with approximately 120,000 known species and many as yet undescribed, this is a virtually untapped source of biological control in modern agriculture. With various complex life histories, often alternating between several hosts and attacking specific life stages, these wasps have not been important sources of biological control in tree fruit since the introduction of disruptive broad-spectrum insecticides. Previous to this, however, they provided almost complete control of many of the leafroller species. Currently, there are almost 40 different wasp parasitoids capable of attacking tufted apple bud moth in Pennsylvania apple orchards. All species appear to be very susceptible to pesticides and are important only in pheromone disruption or orchards with minimal pesticide sprays. Braconid species appear to be most important late in the growing season.

Trichogramma Egg Parasitoid--most commonly employed as a biopesticides obtained from biological supply houses for mass releases into many crops. These tiny wasps complete their development inside a single egg of their moth or butterfly host. Native populations of mostly T. minutum attack many different orchard pests in Pennsylvania (most important are the several species of leafrollers, codling moth, and oriental fruit moth). The life of the adults and the number of eggs laid are greatly increased with the provision of nectar sources and females may then live up to 2 weeks and lay over 80 eggs. Although present during most of the growing season, populations generally do not build to be significantly important in controlling these orchard pests until late summer. Trichogramma is very susceptible to pesticides and is important only in pheromone disruption or orchards with minimal pesticide sprays.