Greenhouse: Biological Control of Aphids in Greenhouse Vegetable Production

I had a question yesterday on the biological control of aphids in greenhouse vegetable production. The following is a good article on the subject from Cathy Thomas, Intergrated Pest Management Program, Pennsylvania Department of Agriculture.

Effective and timely control of aphid populations in greenhouse vegetable production is important due to their ability to develop into large populations quickly. There are many different aphid species in PA greenhouses including, green peach aphid, potato aphid and melon aphid. The aphid that is usually found to infest vegetable crops, especially tomatoes (Solanaceae crops) is potato aphid (Macrosiphum euphorbia). In addition to the ability to transmit viruses, aphids cause direct damage by feeding on plant sap to acquire the proteins and sugars needed for their reproduction. Aphids secrete excess sugars in the form of sticky 'honeydew'. Honeydew supports the growth of black sooty mold that affects plant photosynthesis, possibly reducing plant yields. Removing sooty mold from fruit increases handling time and can possibly render fruit unsaleable.

Life cycle

In greenhouse production, aphids are very prolific. Instead of reproducing by eggs, female aphids (stem mother) give birth to live offspring (3-10/day) which start to feed immediately. Within a week, this offspring will be ready to reproduce. Aphids can have two forms: winged or wingless. As colonies enlarge, aphids develop wings to migrate to less populated areas in the crop.

The most outstanding characteristic for identifying aphids is by the two cornicles ("tail pipes") on the rear of their abdomen. Color is variable among species and is not accurate for identification. As aphids increase in size, they shed their exoskeletons (cast skins). These white cast skins, often mistaken for adult whiteflies, can be found on leaves or stuck in honeydew excretions.

Monitoring

Plant monitoring should begin at the seedling stage and continue through the duration of the crop cycle. Start plant inspection on lower leaves and continue up the plant to the growing tips. As aphids feed on growing tips, the leaves curl, sometimes looking like virus symptoms.

Yellow sticky cards are useful in detecting winged aphids. Hang sticky cards 4 – 6 inches from growing tips. IPM Labs (ipmlabs@baldcom.net) sells a chart with drawings of important insects found on sticky cards. Unfortunately, winged adults on sticky cards may indicate that there are clumped populations already established in the crop and they are migrating to less populated areas. The presence of ants in the greenhouse may indicate aphid development, since the ants feed on the excreted honeydew and thus protect the aphids. When introducing natural enemies, place them in an area protected from ants and control ants with baits or traps.

Biological Control

Several long-lasting and effective biological controls are commercially available for aphid control. Each natural enemy has an effective introduction strategy that will be discussed further in upcoming issues of this column.

Predators for aphid control

Ladybeetles (Hippodamia convergens)– Ladybeetles are sold as adults in pints, quarts and gallons. A general predator, ladybeetles are effective for cleaning up hot spots. They also feed on scales, thrips, and other soft – bodied insects.

Lacewings (Chrysoperla rufilabris) – Lacewings are sold as eggs, larvae. The larvae are voracious predators known as "aphid lions". They will also feed on mealybugs, scales, spider mites and thrips.

Predatory Midge (Aphidoletes aphidimyza)- This midge is sold as the adult to be released in greenhouse. The adult midge lays eggs near aphid colonies and the orange larvae feeds on aphids.

Parasites for aphid control

Caution: Parasites for aphid control are very species specific. Identify the aphid species infesting your crop before ordering from your supplier.

Aphidius colemani – Used to control green peach aphid and melon aphids. This tiny parasitic wasp lays an egg in the aphid. The egg hatches into a larva which spins a cocoon, producing a new wasp. The wasp exits the aphid body, leaving behind a brown
shell called an aphid mummy.

Aphidius ervi – Used to control potato aphids. This parasite has a similar appearance and
life cycle as Aphidius colemani. This parasitic wasp is about twice the size of A. colemani.

Aphelinus abdominalis – Used to control larger aphid species such as potato aphid and glasshouse potato aphid. This wasp is about 3 mm long. The main advantage to using this parasite is that the female adult will parasitize for several weeks and it will also feed on the aphids.

Other effective controls for aphids include: screening vents, removing weeds in the greenhouse and outside greenhouse, inspecting incoming plant material, disposing of plant debris, and avoid growing ornamentals in vegetable production area.

By Cathy Thomas, Integrated Pest Management Program, Bureau of Plant Industry, PA Dept. of Agricluture. cthomas@state.pa.us in the March 2001 edition of the Penn State Fruit and Vegetable Gazette.