Aphids have a complex life cycle involving overwintering eggs hatching in spring into wingless females, which give birth parthenogenetically to young that mature as wingless females. Populations increase rapidly during times of rapid plant growth. Later in the summer, winged females that fly to other plants and often to other plant species are produced. The last winged female generation of the season flies back to the primary host species. In the fall, wingless males and egg-laying females are produced and mated; eggs are laid on the primary host.
Aphids cause two types of damage to raspberries. First, they are piercing-sucking insects, removing nutrients from phloem tissue. Second, their feeding activities can spread disease, notably viruses. These viruses can in turn stunt plant growth, distort and discolor leaves, and decrease flower and fruit production. Plant viruses are credited with the sharp decline in raspberry acreage in recent decades. All stages except the egg in the aphid life cycle are capable of transmitting viruses. The minimum feeding time necessary for aphids to pick up a virus from an infected plant is about 15 to 30 minutes. Aphids can retain the virus for several hours and are very efficient at transmitting the virus from plant to plant; a single feeding probe by a single aphid suffices.
The migration and dispersal of aphid populations, which takes place in June through mid-August, must be understood to appreciate the potential that aphids have for disease transmission. The local dispersal of aphids within rows is accomplished mainly by wingless females early in the season. Winged females are responsible for long-distance dispersal. The maximum distance that aphids travel is unknown, but if an aphid flies up into a stiff breeze it can be carried for miles. Of course, the probability that aphids will establish new colonies decreases rapidly with distance. Aphids also can be carried to new plants by animals, equipment, and even people.
Aphid control is important in reducing the secondary spread of viruses. The virus problem can be reduced in the following ways:
- Eliminate virus-infected wild and cultivated raspberries. Remember that not all viruses show symptoms, and healthy-looking plants may still harbor a virus.
- Plant raspberries that are certified to be virus free, and use proper fertilization, pruning, and irrigation practices. Certified virus-free stock remain infection free for at least 2 years and produce larger crops on healthy, vigorous plants; however, the local spread of a virus after planting must be maintained at low levels. This means roguing infected plants and controlling aphid vectors.
- Control aphids with horticultural oil or safer soap and by conserving natural enemies. Monitor plantings beginning at egg hatch (approximately May). Initiate a spray program when aphids are spotted. Other insects such as ladybird beetles can devour great numbers of aphids. These beneficial insects should be conserved by using insecticides or other tools only when necessary.
- Use varieties that are resistant to aphids. For instance, the purple-red Royalty variety is immune or resistant to two aphid species and the raspberry fruitworm.
- Use virus-resistant varieties.
- Maintain 500 to 1,000 feet between new plantings and virus-infected wild and cultivated raspberries.