Balsam twig aphid
Damage, symptoms and biology
Balsam twig aphids cause curling of the needles and distortion of the shoots, resulting in a gall-like swelling. The aphids secrete copious amounts of honeydew on the shoots, which allows the growth of a fungus, the sooty mold, further reducing the aesthetic appearance of the infested shoots.
Light damage caused early in the season by a colony can disappear as the shoot grows. However, severe damage can be visible for several years, but disappears when the affected mature foliage eventually drops to the ground.
The severity of balsam twig aphid infestations varies from year to year and from tree to tree. During heavy infestations in plantations, the height growth of trees and elongation of annual shoots can be reduced by 10 to 30%.
The life cycle comprises three to four successive generations (multivoltine life cycle) from April to July. The insects overwinter as eggs, which begin to hatch in mid-April, continuing until mid-May. The first generation of nymphs, called stem-mothers (fundatrices), consists exclusively of wingless females. The stem-mothers and their offspring form a colony which grows quickly, feeding on needles of developing shoots.
Adult stem-mothers are viviparous, producing nymphs directly which, for the most part, become winged adults. The latter, also female and viviparous, also produce nymphs, which will become sexual adults. These adults are either male or female and are wingless. After mating, the female deposits one or two eggs on current-year shoots near the base of needles or on buds.
Life cycle (East of the Rockies)
The balsam twig aphid has a holarctic distribution, extending across Europe and North America. In North America, the insect occurs throughout the range of fir, from the Atlantic to the Pacific Ocean. In eastern Canada, the insect was first reported in the 1930s. Since then, its populations have fluctuated dramatically. Several major infestations were reported in natural stands in New Brunswick and Nova Scotia in 1966 and 1967, and in Quebec in 1972 and 1978, when the entire part of the province south of the 50th parallel was infested.
Nevertheless, the balsam twig aphid is a secondary pest in natural stands since it does not cause tree mortality.
Outbreaks are generally of short duration and affect specifically ornamental trees and edge trees or nursery and plantation trees. In Christmas tree plantations, balsam twig aphids can have a major economic impact because they reduce the aesthetic appearance of the trees.
In the integrated management of Christmas tree plantations, the age of the plantation is an important consideration because the aesthetic damage caused by the aphids is visible for only a few years, given the rapid growth of young firs and the annual pruning of foliage. As a result, it does not appear to be necessary to treat a plantation until the trees are two to three years away from commercial maturity. Such a management practice allows beneficial organisms, particularly natural enemies of the aphids, to become established and to proliferate in the plantations, thereby reducing the severity of the infestations and the need for insecticides.
Canadian Forest Service Publications
Diet and feeding behaviour
: Feeds on plant sap.
- Piercing-sucking: Has specialized mouthparts for sucking the fluids from plants, thereby causing deformities or killing the affected plant sections.