European Asparagus Aphid

The European asparagus aphid is a small, blue-green to gray-green aphid about 0.06 inch (1.5 mm) in length. The aphid is often covered with a powdery wax. Unlike most aphids, the cornicles of European asparagus aphid are reduced to practically invisible openings on the abdomen. The cauda, a projection at the very rear tip of the abdomen, is relatively long compared with other aphid species and has sides that are nearly parallel. The antennae are short.

The wingless forms of the aphids like to feed where the needles of the fern attach to the petioles. Their small size and coloration make them difficult to spot even upon close examination. Winged forms often occur in very large numbers that may appear as a large cloud. The aphid overwinters as eggs deposited on the old fern or in cracks in the soil.

Damage from European asparagus aphid is primarily from a toxin that the aphids inject into the plant when feeding. The toxin causes shortened internodes on subsequent growth, resulting in a tufted appearance that is called bonsai growth. While other factors can cause a limited amount of this type of distorted growth, heavy European asparagus aphid infestations produce this distortion in great profusion. Heavy populations also produce massive amounts of honeydew that may lead to considerable ant activity.

Because asparagus is a perennial plant, the important damage is the impact of the European asparagus aphid feeding on the subsequent year's growth. The distorted growth is unable to adequately nourish the plant's crown and it will desiccate after 1 or 2 years feeding by this pest. The toxin may also cause a delay in bud break in spring, followed by a profusion of small spears produced simultaneously. The impact is especially pronounced on newly established or weak plantings, and in seedling beds.

Cleaning fields of crop debris and encouraging natural enemies are important in managing this pest. Monitor field edges regularly to detect the appearance of populations.

Natural enemies, especially parasitic wasps and lady beetles, help control European asparagus aphid populations. Most of the parasites, such as Diaeretiella rapae, have their greatest impact on heavy populations after the damage is done. A species of Trioxys imported and released to control European asparagus aphid has had little success to date. General predators, such as the convergent lady beetle, may feed on some European asparagus aphids, but the European asparagus aphid's rate of reproduction can overwhelm the predators' impact. Encourage natural populations of parasites by delaying pesticide applications where possible.

Mowing, chopping up, and then incorporating ferns during the dormant season may substantially reduce eggs in the area. Burning is also effective where permitted.

Cultural and biological control, and sprays of insecticidal oils and PyGanic are acceptable to use in an organically certified crop.

European asparagus aphid populations start very slowly and in widely dispersed patches, then seem to nearly explode. Populations often begin near field edges, so monitor the edges of fields regularly whenever fern is present. It is best to collect plant samples and shake or beat them on a hard, light-colored surface (the side of a white 5-gallon bucket or hood of a pick-up, for example) to dislodge both the aphids and their natural enemies. Visual inspection of the ferns is not reliable, even for experienced scouts.

No definite threshold has been established and any threshold will vary with the condition of the field and time of the season. A high percentage of plants infested is more important than a high number of aphids on a few plants. The earlier in the season, the more likely a small infestation will become a problem. Waiting for appearance of distorted plants or large amounts of white cast skins under plants may allow populations to reach dangerous levels before the infestation is detected. Treat when numbers of aphids begin to increase faster than beneficials.