European Asparagus Aphid

European asparagus aphid is a small, blue-green to gray-green aphid about 0.06 inch (1.5 mm) long. It 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 aphid likes 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 in very large numbers may appear as a large cloud. The aphid overwinters as eggs that have been deposited on the old fern or in cracks in the soil.

European asparagus aphid is an economic problem primarily in the southern Sacramento Valley and the northern San Joaquin Valley. Damage from European asparagus aphid comes mostly from the toxin it injects 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 growth distortion, heavy European asparagus aphid infestations produce this distortion profusely. Heavy populations also produce massive amounts of honeydew that may attract ants.

Feeding by European asparagus aphid causes more damage to the subsequent year’s crop because it compromises the vigor of the crown. After 1 or 2 years of feeding by this aphid, the asparagus crown weakens and dries out. The toxin may also delay the bud break in spring, only to then cause an eruption of small spears. The damage is especially pronounced on newly established or weak plantings, and in seedling beds.

Clean fields of crop debris and conserve natural enemies.

Natural enemies, especially parasitic wasps and lady beetles, help control European asparagus aphid. Most parasitoids, such as Diaeretiella rapae, are most effective on heavy infestations, but only 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 pest's rate of reproduction can overwhelm the predators’ impact. Encourage natural populations of parasitoids by delaying pesticide applications where possible.

Mowing, chopping up, and then incorporating ferns during the dormant season may substantially reduce the number of eggs in the area.

Cultural and biological control, and sprays of mineral oil (narrow range oil) and pyrethrins are acceptable for use in a certified organic crop.

Monitor field edges regularly. 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 regularly whenever fern is present. It is best to collect plant samples and then shake or beat them on a hard, light-colored surface (such as the side of a white 5-gallon bucket or hood of a pick-up) 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; any threshold will vary with the condition of the field and time of the season. Many infested plants indicate a worse problem than many aphids on a few plants. A small infestation earlier in the season is more likely to become a problem. Waiting to see plant distortion or large amounts of white cast skins under plants may allow numbers to reach damaging levels before the infestation is detected. Treat when numbers of aphids  increase rapidly and there is not enough natural enemy activity to suggest that the aphids will be controlled in a few days.