Aphids

Strawberry aphid is pale green to yellow. Both adults and nymphs have transverse striations (horizontal lines) across the abdomen and are covered with knobbed hairs that are readily seen with a hand lens. These striations and hairs are not found on any of the other aphid species infesting strawberry.

Melon aphid is small, globular, and color varies from yellowish green to greenish black. This species is often the first to migrate into the strawberry fields each season and is the most difficult aphid species to control with insecticides.

Green peach aphid and potato aphid are less common in strawberries than the other species. The green peach aphid is green to greenish yellow and is more streamlined than the rounded melon aphid. Winged adults typically have a black spot on the top of the abdomen that is easy to see with a hand lens.

The potato aphid is much larger than the other species and has both a pink form and a green form in California. The long legs on this species give it a characteristic spiderlike appearance.

Aphid numbers usually peak during late March in Central and Southern California and undergo a natural decline to economically insignificant levels during May and June. In high-elevation nurseries, numbers peak in mid- to late summer. Numbers may continue to increase to damaging levels when spring temperatures are moderate and humidity is high.

In California strawberry production fields, aphids rarely reach damaging levels but occasionally cause yield losses because of honeydew contamination. Honeydew deposition on fruit causes sooty molds to develop and the white skins shed by aphid nymphs to stick to the fruit. This contamination renders the fruit unmarketable as fresh fruit.

Aphids transmit several viruses that can cause significant economic losses in strawberries if the planting remains in the field for several years. While not a serious problem in annual production plantings, aphid transmission of viruses is a major concern for nursery production.

While biological control can help to keep aphid numbers low, insecticide applications may be necessary in Southern California, and occasionally in Central Coast fields, if spring weather is conducive to their development. Insecticides are also applied in strawberry nurseries to prevent aphid buildup and virus spread. In other strawberry fruit production areas, aphids rarely reach damaging numbers and are not treated.

A complex of at least seven species of primary parasites have been reared from aphids infesting strawberry plants. However, the parasites themselves are attacked by a large number of hyperparasite species (parasites of the parasites) that limit the buildup of primary parasites.

Generalist predators such as syrphid fly or green lacewing larvae often provide a greater level of control. Lacewings can be purchased and released to help control aphids, but research is lacking on the efficacy of augmentative releases against aphids. Naturally occurring biological controls can keep aphid numbers below economically damaging levels, such as with the case of the melon aphid in Southern California strawberry-growing regions, so consider parasite and predator numbers before any treatment decision is made.

• Control dust (e.g., with water sprays on driveways or with cereal crops at ends of beds) to facilitate parasite and predator activity.
• Avoid excess nitrogen fertilizer, as aphid numbers tend to be especially high in plants that receive too much nitrogen.
• Some row covers (plastic tunnels or Remay-type enclosures) reduce aphid numbers to below economic levels, but the costs can be a limiting factor for large- or even small-scale plantings.

Use cultural and biological controls and sprays of insecticidal soap, azadirachtin (Neemix), neem oil (Trilogy), and pyrethrin (PyGanic) on organically certified strawberries.

• In strawberry nurseries, consider controlling aphids as soon as they appear on the plants, to reduce the spread of viruses, especially for the earliest generations.
• In Southern California, start taking weekly samples when the first leaf is fully expanded. Remove the oldest trifoliate leaf and record if any aphids are present. It is not necessary to count the aphid numbers. Randomly sample 40 trifoliate leaves per acre and calculate the percent of leaves that have aphids. Apply insecticide if the infestation level reaches 30%.
• In the Central Coast, aphids rarely reach damaging levels. If aphid numbers appear to be increasing, an insecticidal soap spray will help reduce the aphid numbers with minimal damage to natural enemies. Take a newly unfolded leaf from each plant sampled for mites and count the number of aphids. If numbers reach an average of 10 per leaf, apply insecticidal soap.