How to Manage Pests

UC Pest Management Guidelines

DESCRIPTION OF THE PESTS

Strawberry aphid is pale green to yellowish in color. Both adults and nymphs appear to have transverse striations 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 varies in color 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 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 in color 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 observe 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 gives it a characteristic spiderlike appearance.

DAMAGE

Populations of aphids usually peak during late March in central and southern California and undergo a natural decline to noneconomic levels during May and June. (In high elevation nurseries, populations peak in mid- to late-summer.) Populations 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 production. Honeydew deposits on fruit cause 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.

MANAGEMENT

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

Biological Control
A complex of at least seven species of primary parasites have been reared from aphids infesting strawberry plants. Unfortunately, the parasites themselves are attacked by a large group of hyperparasites (parasites of the parasites) that limits the buildup of primary parasites.

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 densities below economically damaging levels, such as with the case of the melon aphid in southern California strawberry-growing regions, so consider parasite and predator densities before any treatment decision is made.

Cultural Control
Some row covers (plastic tunnels or Remay-type enclosures) have reduced aphid populations to below economic levels, but the costs are substantial and the economic viability for large- or even small-scale plantings has not been established. Controlling dust is important to facilitate parasite and predator activity. Aphid populations tend to be especially large in plants that receive an excess of nitrogen fertilizer.

Organically Acceptable Methods
Cultural and biological controls and sprays of insecticidal soap, azadirachtin (Neemix), neem oil (Trilogy), and pyrethrin (PyGanic) are acceptable for use on organically certified strawberries.

Monitoring and Treatment Decisions

• 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. Randomly sample 40 trifoliate leaves per acre and calculate the percent of leaves that have aphids. Treat if the infestation level reaches 30%.
• In strawberry nurseries, consider controlling aphids as soon as they appear to reduce the spread of virus, especially for the earliest generations.
• In Central Coast fields, aphids rarely reach damaging levels. If aphid numbers appear to be increasing, an insecticidal soap spray will help reduce the aphid populations with minimal damage to beneficials. Take a newly unfolded leaf from each plant sampled for mites and count the number of aphids. If populations reach an average of 10 per leaf, treat with insecticidal soap.

PUBLICATION

UC IPM Pest Management Guidelines: Strawberry
UC ANR Publication 3468
Insects and Mites
F. G. Zalom, Entomology, UC Davis
M. P. Bolda, UC Cooperative Extension Santa Cruz Co.
P. A. Phillips, UC IPM Program/UC Cooperative Extension Ventura Co.
Acknowledgment for contributions to the Insects and Mites:
N. C. Toscano, Entomology, UC Riverside

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