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Aphids feed on strawberry leaves, particularly on unfolding leaves. Aphid individuals concentrate on the underside of a leaf along main veins, particularly at the base of a petiole. In higher numbers, aphids inhibit plant growth and development, and sooty mould develops on the abundant honeydew. They could induce plant withering in susceptible cultivars. C. fragaefolii is the major vector of strawberry viruses: symptoms of virus diseases should also be considered when observing the symptoms of aphid atttack.
The severity of symptoms of C. fragaefolii attack varies depending on cultivar resistance. In resistant strawberry species and cultivars, the aphid develops in colonies in low numbers and without visible symptoms on plants. In the susceptible cultivated strawberry cultivars (and the majority of them are susceptible), however, the number of colonies is high, particularly during pronounced peaks (May-June and September-October).
A number of parasitoids and predators have been recorded on C. fragaefolii, but they have failed to give adequate control.
Pari et al. (1993) have reported that the larvae of Chrysoperla carnea (at least 20 larvae per metre) can be used for biological control in protected cultivations, provided that the C. fragaefolii population is reduced to a lower level by using pyrethroids.
Clones of Fragaria chiloensis (Del Norte and Yaquina) are the sources of the strawberry resistance to C. fragaefolii (Shanks and Barritt, 1974). By crossing the clone Del Norte and commercial large-fruited cultivars, the same authors have obtained two resistant but staminate clones. By back-crossing two resistant selections with cv. Totem, Barritt and Shanks (1980) have obtained 14.9% and 25% resistant seedlings, respectively.
By screening nine commercial cultivars (Fragaria x ananassa) and 557 clones of F. chiloensis for resistance, Crock et al. (1982) have concluded that the cultivars have not shown resistance, but cv. Benton consistently supported fewer aphids than the other cultivars. The clone RCP-37 (F. chiloensis) was found to be more resistant than the clone Del Norte.
Because the two-spotted spider mite (Tetranychus urticae) represents a major problem for strawberry pest control in some areas, there are data concerning the existence of strawberry clones which are resistant both to C. fragaefolii and the two-spotted spider mite (Shanks and Barritt, 1984; Shanks and Moore, 1995).
The survival and the production of nymphs are used as the resistance criteria, and Shanks and Garth (1992) have used honeydew production as a valid criterion. According to these authors, each strawberry genotype, which was found to have aphid numbers below 10% of the population on cv. Totem in laboratory evaluation, namely below 25% of the population on cv. Totem in field screening in resistance tests, can be considered as resistant. The cv. Totem is used as the standard for susceptibility.
Rosa rugosa has also been reported as a host resistant to C. fragaefolii (Svejda, 1984).
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:
Significant losses in strawberry production due to C. fragaefolii mainly result from their role as vectors of virus diseases. However, direct damage caused by feeding should not be overlooked. Due to aphid feeding, honeydew and sooty mould, plants decline and gradually, unless aphicides are applied, they die.