Seasonal Populations of the Strawberry Aphid and Transmission of Strawberry Viruses in the Field in Relation to Virus Control in Western Washington1

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Abstract
Apterous strawberry aphids, Pentatrichopus fragaefolii (Cockerell), reach peak numbers in June or July in western Washington. The winged form is most numerous May through July and the number is not related to total population. Both forms may be found at any time of the year and may increase slightly in the fall. The population could not be related entirely to temperature, rainfall, or plant condition but each factor probably has a role. Transmission of mottle (MV), mild yellow edge (MYE), and crinkle (CV) viruses in the field reached a peak in June but was detected even in late December. The rate of dissemination was apparently related more to aphid activity than to aphid number. When Alpine Fragaria vesca L. indicator plants were placed in the field, MV and CV symptoms appeared in the indicator plants in almost equal numbers although CV is extremely difficult to transmit with aphids in the laboratory. The incidence of MYE was much lower even though virtually all the commercial strawberry plants were infected. MYE was detected in combination with MV or MV+CV much more than alone and never with only CV. Aphids collected from virus-infected plants in the field were caged on Alpine indicator plants. Of 114 plants, 53.4% became infected with MV, 25.0% with CV, 3.4% with MYE, and 2.3% with veinbanding virus. The figures are low considering that virtually all the field plants were infected with the first 3 viruses. Alpine indicator plants were set among virus-infected strawberries in the field for 1, 2, 4, 8, or 16 days. The percentage of plants becoming infected with 1 or more viruses was 7, 9, 35, and 82, respectively. Alpine indicator plants were placed 12, 36, 200, 400, or 1275 ft from an infected strawberry field. Nearly three-fourths of those at 12 ft and about 6% at 36 ft became infected. None of the others became infected, perhaps because they were scattered across a pear orchard. When indicator plants were set in a virtually aphid-free strawberry field 350 to 400 ft from an infested field, 23% became infected.

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