Influence of thrips on bronzing of strawberry fruit

Thrips are presumed to be the major cause of bronzed strawberry (Fragaria xananassa Duchesne) fruit in the Midwestern United States. The objectives of this study were to 1) identify thrips species present in Iowa strawberry fields and 2) determine the relationship between the numbers of thrips collected from flowers and fruit and the percentage of mature fruit exhibiting bronzing damage. Thrips were collected from flowers and fruit for three growing seasons at three sites in central Iowa. Approximately 82% of these thrips were eastern flower thrips, Frankliniella tritici (Fitch) (Thysanoptera: Thripidae); the remaining 18 % were primarily Frankliniella fusca (Hinds) (Thysanoptera: Thripidae). Mean number of adult and immature thrips per flower or fruit ranged from 0 to 7 in 2000, from 0 to 22 in 2001, and 0 to 16 in 2002. In 2001 and 2002, the incidence of thrips on both flowers and fruit was common; the mean number of thrips on flowers was 2 to 16 times greater than on fruit. However, in both years <1 % of damage to fruit was attributed to thrips. Based on our results, we conclude that thrips infestations do not necessarily cause bronzed fruit in Iowa strawberry fields.