Ecological studies of Lygus lineolaris and phytophagous thrips in Iowa strawberry fields
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From April to September in 2000 and 2001, Lygus lineolaris (Heteroptera: Miridae) adults and nymphs were collected weekly from strawberry and alfalfa fields to detect parasitoids and fungal infections. No parasitoids were observed from L. lineolaris collected from strawberries in either year. Four parasitoid pupae and one parasitoid larva were reared from 1,326 individuals collected in alfalfa in 2000. One adult L. lineolaris was infected with Beauveria bassiana in 2000. During 2001, 42 unidentified parasitoid larvae were observed in 792 dissected individuals. Frankliniella tritici (Fitch) (Thysanoptera: Thripidae) were collected from strawberry flowers and fruits in 2000 and 2001 from three sites in central Iowa. Mean numbers of F. tritici ranged from 0.3 to 7.4 in 2000 and from 0.1 to 48 in 2001. Significantly higher numbers of thrips were observed on flowers compared to fruits. In 2001, strawberry fruits from one strawberry farm were hand harvested for 5 days during peak strawberry production; less than 1% of damaged fruit was attributed to thrips damage. At this farm, mean number of thrips ranged from 0.1 to 48 per flower. In a greenhouse study, no relationship was observed between numbers of thrips and level of damage of the fruits. In some of the experimental cages, more damage was observed when fewer thrips (0-4) were present compared to higher thrips infestations > 15. The sources of phytophagous thrips in the genus Frankliniella infesting strawberry fields in Iowa are not known. Thrips may overwinter in fields or fly into fields during the spring. In April 2001, soil samples were collected from three strawberry fields and dried in Berlese funnels to determine if thrips overwintered in these strawberry fields. One Frankliniella spp. second instar was collected from 39 soil samples. To determine if thrips were flying into fields, yellow and blue sticky cards were placed in three strawberry fields in central Iowa from April to June 2001. During the first two weeks of May, adult thrips were captured on the sticky cards in strawberry fields indicating that Frankliniella tritici were flying into fields. Compared to blue sticky cards, yellow sticky cards caught significantly higher numbers of thrips in strawberry fields.