Damage to Watermelon Seedlings Caused by Frankliniella fusca (Thysanoptera: Thripidae)

The spring watermelon [ Citrullus lanatus (Thunb.) Matsum & Nakai] crop in North Central Florida does not normally suffer much direct feeding damage from insects. For the past five years, however, many young plants have been found with scarred and distorted leaves. The most heavily damaged plants appeared to be stunted as well. Thrips were found on seedlings, and an experiment was performed to determine if the observed damage was caused by their feeding. Watermelon seedlings showing typical damage, thinned from a 2-ha field at the Central Florida Research and Education Center, were chilled and then searched for thrips. Thrips were collected by aspirating into medicine droppers; 30 to 40 were sealed into individual droppers with Parafilm. Thrips were then released onto watermelon seedlings (‘Charlee’), most having two true leaves, planted in groups of eight to 10 in five 12-liter pots. Plants were covered with cast acrylic cylinders that fit inside the pot rim, the open end covered with nylon organdy. Plants were placed in an insect rearing room kept at 23 ∠ 2 ° C with a photoperiod of 16: 8 (L:D). Some thrips were preserved in 70% ethanol for identification. Plants were observed as they grew, and photographs were taken at intervals to record damage. Damage produced by thrips was identical to that found in the field (Fig. 1). Plants without thrips remained symptomless. Damage to infested plants (silvery, clear areas on leaves, crinkling, and chlorotic spots and streaks) became apparent as each new leaf expanded, suggesting that feeding was occurring mainly in the terminal buds. Some minor feeding damage also occurred on the upper surface of cotyledons and on already-expanded leaves. Thrips used to infest plants were identified as Frankliniella fusca (Hinds), the tobacco thrips. Recently, this same thrips was identified from badly damaged watermelon seedlings growing in a greenhouse at the Central Florida Research and Education Center. F. fusca is a polyphagous species that has been reported to cause direct damage to