Feeding Behavior of Toxotrypana curvicauda (Diptera: Tephritidae) on Young Papaya Seeds

Abstract

Toxotrypana curvicauda Gerstaecker (Diptera: Tephritidae), the papaya fruit fly, is a major pest of papaya fruits in tropical America and as such, attempts have been made to rear larvae of this fly on semiartificial media (R. M. Baranowski, personal communication). Such efforts have not met with success, partly because first instar larvae feed on young seeds in the fruit's cavity while later instars feed on fruit pulp (Knab and Yothers 1914). Since seeds may play an important role in larval nutrition and the subsequent development of rearing media, we studied the feeding preferences of first instar larvae for different papaya seed layers. The investigation utilized fruits and insects collected in Dade County, Florida, during May 1983. In a preliminary study, immature (10 cm diameter, green) papaya fruits were halved and newly hatched larvae (derived from eggs incubated at 260C+ 10C, 75% RH) were placed on the seeds. -The larvae were observed for one hour with particular attention paid to larval feeding behavior on the gelatinous endosperm, the testa (which surrounds the endosperm), and the exterior layer, the sarcotesta (Fisher 1980). Larvae penetrated the seeds in two ways: a) by excavating the micropylar region, or b) by cutting a 0.2-0.3 mm perforation in the sarcotesta. Thereafter, the sarcotesta was not eaten further but the testa and endosperm were consumed. Subsequent examination of infested fruit from the field confirmed that the bulk of sarcotesta is not consumed by larvae, whereas the testa and endosperm are. To test the preference of first instar larvae were placed on sarcotesta, testa and endosperm of bisected seeds. The seeds were taken from immature fruit and embedded cut side up in agar (3 g/200 ml) in covered petri dishes. Feeding activity was monitored at 0, 2, 8 and 24 hrs. Tests on each seed layer were replicated 12 times, with each replicate consisting of 1 individual/seed layer/petri dish. Twelve replicates were also run on whole seeds imbedded in agar. On intact seeds, only 8% of the larvae placed on intact seeds began feeding immediately (on the sarcotesta), but by 24 hrs 92% of the larvae had already penetrated the seed and were feeding on the endosperm. Larvae placed on the sarcotesta of cut seeds were seldom observed feeding on this tissue, with the most feeding activity at 8 hrs when 33% of the individuals were feeding: After'24 hrs, 50% of the larvae remained on the sarcotesta but were not feeding on it, 17% 'were on the sarcotesta and feeding on it and 33% had moved to agar for food. In contrast, the percentage of larvae feeding on the testa or endosperm ranged from 50-to 92% throughout the experiment. These results suggested that early first in'star larvae are discriminating feeders, preferring the testa and endosperm over'the sarcotesta. To test