A remarkable reduction of the post-hatching survival rate caused by adopting the furcellaran solution (a viscous liquid, hereafter FS) as an egg-seeding agent has become a problem during development of mass-rearing methods for the West Indian sweet potato weevil (Euscepes postfasciatus) using an artificial diet. We hypothesized that FS physically inhibits the feeding of hatched larvae on the diet, and predicted that removing and/or drying FS after egg seeding would improve the survival of E. postfasciatus. To confirm this prediction, we examined the effects of two post-eggseeding treatments (placing absorbent paper on the diet surface and ensuring air permeability of the rearing tray) on the survival and development of the weevils. Neither of the two treatments worked well individually, but they exhibited a clear synergistic positive effect on weevil survival. Their survival rate jumped from 2% to 40% when these two treatments were combined, without any adverse effects on the developmental period and the adult body size. These treatments thus enabled the FS-based egg-seeding method, which is much simpler than previous methods, clearly contributing to the establishment of a mass-rearing system for E. postfasciatus.

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We examined the effects of powdered plant material as an ingredient of the artificial diet for egg collection, the dehydration method for preparing plant material (freeze or thermal dry), and timing of mixing the plant material in diet preparation procedure (pre-or post-heating) on the number of eggs obtained, adult survival rate, and egg hatchability of Euscepes postfasciatus, a serious pest of sweet potato (Ipomoea batatas). The type of plant material did not significantly affect the adult survival and hatchability but markedly affected the number of eggs obtained. Diets with the leaves and vines of the host plants of E. postfasciatus (sweet potato and a wild host, Ipomoea pes-caprae) realized twice the number of eggs than diets with the storage root of sweet potato, indicating an advantage of the former parts of Ipomoea plants as an ingredient of the diet for egg collection. The drying method for the plant parts and timing of mixing the plant material did not significantly affect the number of eggs obtained and other parameters, suggesting that some unknown factor(s) of leaves and vines inducing good oviposition of E. postfascuatus may be stable to heat treatment.

Abstract:In the mass rearing of Euscepes postfasciatus using artificial diets, a method for estimating the number of eggs is essential for monitoring egg-collection efficiency and controlling egg-seeding density on the diets. To develop a simple method, we first analyzed the relationship between the volumes of eggs aspirated into a 1-or 2-ml measuring pipette and the number of eggs in it. A significant difference in the pattern of increase in egg number, along with the increase of volume, was found between the two pipette types. Linear and quadratic regressions fitted well for the 1-and 2-ml pipette data, respectively (R 2 Ͼ0.97). To extend the egg-number estimation to a larger quantity of eggs, we performed similar analyses using a 5-ml measuring cylinder. Here, the number of eggs contained was determined using the above measuring pipettes and regressions. A linear regression of the estimated number of eggs on volume fitted well (R 2 ϭ0.99). The results show that the number of E. postfasciatus eggs can be estimated precisely using measuring pipettes or cylinders. This study also showed that the pattern of increase in egg number differs depending on the measuring equipment, which should be considered for establishing egg-number estimation methods for other insects.