EFFECT OF STORING AT LOW TEMPERATU RES Oxydia trychiata (Guenée) EGGS UNPARASITIZED AND PARASITIZED BY Telenomus alsophilae VIERECK

Abstract

Storing insect eggs at low temperatures for use in rearing egg parasites is done to interrupt the insect development and, when parasitized, to stop the parasitoid development. This tech­nique facilitates the synchronization of insect rearing and field parasite re­leases. The objectives of this research were: 1) to study the effect of cold storage (-6.76°C) on unparasitized Oxydia trychiata eggs placed in double plastic bags, the air was removed from each bag, and the necks of the bags were twisted and tied. Eggs were also stored in glass containers tightly sealed. Eggs were maintained under freezing during a variable number of days (15, 30, 45, 60, 80, and 100) before being exposed to Telenomus alsophilae. 2) to study the effect of cold storage (-6.76°C) on parasitized O. trychiata eggs with different development ages of parasitization (1, 2, 3, 4, 7, 15, 20 and 23 days); these eggs were kept in plastic bags during 15 days in a freezer. Also, 23-days old immature parasitoids were tested using different freezing times (15, 30, and 45 days). Another experiment was conducted to find out the effect of refrigeration conditions (4.15°C) using 23-days old parasitized eggs stored in a desiccator (70°/oRH) during 15, 30, and 45 days. All the experiments were set up in a complete randomized design with four replica­tions. The experiment with unpara­sitized eggs was set up in a factorial array 2 x 6. The deleterious effect of cold storage (-6.76°C) on the unparasitized eggs was manifested by the shrinking of eggs, granules inside the eggs, and the change in color from yellow to brown. These signs were more frequent in eggs stored in glass containers than those stored in plastic bags, especially after 45 days of cook treatment. Parasitoids developed only in eggs that remained normal in appearance. Data recorded were total parasitism, emerged parasites, unmerged parasites, and those that died in the larval stage. With the exception of the unmerged category, there were significant dif­ferences (P<0.01) among treatments when unparasitized eggs were exposed to cold storage. Total parasitism varied from 91.85°/o at 15 days to 20.22 at 100 days. The emerged parasites varied from 74.51% to 16.48°/o for the same days; the unmerged-parasite category varied from 19.8% at 30 days to 3.74°/o at 100 days and for those eggs where the parasite died in the larval stage varied from 8,15% at 15 days to 79.78°/o at 100 days. In general, parasitism decreased as storing time increased, a regression analysis showed a linear trend for al) the categories. The T. alsophilae sex ratio favored females and was not signifi­cantly affected by the treatments. Parasitized eggs of O. trychiata were highly susceptible to cold treatment in the freezer (-6.76%) and in the refrigerator (4.15°C). None emerged from those stored in the freezer but when stored in the refrigerator only a proportion (25.62%) of immature parasites emerged as males in the 15-day cold treatment. The results of this study indicate that O. trychiata eggs can be stored at -6.76°C for a long period of time (one to two months) and remain viable for T. alsophilae development. The storage method using double plastic bags is more efficient and economic. Cold treatments should be tested against other insect species that need to be used for egg parasite rearings.