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Management of Potato Tuber Moth Phthorimaea Operculella (Zeller) in Potato Seed Storage in Thailand
Dissertation Abstract:
This study looked for alternative methods in controlling the potato tuber moth (PTM, Phthorimaea operculella (Zeller)) population in Thailand at an econom ically non-damaging level.
The potato storage areas studied were situated in Chiang Mai Province, Sansai Fang, and Fang Horticulture Station from June to October 1992. Fifty tubers were randomly selected. Damaged tubers and each developmental stage of PTM were counted and examined. The duration of the different PTM developmental stages was also recorded.
Coincidentally, parasites and predators were also observed. Three storages and one potato warehouse were used for the table potato evaluation from May to October 1992 and May 1992 to April 1993, respectively. Ten kilograms of potatoes were randomly selected from each type of storage monthly. Evaluation was done in the same manner as that for seed storage.
On the other hand laboratory studies were conducted at the Department of Agriculture in Bangkok at 27°±2°C and 80±10%RH;LD = 12:12.
Results showed that very low PTM damage occurred in new seed storage. In the warehouse where potato was continuously stored, there was a high percentage of damaged tubers (86.36%). PTM sex ratio (male to female) in the storage ranged from 1:0.75 to 1:1.55.
Natural enemies of PTM were not found in the farmers' and Department of Agriculture's storage areas. PTM larvae consumed an average of0.22 grams of potato. At 15°, 20°, 30°, and 35°C, PTM had a low rate of development and rates of adult emergence were 1.0, 9.40, 10.21, and 4.35 percent, respectively.
The fecundity at 20°, 30°, and 35°C were as low as 36.62, 32.65, and 1.35 eggs per female, respectively, while at l5°C, very few adults (only 2 males) emerged. In these temperatures, the net reproductive rate, capacity for increase, and finite rate of increase were low compared with 25°C, controlled (23° ± 2°C and uncontrolled room (25° -29 °C) temperatures. The most suitable temperature for PTM development was 25 "C.
Among the eight Bacillus thuringiensis (B.t.) products evaluated, Centari, Baccut-P, Delfin, and Bactac II were the most effective in reducing PTM population. Among these treatments, Delfin had the least cost. Hence, Delfin (B. thuringiensis Berliner var kurstak) at the rate of 10 grams per 20 Iiters of water was recommended for PTM control in seed storage.
The granulosis virus (GV 25 LE) at 25 larval equivalent per liter of water was effective in controlling PTM population.
There were no s ignificant differences between parasitization of three trichograma species to PTM egg and C. cephalonica. Adult emergence was 83.93-93.97 percent and 86.86-89.28 percent in PTM and Corcyra cephalonica eggs , respectively. Among these, Trichogramma nubilale was the most effective in parasitizing PTM eggs (32.41% ). However, very low parasitization occurred in PTM eggs caged with these three thrichogrammatids (less than 10% ).
In the seed storage, PTM damage was low in the first month but significantly increased in the second and the third month. However, damaged tubers did not exceed 10 percent, while untreated tubers dipped in water and control had 17 percent damage in the third months of storage period. AII of these treatments had positive value of return on investment compared with the untreated potato tubers (dipped in water and control) which had negative results. The highest value of return on investment was GY +Delfin (64.75%). The other effective treatments were Delfin +rice husk, GV, GY +rice husk, and GY +carbaryl. The netted and rice husk also has a positive value of return on investment.