Chemical Control of the Potato tuber moth, Phthorimaea operculella (ZELLER) (LEPIDOPTERA: GELECHIIDAE) on stored Potatoes.

An experiment was conducted to test the efficacy of five insecticides as protectants against the potato tuber moth on stored potatoes. Test samples were dipped in solutions of fenithrothion, decamethrin, diazinon, methamidophos, profenofos at concentrations of 10, 25, 300, 500 and 3750, respectively and a check dipped in plain water. The treated potatoes, exposed to infested tubers, were kept for 90 days in a room having a temperature of 20-25° C and 30-35% relative humidity. Profenofos with a mean percent infestation of 2.33 gave significantly better control than the check and the remaining insecticide treatments. Residue analysis for the insecticides in the treated potatoes after 90 days of storage indicated 6 ppm for diazinon and none for the other insecticides. One of the principal limiting factors in the production and storage of potatoes is the potato tuber moth, Phthorimaea operculella (Zeiler). At present no adequate methods have been devised to store seed and/or table potatoes free of this pest in Ethiopia. In most areas great quantities of potatoes are available during and for some weeks after the rainy season (June, July and August). But supplies soon dwindle and become limited later in the year resulting in large price fluctuations. It is obvious that tubers which are to be used for next season's planting need to be stored for some time Elimination of infested tubers before storage could minimize the risk of a total loss; but adult moths could migrate from field to store and cause damage therein (7). Destruction of such tubers in itself may not ensure clean potatoes. To-date, chemical control is one of the most effective means of protection against this insect. Historically, a wide variety of chemicals and formulations have been used to protect potatoes in store. Fumigation with carbon disulphide (CSo) was recommended more than 50 years ago by Steward (10) Delassus (4) recommended paradichlorobenzine (PDB) which killed all larvae and freed tubers from infestation without affecting their germination Methyl bromide (CH3Br) was first tried to control P. operculella by Mackie and Carter (9). Walker and Anderson (11) reported fumigation of infested potatoes with this chemical and effective control of all stages was achieved with no residue problems. DDT is the most widely used contact insecticide but the residue after six months was not completely removed by washing or boiling (1). Chandhuri (3) recommended the use of DDD which gave excellent protection to sound tubers and consequently much safer table potatoes than was the case with DDT. Another excellent contact insecticide, according to Lloyd (8) was Gamma BHC which gave complete protection to tubers directly dusted and stored in treated bags and boxes. However, its toxicity to higher animals was reported to be two and half times that of DDT (2). Al-Ali and Talhouk (1) recommended carbaryl for use on stored table potatoes on account of its lower mammalian toxicity than DDT and Gamma BHC. It is the objective of this experiment to compare the relative safety and efficacies of the five insecticides as protectants against the potato tuber worm in stored potatoes.