Minimum gamma irradiation dose for phytosanitary treatment of Exallomochlus hispidus (Hemiptera: Pseudococcidae)

Exallomochlus hispidus (Morris) (Hemiptera: Pseudococcidae), is a polyphagous pest, attacking a wide range of plants, including mangosteen (Garcinia mangostana L.; Malpighiales: Clusiaceae) in Indonesia. The pest’s present distribution is restricted to South East Asian countries, and it has, thus, become a quarantine pest hindering the export of fruits, vegetables and ornamental plants from the region. A series of efficacy tests were conducted to determine the dose of gamma radiation required as a phytosanitary treatment against this pest. In each efficacy test approximately 50 mealybugs were inoculated on a squash (Cucurbita moschata Duchesne ex Poir.; Cucurbitales: Cucurbitaceae) fruit and gamma irradiated at various doses in 4 replicates. The mortality and progeny production responses were observed. Tests to determine the most radio-tolerant stage were conducted at doses of 100, 200, 300 and 400 Gy on 1st, 2nd and 3rd instar nymphs and pre-oviposition adults. These tests showed that irradiation inhibited the development of the insects besides inducing mortality and sterility. The 400 Gy generic dose used for arthropods caused only 84.6% mortality. Based on the mortality response, adults and 3rd instar nymphs were found to be the most tolerant stages. Efficacy tests were subsequently conducted on adults at doses of 50, 75, 100 and 125 Gy. These increasing doses did not significantly increase mortality but significantly increased sterility. No treated adult produced progeny except in 1 replicate at a dose of 50 Gy. To determine the minimum dose that would be required to use irradiation as a phytosanitary treatment, other tests on adults were conducted at doses of 40, 60, 80, 100 and 120 Gy and the percent sterility of individual adults was observed and recorded. The results and the probit analysis of the sterility data showed that ED50, ED99 and ED99.99683 (probit 9) the sterility doses were 85.5, 99.4 and 110.7 Gy, respectively. It is concluded that when sterility is the measure of treatment efficacy, the minimum dose for phytosanitary treatment against E. hispidus should be 111 Gy. This is lower than the doses of 200 to 250 Gy necessary to prevent the development of the gray pineapple mealybug, Dysmicoccus neobrevipes Beardsley (Hemiptera: Pseudococcidae).

Keywords: effective dose, export, mangosteen, fruits, vegetable