STUDIES ON SOME ECOLOGICAL ASPECTS IN RELATION TO CONTROL OF Spodoptera eiigua (Hb,) ON ONIONS IN SRI LANKA

Abstract

Biological studies of Spodoptera exigua (Armyworm) in tbo Northern Province oi Sri Lanka, revealed that the pre-oviposition, oviposition, incubation, larval stages, pre-pupal and the pupal periods lasted 1, 1, 1.25, 1.5 and 5.25 days respectively, in captivity. The average egg mass had 25 eggs, hatching wituin two days. The second to fifth instars measured 3.75, d.75 , 13.75 and 2o.5-mm in length. The sex-ratio of the adults was 1.4:1. Ecological studies revealed the preferred age of the onion crop to be 15 days. The site of oviposition was found to always be on the upper half of the leaf, irrespective of crop age. The first instar larva took about 40 to 120 minutes to enter an onion leaf. The third instar moved out of the leaves after one week. Generally tlie later larval stages fed externally. The maximum distance a larva migrated from the site of the egg mass was found to be 2d cm. Fields planted to the Vethala Red onion had greater leaf damage than those of small red onions. Obstructions such as hedges and tall plants prevented the infestation migrating to adjoining fields. Five crop hosts, onions, chillies, beet root, Boerhavia and castor were evaluated in cror preference studies. For egg laying the moths preferred onion, with the Vethala Red variety being preferred over the small reu onion. However, beet root was found to he the preferred host in the feeding studies using third instar larvae. Though the survival of larvae was better on Boernaviu than on beet root, body weigbL was significantly higher on the latter. If temperatures between 25 C - 29 C occurred during a particular week, in the onion season, light trap catches were significantly higher three weeks later. The Jaffna peninsula had a history of heavy reliance on pesticides. LD.S0 values of seven insecticides applied topically on ill instar larvae were found to be lenvlerate 45, periuethrln 137, acepbute 171, profenofos 311, diazinon 499, methyl parathion 69U and monocrotopbos 7d7 mg/kg. From comparison of data elsewhere the r population in tbe Jaffna peninsula appeared to have developed resistance to Methyl paratbion (having a record of long use) and a certain degree of cross-resistance to acephate which had very limited use in the control oi this pest.