Feasibility study of using Pygmy waterboatman, Micronecta grisea, instead of Temephos larvicides in controlling of Aedes aegypti larvae

Abstract

Pygmy waterboatmen, Micronecta grisea, were collected and used to establish laboratory cultures in order to study the predation rates and feeding behavior of nymphal instars (N) and adults against third instar larvae (L3) of Aedes aegypti to assess their potential for biological control. The body length, head length and head capsule size of 330 nymphs and 71 adults of M. grisea, collected from Bang Bua Thong District, Nonthaburi Province, Thailand, were measured using a stereo microscope. Five discrete nymphal instars (N1-N5) plus adults could be classified by body length; the 1st (N1; 0.54-0.65 mm), 2nd(N2; 0.69-0.84 mm), 3rd(N3; 0.9-1.11 mm), 4th(N4; 1.29-1.56 mm) and 5th(N5; 1.74-1.98 mm) nymphal instars plus adults (2.07-2.43 mm). Nymphs classified as three discrete size categories, small (N1&N2), medium (N3&N4) and large (N5), including adults were examined for predation rates and prey handling times when fed L3 Ae. aegypti at different predator to prey ratios, Prey searching and handling times decreased with increasing size of M. grisea and were consisten with the Type II functional predator-prey response. The 5th nymphal instar and adult of M. grisea were tested the ability of M. grisea to feed on L3 Ae. aegypti to examine their efficiency. Temephos zeolite granules, applied in water at 1 ppm, were also tested against Ae. aegypti larvae. The number of live mosquito larvae left was recorded and found that, within 24 hours, temephos and adult showed the highest mortality percentages (100% and 99%, respectively), whereas the fifth instar nymph killed 64% of the mosquito larvae. Feeding behavior of M. grisea was observed during the feeding tests. Compared with adult of M. grisea, the fifth instar nymph took more time feeding on Ae. aegypti larvae. When using one adult M. grisea, within 72 h, the highest larval mortality percentages occurred at the lowest prey densities which also showed in further feeding tests in small scale containers, exhibited higher prey mortality (percentages) at the lower prey density. The effects of temephos on M. grisea were also tested and results revealed complete mortality of M. grisea within 8 hours of exposure. As larvicides applied in the field are toxic to M. grisea, but M. grisea's satisfactory results provided excellent control against Ae. aegypti larvae, controlling methods should be safe in the areas as well as promoting benefits for conservation of effective predators to control mosquito population in the nature.