EFFECTS OF INSECT GROWTH REGULATORS (IGRs) ON THE EMBRYO AND LARVAE OF CHIRONOMUS COSTATUS AND AEDES ALBOPICTUS

Abstract

Effects of two insect growth regulators (IGRs), namely methoprene and hydroprene on various aspects of embryonic and post-embryonic development of Aedes albopictus (Skuse) and Chironomus costatus (Johannsen) were investigated. Photographic studies of the developing embryos of C. costatus were carried out. The important stages during embryonic development, such as two and four pole cells stage, cleavage, blastulation, gastrulation, germ band elongation, segment delineation, dorsal closure etc. were photographed and described in detail. Early embryos of A. albopictus and C. costatus were treated with 0.01, 0.05, 0.10, 0.50, 1.00 and 5.00 ppm concentrations of IGRs, and embryonic development was monitored. Mortalities were recorded at different stages of development until hatching. Hatching abnormalities were identified with each treatment. A similar study on effects of IGR treatments at various stages of embryonic development was carried out. In A. albopictus, methoprene caused higher embryonic mortality (92.8%) than hydroprene (78.7%), whereas in C. costatus 100% embryonic mortaltiy was recorded with both treatments. Hatched larvae died some time after hatching, with both methoprene and hydroprene treatments. Most of the larvae showed hatching abnormalities, such as partial hatching, partial side hatching and swollen embryos subjected to the IGR treatments. Larvae of A. albopictus and C. costatus were treated with above mentioned concentrations of methoprene and hydroprene Mortalities were recorded at various stages of post-embryonic development until adult emergence. In A. albopictus, when treatment started at early instars, high mortality was noted in the early unmelanized pupae (EWP). A second high mortality was occurred in the developed pupae (LBP), when older instars were treated with IGRs. In this second high mortality, some of the pupae did not emerge completely and were found dead on the water surface. In C. costatus, high mortality took place in the late 4th instar; this instar had wide thoracic (W.Tstage) segments in which wing buds and legs had completely developed. Some of the larvae had exposed wing buds, legs, antennae and their abdomens were cylindrical with anal and ventral gills. These were identified as larval-pupal intermediates. Morphometry and body weight were measured. Methoprene (7.09 ± 0.04 mm) and hydroprene (7.54 ± 0.15 mm) treated larvae of A. albopictus grew to larger size than the control (6.08 ± 0.13 mm). Similarly pupae from treatments grew to larger size than the control. The values obtained were 5.51 ± 0.09, 5.75 ± 0.06 and 4.17 ± 0.07 mm with methoprene, hydroprene and control respectively. All the treatment values were significantly (p<0.05) higher than the control. Considerable weight gain was obtained in late 4th instar and pupae of A. albopictus with IGR treatments. The larvae grew bigger and gain mean body weight up to 2.43 ± 0.01 and 2.64 ± 0.01 mg wet weight / individual. These values are significantly (p<0.05) higher than the control (2.09 ± 0.04 mg wet weight/ individual). In pupae, body weight measured was 1.96 ± 0.03, 2.13 ± 0.04 and 1.71 ± 0.04 mg wet weight / individual with methoprene, hydroprene and control respectively. Similarly in C. costatus IGR treatments increased body weight, and values measured were 5.89 ± 0.02, 6.34 ± 0.01 and 5.57 ± 0.01 mg wet weight / individual with methoprene, hydroprene and control respectively. Between treatments hydroprene caused more body weight than methoprene. Trehalase activity was increased in both insects with IGR treatments. Trehalase activity in A albopictus larvae was 3.56 ± 0.04, 3.71 ± 0.06, and 2.23 ± 0.03 µg glucose/ µg protein/ 30 min with methoprene, hydroprene and control respectively. In pupae, such activity was recorded as 2.68 ± 0.02, 31.4 ± 0.04 and 2.00 ± 0.02 µg glucose / µg protein / 30 min with methoprene, hydroprene and control respectively. Similarly in C. costatus, trehalase activity measured was 2.76 ± 0.02, 2.94 ± 0.03 and 1.92 ± 0.02 µg glucose/ µg protein/ 30 min with methoprene, hydroprene and control respectively. In all treatments, trehalase activity was significantly (p<0.05) higher than the control. Carboxyesterase (CE) activity significantly (p<0.05) increased in A. albopictus larvae with methoprene and hydroprene than the control (0.20 ± 0.002, 0.19 ± 0.004 and 0.11 ± 0.002 µg naphthol / µg protein). On the other hand CE activity was significantly (p>0.05) reduced in pupae of A. albopictus, and larval-pupal intermediates of C. costatus. CE activity measured was 0.09 ± 0.002, 0.10 ± 0.003 and 0.11 ± 0.002 µg naphthol / µg protein in pupae, and 0.13 ±0.003, 0.13 t 0.002 and 0.15 ± 0.001 µg naphthol / µg protein in LPI with methoprene, hydroprene and control respectively. Salivary gland esterase activity in both insects with IGR treatments was found to be similar to that of CE activity. Acetylcholinesterase activity significantly (p<0.05) declined in both insects with methoprene and hydroprene treatments. Methoprene caused significant (p<0.05) reduction in AChE activity compared to hydroprene.