Entomocidal studies of plant materials against maize weevil, Sitophilus zeamais (Motsch) (Coleoptera: Curculionidae) and its parasitoid, Anisopteromalus calandrae (Howard)

Abstract

The control of maize weevil, Sitophilus zeamais (Motsch.) (Curculionidae: Coleoptera) is mainly achieved by the application of synthetic insecticides in stored grains which poses extreme environmental and health hazards, development of resistance, high cost and residual effects in stored grains. Recent development in health sciences and public awareness about ill effects of insecticides necessitate to find out some alternative methods which are soft on environment, safe for health and have least chances of development of resistance. Plant derivatives are emerging as alternatives to conventional chemical insecticides as they have rich sources of bioactive molecules. Lots of work has been done on plant derivatives and their insecticidal activity, however it always leaves room for evaluating indigenous plants. In the present investigations, different parts of six local plants were tested in a series of experiments to evaluate the insecticidal and growth regulating properties on maize weevil and its parasitoid (Anisopteromalus calandrae) under lab conditions. Ground plant parts, aqueous and 5-organic solvent extracts of Neem seed (Azadirachta indica) (Meliaceae), Bitter cress succulent fruit (Caralluma turberculata) (Asclepiadaceae), Garlic bulbs (Allium sativum) (Amaryllidaceae), Turmeric rhizome (Curcuma longa) (Zingiberaceae), Bitter Apple fruit (Citrullus colocynthis) (Cucurbitaceae) and Ak leaves (Calotropis procera) (Apocynaceae) were used at six different concentrations of 0.5,1, 1.5, 2, 2.5 and 3%, respectively. Pure ground parts in six different doses ranging from 0.5g-3.0g 100g-1 of seed were tested, whereas aqueous and extracts of organic solvent of Acetone, Hexane, Ethyl acetate, Ethanol and Methanol were tested at 0.5,1, 1.5, 2, 2.5 and 3% concentration (v /w), against maize weevil. The side effects of effective plants extracts were also investigated on the behavior and survival of maize-weevil parasitoid, Anisopteromalus calandrae. All experiments were conducted at constant conditions of 27±20C, 65± 5% R.H. in the laboratory of Entomology Department, Gomal University, Dera Ismail Khan. The plant powders, aqueous, organic solvents extracts of Acetone, Hexane, Ethyl acetate, Ethanol and Methanol were evaluated against the test insect for days to adult emergence, inhibition of adult emergence, adult longevity and sex ratio, percent grain infestation and grain-weight loss. The newly emerged ten pairs of adult weevils were exposed to treated grains in 200 ml transparent plastic jars covered with muslin cloth. The results revealed that all the plant extracts carry varying degree of biological effects against maize weevil. The adults of maize weevil preferred untreated maize grains and settled significantly more on untreated grains. A. indica seed powder at highest concentration repelled 100% test insects followed by C. longa (76%) after 72 hours exposure period from the treated grains whereas; C. procera was found least effective showing only 39% repellency of the test insect. The experiment on the entomocidal evaluation of selected plant materials against maize weevil revealed that all plant-powders were more effective compared to plant aqueous extracts. The mortality was significantly in dose and time dependent manner. xi Among the evaluated treatments, A. indica plant powder showed the highest cumulative adult mortality (100%) followed by C. longa (90%) at (3% w/w) at 7 days after exposure period whereas, minimum mortality of 24% was recorded in C. procera compared with untreated check (3%) where none of the powders were applied. The studies on varietal screening revealed minimum (23.8) days to adult emergence of maize weevil were recorded when reared on variety Azam (white); whereas; maximum (34.2) days were recorded on variety Sadaf. Total number of adults emerged were maximum (91) in Azam (white) while minimum (62) in Sadaf (yellow). Percent infestation of maize kernels was maximum (39.01%) in Azam (white) while minimum (25.80%) in Sadaf (yellow). Percent weight loss was maximum in Azam (white) (26.12%) while minimum (16.88%) was in Sadaf (yellow). Maximum adult longevity of 68.4 days was recorded in Azam (white) while minimum (53.4 days) on Sadaf (yellow) variety. The investigations of different organic solvents extracts prepared from six selected plants revealed that Hexane extracts were found most effective followed by Acetone, Methanol, ethanol, whereas; ethyl acetate were found least effective. All tested extracts prepared in hexane had significant (P<0.05) effect on the biology of maize weevil however, A. indica and C. longa extracts prepared in hexane @ 3% concentration were most effective showing least infestation (1.74 and 2.22%), minimum weight loss (0.72 and 1.03%), less adult emergence (7.00 and 9.80) and maximum days to adult emergence (59.40 and 54.40 days, respectively). The experiments to investigate the supplementary bioactivities of turmeric (Curcuma longa) hexane extracts and parasitoid, Anisopteromalus calandrae against maize weevil revealed that higher concentrations of 2.5 and 3% (v/w) have effect on settlement of A. calandrae adults on treated maize grains, whereas, corn kernels treated with turmeric extracts at 0.5,1,1.5,2 and 2.5% concentrations had no effect on the development of the parasitoid and these treatments were non-significantly different than untreated check. However, 3% treatment showed prolongation in days to adult emergence of parasitoid (29.11 days) compared with control 24.60 days, which is statistical significant. No significant differences were observed among the treatments when A. calandrae were offered maize weevil for parasitization on maize grains treated with all the tested concentrations of turmeric hexane extracts compared to untreated check. Treated corn kernels with turmeric hexane extracts by six concentrations in combination with the release of five pairs of parasitoid per treatment reduced the population of maize weevil by 64.24, 66.42, 72.71, 80.87, 86.30 and 90.64% respectively. Similarly, combination of neem seed hexane extracts and parasitoid, A. calandrae proved to be most effective in reducing the emergence of maize weevil from treated grains. The neem seed hexane extracts were found completely safe to adults of A. calandrae having no effect on the survival of parasitoids. The maize grains treated with hexane extracts at 0.5, 1, 1.5, 2, 2.5 and 3% along with the release of five pairs of adult parasitoids per treatment inhibited the emergence of maize weevil by 68.52, 71.78, 78.07, 84.08, 88.44 and 92.25%, respectively. Hence, plant extracts and powders of A. indica and C. longa along with parasitoid (A. calandrae), is compatible in grain protection practices for the safe management of maize weevil.