Cotton leafhopper Amrasca devastans (Distant) a sucking pest of cotton of the family Cicadellidae and Cotton scale or mealy bug Phenacoccus sp. of the family Pseudococcidae infest a wide range of crops including Solanum melangena and Habiscus esculentus. All the stages of both the species are injurious to crops and the infestation may be very prominent and characterized by leaf curl, stunted growth and white scales, which cover the nodes, twigs and bolls. They exhibit three types of damage in all i.e. 1st direct damage by sucking the plant sap, secondly both pests are capable of transmitting viral and other infectious disease pathogens in the fields, thirdly the secretion of honey dew by Phenacoccus sp. which assists in fungal growth in the fields.
Presently horizontal and vertical life budgets and survivorship curves of A. devastans and Phenacoccus sp. is drawn monthly starting from May 05 to August 06 in lab and compared with that of field data following Southwood (1978). The life table is used to determine the most vulnerable stage and key limiting factors, affecting the mortality both in laboratory and in the field. The survivorship curves constructed both from laboratory and field data were found to be apparently resembling with the type III and IV of Slobodkin (1962) with some deviations. The most vulnerable stage in the life table of A. devastans was found to be the initial 1st , 2nd and 3rd instars and for Phenacoccus sp. it was 1st and 2nd and to some extent the 3rd instars, Study of population dynamics recorded high density of all the stages of both the pests during May 05 through Aug 05 and Apr 06 to Jul 06 whereas low density of all the stages for both the species were found during Nov 05 through February 06. The main limiting factors affecting the mortality in laboratory were found to be very low temperature and dry weather condition associated with fungal infestation, which were also true for the field data along with the very high temperature and heavy rain fall and poor food supply. Correlation of population density with minimum mean temperature and mean percent humidity showed that both the factors in certain limits have strong positive and significant correlation with all the stages of cotton leafhopper while very weak, positive and non significant correlation was found with these factors and different stages of Phenacoccus sp. Maximum mean temperature showed positive non significant correlation with the population density of both the pests.
Toxicity of a synthetic pyrethroid lambda cyhalothrin, a biocide abametin and a neem extract biosal were determined by topical and filter paper impregnation method. The LC50values were calculated by log-probit graph paper. The range of mean % mortality and P-values were determined statistically at 95% confidence limit
LC50 of lambda cyhalothrin was found to be 0.05 µg / cm2, 0.125 µg / cm2 and 0.65 µg / cm2.against the 2nd and 3rd , 4th and 5th and adults of A. devastans. The LC50 of lambda cyhalothrin was found to be 0.65 µg /cm2 and 1.2 µg /cm2 against 2nd and 3rd and newly moulted adults of Phenacoccous sp respectively.
LC50 of abamectin was found to be 0.018 µg /cm2, 0.03 µg /cm2and 0.04 µg /cm2, against the 2nd and 3rd, 4th and 5th and adults of A.devastans. The LC50 of abamectin was found to be 0.68 µg /cm2 and 1.17 µg /cm2, against 2nd and 3rd and newly moulted adults of Phenacoccous sp respectively.
LC50 of biosal was found to be 45.0 µg /cm2, 90.0 µg /cm2and 120.0 µg /cm2, against the 2nd and 3rd, 4th and 5th and adults of A. devastans. The LC50 of biosal was found to be 36.0 µg /cm2 and 157.0 µg /cm2, against 2nd and 3rd and newly moulted adults of Phenacoccous sp respectively.
In all the toxicity experiments with A. devastans the potency of pesticides was found to be as abamectin > lambda cyhalothrin > biosal. The potency of pesticides against the 2nd and 3rd instars of Phenacoccus sp was lambda cyhalothrin > abamectin > biosal and against the newly moulted adults was abamectin > lambda cyhalothrin > biosal.
Determination of activity of cholinesterase in adults of A. devastans and newly moulted adults of Phenacoccus sp. Showed inhibition with all three pesticides. Inhibition in activity of cholinesterase was found to be 11.26%, 10.54% and 8.3% in A. devastans treated with lambda cyhalothrin, abamectin and biosal respectively. In case of newly moulted adults of Phenacoccus sp. the inhibition in activity of cholinesterase was found to be 20.32%, 11.59% and 5.89% in insects treated with abamectin, biosal and lambda cyhalothrin respectively.
The total protein contents in adults of A. devastans were decreased to 8.03% and 46.1% after the treatment with lambda cyhalothrin and biosal, whereas it became increased to 125.8% after the treatment with abamectin as compared to the control. The potency of insectides to inhibit the total protein contents in adults of A. devastans was biosal > lambda cyhalothrin. The total protein contents in newly moulted adults of Phenacoccus sp. were decreased to 13.04%, 23.19% and 28.89% after the treatment with biosal, abamectin and lambda cyhalothrin. The potency of pesticides to inhibit the total protein contents in newly moulted adults of Phenacoccus sp. was lambda cyhalothrin.> abamectin > biosal.