Combined Effects of Predator, Xylocoris Flavipes (Reuter) and Bacterium, Spinosad for Control of Stored Product Insect Pests

Abstract

The flat grain beetle, Cryptolestes pusillus (Schon) (Coleoptera: Cucujidae) and the lesser grain borer, Rhyzopertha dominica (Fabricus) (Coleoptera: Bostrichidae) are the serious insect pest of stored commodities. C. pusillus is an internal and R. dominica is an internal feeder of whole wheat seed, flour, etc. The hemipteran predator, Xylocoris flavipes (Reuter) predates the eggs, larvae and pupae both of the pests in storage condition and checks their population in considerable level. A newly reduced risk broad-spectrum bacterium, Spinosad is effectively control the population of both C. pusillus and R. dominica. The effects of different life stages of both hosts on the biology of X. flavipes under laboratory condition were assessed. The influence of Spinosad on both hosts and predator and also the combined effects of X. flavipes and Spinosad on the population of C. pusillus and R. dominica were subsequently investigated. The nymphs 1st up to 5th instar and adults of X. flavipes were found efficient to survive on eggs, larvae 1st up to 4th instar and pupae of the both insect pest C. pusillus and R. dominica. The mean duration of developmental period through five nymphal instars on eggs, larvae 1st up to 4th instars and pupae of were 15±2.00, 20±0.00, 22±0.58, 18±1.00, 14±1.15 and 12±1.15 days in C. pusillus and 18±1.00, 20±0.58, 16±2.00, 14±1.15, 12±1.15 and 13±0.58 days in R. dominica respectively. The adult female X. flavipes survived longer than the male. Average consumption rates of each nymph 1st up to 5th instar and adult stage of X. flavipes was found highest on eggs, 1st and 2nd instar larvae but lowest on 4th instar larvae and pupae of the both insect pests. The egg of R. dominica was more preferable than that of C. pusillus. Moreover, 1st, 2nd and 3rd instar larvae of the both host insect were more preferable to the predator than the other stages. The female predator always consumed more individuals of both the pests than the male. Average survivability rates of nymphs 1st up to 5th instar and adults were maximum on 1st and 2nd instar larvae and minimum on 4th instar larvae and pupae. The size of the female predator was found larger than the male at all stages studied. Based on ratio 1:1, sex ratio was found the best (male and female almost equal in number) on 1st and 2nd instar larvae comparatively than that of other stages. Developmental period, adult longevity, consumption rates, survivability rates, size and sex ratio of X. flavipes always significant (P<0.001) in different life stages of both the insect pests. The average percentage of egg hatchability (±SE) was the highest as 25.00±1.15 at 0.491μl/cm² and the lowest 5.00±1.02 at 7.863μl/cm² when C. pusillus was applied different concentrations of Spinosad and the results found highly significant (P<0.001). On the other hand, average (±SE) mortality of larvae, pupae and adults were found the highest as 14.00±2, 8.33±0.88 and 15.33±1.22 respectively at 7.883μl/cm² after 72h and the lowest 3.33±0.88, 1.67±0.33 and 5.00± 0.58 at 0.491μl/cm² Spinosad concentrations after 24h of exposure. The larvae (72h LC50 was 0.1755007μl/cm²) and adults (72h LC50 was 0.839572μl/cm²) were found more susceptible than pupae (72h LC50 was 35.94058μl/cm²). Effects of separate concentrations of Spinosad on different stages of R. dominica were investigated and found that average percentage of egg hatchability (±SE) was the highest as 15.00±1.14 at 0.491μl/cm² and the lowest 0.33±1.03 at 7.863μl/cm². The effect of different concentrations on egg hatchability was highly significant (P<0.001). Average (±SE) mortality of larvae, pupae and adults were 13.33±0.88, 8.33±1.45 and 17.33±1.20 at 7.863μl/cm² after 72h and the lowest 4.67±0.33, 1.00±0.58 and 6.67±0.88 at 0.491μl/cm² after 24h of exposure periods. Larvae (72h LC50 was 0.5433412μl/cm²) and adults (72h LC50 was 0.466328μl/cm²) were found more susceptible than pupae (72h LC50 was 22.0538μl/cm²). Different concentrations of Spinosad on different life stages of X. flavipes were investigated and recorded average percentage of egg hatchability (±SE) was highest 35.00±1.73 in control (untreated) and lowest 25.00±2.12 at 7.863μl/cm² concentrations. At 1.966, 0.983 and 0.491μl/cm² concentrations, the egg hatchability was almost similar like in control medium. Effect of different concentrations on egg hatchability was not significant (P<0.001) comparatively than that of control medium. Average (±SE) mortality of nymphs and adults was found highest as 6.67±1.76 and 5.00±0.45 at 7.863μl/cm² after 72h but lowest 1.00±0.58 and 1.67±0.67 at 0.491μl/cm² Spinosad concentrations after 24h of exposure. A nymph (72h LC50 was 73.82966μl/cm²) was found more susceptible comparatively than that of the adult (72h LC50 was 331.5098μl/cm²). Moreover, at 0.491, 0.983 and 1.966μl/cm² concentrations after 24h to 72h of exposure, survivability of adults were found 94 to 84% respectively. The adult population of C. pusillus and R. dominica was significantly (P<0.001) reduced by X. flavipes and different concentrations of Spinosad alone and their combinations after 3, 6, 9 and 12 months of storage than those of control medium. After 3 months, from Spinosad concentrations reduced the population from 57.69 to 66.12% compared to control. In combination this range was from 42.86 to 51.56%. After 6 months, this reduction percentage rate was increased in both concentrations and combinations. After 9 months, this reduction percentage rate was increased continuously and after 12 months the effect of both concentration and combination were found more effective compare to early storage period. In case of the predator alone, the population of the both insect pests was higher than those of concentrations and combinations. The population of both insect pests was found more susceptible to the combinations comparatively than that of concentrations. The population of R. dominica was found more susceptible to different concentrations and combinations than that of C. pusillus at all storage periods.