Analysis regression of the effects of Cotesia kazak Telenga (Hym.: Braconidae) on population densities of Helicoverpa armigera Hubner (Lep.: Noctuidae) and crop injury indices
2010
Ghadiri Rad, S. | Ebrahimi, E.
Helicoverpa armigera Hubner, as the main insect pest of tomato is a polyphagous multivoltine insect with a high fecundity and mobility. Larvae feed on fruits and cause a significant yield reduction. Cotesia kazak Telenga is an active parasitoid in all unsprayed fields of Golestan province with significant densities of pest larvae. Two unsprayed tomato fields, each of them 5000 m2 in area and located in vicinity of Kordkuy (30 Km West of Gorgan) were sampled weekly to determine the efficiency of C. kazak. During late May to mid August 2006, a visual count method was conducted to determine H. armigera and C. kazak population levels. Crop injury indices and population density of pest including numbers of damaged and undamaged fruits per plant, percentage of infected fruits and numbers of pest larvae per plant were set as dependent variable (y). Population density of parasitoid consisting numbers of parasited larvae, percentage of larval parasitism and numbers of parasitoid pupae were considered as independent variable (x). Analysis regression indicates that there are positive correlations between the numbers of parasited larvae (y= 23.74x+7.19, t=3.25, p=0.01, ȕ=0.73) and the numbers of parasitoid pupae (y= 23.32x+7.85, t= 2.82, p=0.05, ȕ= 0.68) with the numbers of undamaged fruits in each plant. The comparison of standard estimated coefficients (ȕ) shows that numbers of parasited larvae has a much more effect than numbers of parasitoid pupae. In addition to effects of extensive spraying of crop on parasitoid populations, present concern is releasing Habrobracon hebetor in tomato fields which may induce an intraspecific competition and decreasing the natural efficiency of C. kazak. Identification of parasitoid species was done by Prof. K. van Achterberg (Netherlands).
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