Effects of forest spraying with two application rates of cypermethrin on food supply and on breeding success of the blue tit (Parus caeruleus)

Abstract

AbstractA field study was carried out in the springs of 1988 and 1989 in deciduous oak forests of west‐central Spain to assess the effects of two aerial sprayings with two different application rates of Cypermethrin, a synthetic pyrethroid, on several reproductive parameters (nest abandonment, nest success, clutch size, hatching success, nestling mortality, daily survival rate, and nestling weight) of the blue tit (Parus caeruleus). These variables were analyzed according to reduction of arthropod density in canopy trees after treatments and results were compared with a control plot (plot A) without application. The plot sprayed in 1988, with a dust formulation of cypermethrin at 75 g active ingredient (a.i.) per hectare (plot B), showed a nearly 100% mortality of lepidopteran larvae, the most important food resource for breeding blue tits, as well as for other arthropods. The drop in food supply in this plot coincided with the hatching or early nestling stage of the blue tit population. There were statistically significant differences between the control plot and plot B in nestling mortality, daily survival rate, nest success, and nestling weight. In the treated plot, nestling mortality was 81%, 65% of nests were unsuccessful, and mean nestling weight was 1 g less than that in the control plot. In the control plot, nestling mortality was 6% and all nests were successful. A second plot was sprayed in 1989 with an ultra‐low‐volume formulation of cypermethrin at 3.75 g a.i. per hectare, reducing 90% of the caterpillars but not affecting the availability of other arthropods. Blue tit nestlings were older than one week at the moment of the spraying. In this plot, nestlings from late nests showed a higher mortality and a lower daily survival rate than those in the control plot, the only statistically significant differences in breeding parameters between the plots. We suggest that the negative effects of cypermethrin spraying on breeding success of the blue tit in both treated plots are due to indirect effects (shortage in prey availability) rather than to direct effects (toxicity).