Seasonal biology and reproductive strategy of predatory mirid Philophorus gallicus (Hemiptera: Miridae) in the pear crop in the Mediterranean area

Abstract

The pear tree (Pyrus communis L.) is one of the main crops in northeast Murcia. The pear psylla, Cacopsylla pyri L. (Hemiptera: Psyllidae), is a major pest of this crop. Resistance to pesticides evolved by C. pyri as well as the restriction of the use of several potentially harmful chemicals have led to an increasing interest in Biological Control in the pear crop. Preliminary surveys on entomological fauna associated with pear orchards in the municipality of Jumilla (Murcia) showed ants, spiders, and the mirid Pilophorus gallicus (Hemiptera: Miridae) are the most abundant natural enemies, the latter being of special interest for the control of the pear psylla. In the present work the seasonal biology of P. gallicus has been studied for the first time. The main objectives were to analize the effect of temperature on its biological parameters, to determine the mechanisms adopted by the species to overcome unfavorable periods, and the factors that regulate those mechanisms. The results show that P. gallicus performs poorly at low temperatures and has a moderate tolerance to high temperatures (30ºC); its lower thermal threshold was estimated at 12.01ºC (N1 to Adult) and the optimum temperature at 25ºC. This mirid has a multivoltine cycle and enters an embryonic diapause to survive the cold winter period, involving the absence of active forms (nymphs, adults) in winter. We did not find evidence of summer diapause as a mechanism of protection against the extreme temperatures occurring in the area of study. Photoperiods below the critical level - estimated at 13 hours and 53 minutes - induced a diapausing laying in females in early autumn. The direction of change in daylength (increasing or decreasing) had no effect on the incidence of diapause in this species. The ability to reverse the induction of diapause was demonstrated in females, switching from a diapausing laying to a non-diapausing one after being transferred to long photoperiods. On the contrary, diapausing eggs required a period of low temperatures (e.g. 90 days at 6ºC) to resume their development and hatch. The study of females individually revealed the existence of three reproductive strategies: diapause inducing, non-diapause inducing, and mixed. The variation in the proportion of these strategies along the growing season was modulated by environmental conditions. In particular we observed (a) a distribution of the diapausing laying along the growing season and not exclusively at the arrival of autumn, suggesting a bet-hedging strategy that increases the probability of persistence of populations in the eventuality of unpredictable adverse conditions; (b) the presence of non-diapausing laying in autumn, indicating the possibility of extending its period of activity in case warm autumns take place. This variability allows the species to adapt to unexpected changes in the Mediterranean climate. These results are useful for the understanding of the ecology of P. gallicus and its management in biological pest control programs. Finally, this work proposes some guidelines for the use of diapause as an important tool for the biological control industry.