Pith diameter preferences and their impact on fitness parameters of the digger wasp Pemphredon lethifer: a potential biocontrol agent against aphids

The digger wasp Pemphredon lethifer nests in pithy twigs. As the species preys exclusively on aphids, it could be a useful biocontrol agent. However, little is known about the wasps’ nesting preferences and their impact on relevant fitness parameters. One important factor in this regard is the optimal pith diameter. We offered twigs of four different pith diameter ranges between 2.5 and 10.5 mm in field trials and under controlled lab conditions to analyse choice and fitness parameters of the nesting females and their offspring. Under field conditions, the number of nests increased with pith diameter, with significantly more nests built in the largest diameter category. In the laboratory, most nests were built in the three largest categories (> 4.5 to 6.5; > 6.5 to 8.5; > 8.5 to 10.5 mm) compared to the smallest (> 2.5 to 4.5 mm). In the field, pith diameter category influenced the sex ratio of offspring, resulting in more females in nests of category > 6.5 to 8.5 mm. Other parameters, such as nest length or time period of nesting, influenced mortality, parasitism and number of offspring. In conclusion, to promote nesting of P. lethifer , nest boxes should be equipped with pith diameters of 6.5 to 8.5 mm and tube length should be considered to reduce mortality. Efficacy of P. lethifer is expected to be the highest early in nesting season.