1. Fisher's sex allocation theory predicts equal investment in sons and daughters. However, in a central European population of the beewolf Philanthus triangulum F. (Hymenoptera: Sphecidae), the sex ratio of parental investment is heavily biased towards males.

2. Beewolf females hunt for honeybees and carry them to the nest in flight. In order to fly with the additional load, females must have a certain minimum size that can be attained only with at least three bees as larval food. In contrast, sons require only one bee. Storage of bees might be constrained by an increasing risk of parasitism and fungus infestation. Therefore the female needs to gather the necessary bees to produce a daughter within a short period of time. Under bad weather conditions, females might too often not be able to accomplish this and might be compelled to produce an excess of less costly sons. This hypothesis was tested by investigating the temperature dependence of the hunting and provisioning behaviour of beewolf females.

3. Under semi-field and field conditions, the duration of the daily activity period and the number of bees a female brought to the nest correlated positively with two measures of ambient temperature, maximum air temperature, and temperature sum during the activity period. The duration of hunting trips but not the duration of pauses between hunting trips correlated negatively with temperature. An estimate of the investment ratio was not significantly correlated with ambient temperature. The proportion of the variance in investment ratio that was explained by ambient temperature was less than 10%.

4. In the laboratory, temperatures affected hunting success but females did not produce more daughters at higher temperatures.

5. In a subtropical climate in southern France, the population investment ratio was biased towards males to the same extent as in central Europe.

6. Thus, temperature affects several aspects of hunting behaviour, but low temperatures do not lead to an overproduction of male offspring and thus are not a constraint that could explain the observed male-biased investment ratio.