Overview: Six of our seven planned experiments were successfully conducted. Results have been disseminated through 8 publications and at conferences, with a similar number of publications expected in future.
Main results so far:
1. Parsons et al. (2017) reported a new set of 24 genetic markers that we developed for the sweat bee L. malachurum so that we could determine whether the queen, or one of her workers, has produced each of the offspring.
2. Pennell et al. (2018) is an important review where we hoped to stimulate research focussed on caste antagonism. The review characterised antagonism, discussed how it might be resolved during evolution, suggested approaches to study it and reviewed the empirical evidence.
3. Couchoux & Field (2019) relates to objective (4) above. In social animals, parents might manipulate characteristics of their offspring in their own interests. In paper wasps, the first offspring produced are smaller than the queen and become workers. We investigated whether paper wasp queens benefit by producing small daughter workers. We used cross-fostering to create size mismatches. Before cross-fostering, there was a positive correlation between queen and worker body sizes: larger queens had larger workers. After cross-fostering, the correlation was absent. We then recorded foraging, reproductive effort and aggression. Queens were less likely to attack relatively larger workers, but larger workers did not forage less, did not invest more in ovarian development, and were not more aggressive themselves. But because small workers were no less successful foragers, producing a larger number of smaller workers may optimize colony work effort. Thus, queens may limit worker size strategically to optimize foraging, with limited evidence of size-based queen-worker conflict.
4. Parsons et al. (2017) analyse choice of social partner in paper wasps. Results suggest that kin groups form via a rule-of-thumb where wasps simply select nearby partners, rather than via kin recognition.
5. Pennell et al. (2021) showed that an unexpectedly large proportion of L. malachurum nests were orphaned: the original queen had died or been usurped. Colonies with original queens invested more in female offspring than did orphaned colonies (Pennell & Field 2021). Our data also suggested queen coercion – nests with larger queens produced sex ratios closer to the queen’s optimum.
6. Boulton & Field (2022) showed that social populations of the polymorphic H. rubicundus have denser antennal sensilla involved in olfactory perception than solitary populations. Sensilla density appears to be developmentally plastic: bees transplanted from north to south produced offspring (which developed in the south) with more olfactory sensilla.
7. In small colony social insects, individuals may switch from non-reproductive to reproductive roles (e.g. after orphaning). To investigate plasticity, Price & Field (2022) promoted L. malachurum workers experimentally to reproductive roles. Remarkably, we found worker-queens produced the same number of offspring as original foundress-queens, and worker-queens monopolised reproduction to the same extent. However, non-reproductives foraged less with worker-queens, producing less offspring biomass.
8. Gruber & Field (2022) tested an important assumption underlying a model concerning how eusociality evolves (Seger 1983 Nature). The model requires first brood males to survive and mate with females of the second brood. We obtained high estimates of male survival in H. rubicundus, supporting the model.
