A kleptoparasitic beetle larva (Hymenorus dorsalis) exploits vertical division of labor in nests of the Florida harvester ant, Pogonomyrmex badius

Pogonomyrmex badius workers divide transport tasks across subterranean nest space. After foragers deposit seeds and insects in shallow entrance chambers, younger workers transport these items downward to storage and brood chambers. We found that larvae of the beetle Hymenorus dorsalis exploit this temporary caching behavior, and intercept nearly all incoming insect prey before it is delivered to ant larvae. In field colonies, ~ 84% of beetle larvae consumed insects collected by host foragers. When beetles were absent, host seed consumption remained the same, but 31% more ant larvae fed on insects. In the lab, H. dorsalis readily fed on host brood, dead insects, seed-pulp, and seed coats. However, they could not open the smallest whole seeds collected by host ants. Pogonomyrmex badius nests in our study ranged from 40 to 270 cm deep, with 124 to 9,596 workers, and 0–31 beetles. Across populations, ~ 18 to 63% of nests were parasitized (n = 97 excavations, 219 beetles), and ~ 95% of H. dorsalis recovered were larvae. Beetle abundance was not correlated with worker number, but immature colonies were more likely to host beetles at one site. Colonies with < 2000 workers also stored relatively more brood in the top 20 cm top of their nests, where beetle larvae co-occurred (avg. beetle depth = 7.60 cm). Smaller colonies may be more susceptible to brood predation by H. dorsalis and incur a greater per capita cost when parasitized. Alternatively, parasitized colonies may be smaller because of the high nutritional costs imposed by H. dorsalis larvae.