1981] 
Mac Kay — Nest Phenologies of Pogonomvrmex 
65 
The factors influencing the determination of sex ratios in the 
Hymenoptera are currently of much interest (Herbers 1979). 
Experimental manipulation of food input and excavation of 
colonies of known age may provide information on the factors 
which determine the sex ratio in a harvester ant nest. 
Summary 
This investigation compares the phenologies of foraging and 
reproduction in three species of Pogonomvrmex harvester ants 
along an altitudinal transect in southern California, USA. Periodic 
excavations of 126 nests of the three species, P. montanus, P. 
subnitidus, and P. rugosus, reveal that seasonal changes occur 
within the nests. The three species have similarities in the physical 
environment of the nest although P. montanus, the highest altitude 
species, has lower nest temperatures. Both P. montanus and P. 
subnitidus are snowbound during part of the season. Egg laying 
begins in late April or May; development to adult requires five to six 
weeks. The brood reach maximum numbers in late July to late 
August. Most of the larvae and pupae formed in the first part of the 
season become reproductives. Mating flights begin in late July and 
are completed by the first part of September. The highest reproduc- 
tive populations occur in mid August. 
Much of the nest population is in the upper levels of the nest 
during the summer and in the lower levels during the winter. During 
the summer, temperature and humidity gradients exist in the nests 
with deeper levels being cooler and moister. These gradients may 
account for the placement of the brood in the lower levels. There is 
no evidence of segregation of the larvae and pupae within the nest, 
which has been reported by other investigators. 
All three species demonstrate similar seasonal changes in foraging 
patterns, with much activity in the mid summer and no activity 
during the winter. Only about 20% of the nest population is 
involved in foraging. Individual foragers make up to 9 or more 
foraging trips per day. The ants utilize a wide variety of food items, 
although most materials are either seeds or plant parts. There is a 
considerable seasonal change in the food composition of P. 
montanus and P. subnitidus. 
The highest altitude species, P. montanus, allocates more energy 
to reproduction than do the mid or low altitude species. The nests 
