1985] 
Brockmann — Digger wasps 
323 
(Chrysis) (they certainly enter Sphex nests frequently) (see also 
Evans, 1966a). The most common marauders, however, are conspe- 
cifics which regularly dig up the temporarily filled nests of neigh- 
bors. They sometimes discard the contents of the brood chamber 
when they encounter it or they may take over the nest and use the 
provisions for their own offspring (Brockmann and Dawkins, 1979). 
Occasionally a wasp will dig her burrow into the completed nest of 
another female, and when she does she pulls out the contents (be 
they provisions or pupa), flies up and discards them away from the 
nest. In a study of nest-site selection in Sphex ichneumoneus 
(Brockmann, 1979), I observed wasps biting repeatedly at different 
spots on the substrate. They would finally begin to dig when they 
encountered a spot that gave way to their chewing, i.e. a spot that 
was slightly less compacted. Therefore, it seems likely that one 
important selective pressure acting on the final-closure behavior is 
reducing the chance that another female will dig in the exact same 
location. This may be particularly crucial in species that regularly 
nest in aggregations. 
Once the survival value of packing is known, then it may be 
possible to assess the value of different packing methods to different 
species of wasps. Far more information is needed on the nature of 
the nesting substrate, the details of the packing behavior, the conse- 
quences of the various packing methods on different soils and the 
behavior of parasites, predators and conspecifics when digging 
under different conditions of compaction. 
Acknowledgments 
I would like to thank Benjamin Beck for reading an earlier ver- 
sion of this manuscript and Howard Evans for identifying speci- 
mens. Cheryl Hughes and Paloma Ibarra prepared the drawings 
from slides and films. Giulio Genoni translated papers written in 
German. I also wish to thank Orin Gelderloos and the University of 
Michigan-Dearborn, Richard and Betty Brinckerhoff of Exeter, 
New Hampshire, Harry and Olga Olson of Rensselaerville, New 
York, Gary Wagenbach and Carleton College of Northfield, Minne- 
sota, the Southwestern Research Station in Portal, Arizona and the 
Archbold Biological Station in Lake Placid, Florida for providing 
the opportunity to study digger wasps. Equipment and advice were 
provided by Jack P. Hailman and funding was provided by the 
