1980] 
Hook & Matthews — Biology of Oxebelus 
31 
include Oxybelus subulatus (Peckham, 1977), Stizus (Megastizus) 
texanus Cresson (Gillaspy, unpubl.) and Tachytes distinctus Smith 
(Lin, 1963; Lin and Michener, 1972). Stizus texanus nested near 
Matomoros, Mexico and males defended nest territories, copulating 
with provisioning females. Tachytes distinctus defended nest territories 
but did not attempt to mate with provisioning females. Research on 
Trypargilum suggests that male guarding helps reduce cleptoparasitism 
and supersedure by other wasps and bees (Krombein, 1980). Males 
may also assist in nesting activities and females of at least one species 
will not provision unless a guard is present (Coville and Coville, 
1980). 
Peckham (1977) provides quantitative data on the effect of male O. 
subulatus in reducing miltogrammine cleptoparasitism. Unguarded 
nests had 33% (examined 85 cells) cleptoparasitism while guarded 
nests had only 18%(n=84). “Thus the presence ofthe male appears to 
have significantly reduced miltogrammine cleptoparasitism.” The 
low rate of cleptoparasitism in O. sericeus at the Boiler site can be 
taken as indirect evidence that guards in this species reduce 
cleptoparasitism. 
Earlier stages in the evolution of nest guarding as found in 
Oxybelus sericeus may be found in other Oxybelus. Several Oxybelus 
( bipunctatus, subcornutus Cockerell, emarginatus, Peckham et al., 
1973 and pers. obs.; decorosus, uniglumis, Peckham and Hook, in 
press) fit this pattern whereby males patrol the nest area, indiscrimi- 
nantly chasing nearby moving insects. Peckham et al. (1973) found 
only 6% fly cleptoparasitism in an aggregation of O. bipunctatus, and 
attributed this low figure partially to male harassment of miltogram¬ 
mine flies. 
In summary, we suggest the following scenerio for the evolution of 
nest guarding in sphecid wasps. Initially males cue in on females who 
are actively provisioning nests and who remain continuously receptive 
to copulation attempts. Under certain ecological conditions the 
benefits resulting from defense of an area (territory) within a nest 
aggregation will exceed the costs to males. Sperm precedence may be 
an important factor in this context, if it occurs, for the last male to 
inseminate a female prior to oviposition will have increased fitness 
relative to other males. An incidental but important effect of the 
male’s presence and activity is to reduce levels of cleptoparasitism 
(and perhaps supercedure) thereby improving a female’s nesting 
success relative to females inseminated by non-territorial males. It 
