Both nest site availability and density impact rates of colonization of solitary cavity-nesting hymenoptera insects

Background/Question/Methods

Cavity-nesting insects utilize preexisting cavities found above ground.  Nest site selection can be an important factor in the fitness of these types of insects because it influences conditions experienced while larvae are forming, determines the amount of resources available for the female when creating the nest, and thus determines the amount of resources available for the offspring after emergence.  Competition for nesting resources was hypothesized to be a limiting factor to the colonization rates of insects in a given area. Therefore, nest colonization rates will increase with the number of cavities available.  However, density of nests is also important. Insects may prefer to spread out their various nests to reduce the risk that one threat could harm all nests, therefore increasing the survival of all their offspring. To test these hypotheses, we manipulated both the quantity and density of nest sites within artificial nest boxes, which in previous years yielded colonization mostly from bees and wasps.  In March 2014, we set-up three boxes, one of each type of treatment, at eight sites across Morristown National Historical Park in New Jersey. The treatments included: nest boxes with High nest density and High nest quantity (HH), with 24 total nest holes; nest boxes with High nest density and Low nest quantity (HL), with 12 nest holes condensed to the center of the box to keep the same density as HH boxes; and nest boxes with Low nest density and Low nest quantity (LL), with 12 nest holes spread out within the box. Boxes were censused in September 2014. 

Results/Conclusions

Colonization rate by cavity-nesting insects varied significantly among the nest density and quantity treatments.  The high density, high number (HH) boxes showed the greatest colonization rate whereas the high density, low number (HL) boxes showed the lowest colonization rate.  While both of these treatments had a similar density of nests within the box, they did differ in the amount of nests they provided the insects. We found support for our hypothesis that nest colonization rates would increase with the number of cavities available. We also found support that density impacts nest site selection as the low density, low number (LL) boxes had a higher proportion of colonized nests than the high density, low number (HL) boxes. Our results indicate that the configuration of nest holes in the boxes impacts colonization with Hymenoptera having a preference for nests that are spaced out.