Rates of Parasitism in Solitary Native Bees

In recent years, honey bee (Apis spp.) populations have been droping dramatically. They have been experiencing many stresses which have resulted in a wide spread phenomenon called Colony Collapse Disorder. Because honey bees are a major pollinator of our crops, this disorder is threatening to tear apart our agricultural systems. Without these systems, we won’t be finding any more fruits or vegetables in our grocery stores our our dinner tables. Consequently, there is now an increased interest in studying alternative pollinators such as flower flies and solitary bees. Although ecologist have been studying some of the groups for many years, not enough is known about them in order to use them in a commercial setting.

Osmia sp. on a nesting block
Osmia sp. on a nesting block

One group of insects that are of particular interest are mason bees (Osmia spp.). These bees are commonly found nesting in preexisting holes in logs and hollow reeds. Females will build up a cache of pollen and nectar in these per-existing holes, deposit an egg and then seal the nest (Michener 2000). However, often times parasites, such as parasitoids and kleptoparasites, will invade the nest and lay their own eggs (Wcislo and Cane 1996). The parasite egg will usually hatch before the host and eat the host egg. Once the host has been eliminated, the parasite larvae will feed on the pollen stored in the nest. Unfortunately, parasitism rates can be very high, with 15% of nests parasitized (Steffan-Dewenter and Schiele 1993). Although previous studies have looked at what factors affect rates of parasitism (Goodell 2003, Rosenheim 1990), conflicting results have emerged. Studies have reported both positive and negative density dependent rates of parasitism in solitary bees (Rosenheim 1990, Wcislo 1984).

A nesting block with a paper straw sticking out.
A nesting block with a paper straw sticking out.

Adam Groulx, the graduate student I worked with this summer, set out to untangle some of these results. We setup up two experiments to test two different factors that may affect rates of parasitism in mason bees.

Nest Density
The first experiment was aimed at seeing how the density of nests affects rate of parasitism. We setup up three sites with each site containing a low density and a high density replicate. The high density replicates consisted of two large nesting blocks with 32 nests each, attached to a single tree. The low density replicates consisted of 4 small nesting blocks with 16 nests each, attached to four different nests seperated by about 10m.

Cuckoo Wasp (a kleptoparasitic wasp)
Cuckoo Wasp (a kleptoparasitic wasp)

Resource Availability
The second experiment was aimed as seeing how the availability of floral resources affects rates of parasitism. We setup four sites with a total of 80 nests at each site. Each site was selected so that the nesting blocks were located along the margin between a field and a meadow. We applied two treatments: low and high resource availability. The low resource availability treatment involved covering all of the flowers in the field within a 25m radius with row covers. The material acted to stop pollinators from access the flowers. The high resource availability treatment did not have any covers. In order to reduce site affects, we decided to move the row covers between sites at least once.

A Resource Availability Site Covered with Row Covers
A Resource Availability Site Covered with Row Covers

Observations
Our work simply involved monitoring the nesting blocks for parasites. When we noticed that there was significant bee activity, we stood by the blocks and recorded bee and parasite behavior for about 45 minutes. We also dissected all the nests and recorded whether a nest was parasitized or not. We also took pollen samples from the first and last cells of each nest to compare pollen use with wildflower phenology.

A parasitized Osmia nest cell. The large egg is the host and the small egg is the parasite (Sapygid wasp egg).
A parasitized Osmia nest cell. The large egg is the host and the small egg is the parasite (Sapygid wasp egg).

Results
Unfortunately field work is a finicky business. We were unable to get enough bees in the resource availability experiment to obtain adequate results. Sometimes the bees just don’t want to nest where you want them to. On the other hand, we did get plenty of bees in the density experiment blocks. This provided us with a good deal of data that is currently being worked on.

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