Saturday, September 04, 2010

Farm Bulletin: Consider the Grass-Carrying Wasp

I was first captured by contributor Anthony Boutard's writing when I read his description of the barn owls on his farm: "Barn owl school started this week. You know the semester has started when you enter the barn and the ground is littered with dead mice and voles. We entered the barn early one overcast morning this week and disrupted class. The adults skedaddled into the box, leaving three of the immature birds fluttering around the barn in a state of panic. The fourth was calm and just watched us." I've been a fan ever since.

Ecce Isodontia

Against the dark foliage of the Douglas firs, long yellow wisps of dry grass float by against the wind. The scene could, at a glance, be one of Miyazaki's haunting animations. The creature animating each piece of grass is called the "Grass-carrying Wasp." These thread-waisted wasps belong to the genus Isodontia, a taxon distinguished by the grass-carrying habit. They are part of the larger Sphecid family which includes the mud daubers and digger wasps. The Sphecids are all solitary predators.

A grass-carrying wasp caught in the activity for which it was named.

The grass-carriers use existing cavities to build their nests, and those cavities may be part of your house. On our house, they nest in the space between the corner-boards and the shiplap siding. For the fields, we have built boxes with hundreds of bamboo segments to provide nesting habitat for cavity-nesting bees and wasps. We favor bamboo because each pole will have a range of cavity sizes, so we attract everything from the tiniest mason bee to relatively large wasps. They coexist happily. Bamboo is also more sanitary over time than the commonly used paper tubes, and it never gets soggy during the winter monsoons.  

The female Isodontia constructs the nest by lining the cavity with grass, and then provisions it with young grasshoppers, katydids and crickets. The wasp injects venom into the prey and cradles the insect between its legs, grasps  the antennae with its mandibles, then flies to the nest. She drags the insect into the hole, executes a U-turn, and comes out head first. A neat feat given the space constraints. Her wings are pretty ragged by the end of the season.

The poor prey must be thinking, "I have a bad feeling about this…"

She lays her eggs among the prey, then plugs the nest with more grass. The grass is carried the same way as the prey, one end in the mandibles and the other end cradled in the legs. As the grass blade is several times longer than the wasp, it streams behind her.

The eggs hatch and the larvae feed on the stored insects and then pupate over the winter, emerging next summer as adults. The last eggs laid grow into males and exit first, followed a week or so later by the females. Mason bees follow the same pattern. The males are smaller and built for the nuptial rites alone, while the larger female is left with all of the post-nuptial chores.

Hers is not easy work. As the wasp approaches the cavity, a yellow-jacket swoops down, hoping she will drop her prey. It hazes her whether she bears a cricket or a blade of grass, clearly unable to distinguish between her loads. At the nesting box, other smaller wasps scurry around. They may be "cuckoo types" that lay an egg on her prey in her absence, or parasitic wasps that feed upon the developing larvae themselves.

We seem to have at least two species nesting here. Some of the the nests are plugged neatly, looking just like the end of a factory-made cigarette. Other nests are plugged with the grass sticking out, Cheech Marin style. The wasps we see at the moment, with their reddish legs and wings, are probably the species Isodontia elegans. The adults are survive on nectar.

Like all other wasps, our grass carriers can inflict a painful sting. We live among a large number of wasps and bees on the farm, and the only one to sting us unprovoked is the domesticated honey bee. When nectar is short in late summer, honey bees become very aggressive. Over the course of the summer, we will be stung many times by the "gentle" honey bee. Other wasps and bees steer clear of us, and only attack when we actually disturb their nest.

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