Pollinators: The Mason Wasp and the Caterpillar

Mason wasp, Euodynerus hidalgo and caterpillar

Recently I found a wasp on a sunflower wrestling a caterpillar almost as big as herself into submission. The caterpillar had made considerable damage to the flower head as shown on the pictures. Finally the hunter managed to grip its prey by the throat, if that is where caterpillars have a throat. Carefully tucking her prey under her body, she flew away, presumably to her nest to feed her brood with it.

Caterpillar held firmly behind the head

I identified the hunter as one of the mason wasps, so called because they use clay to build nest partitions. It was a rather small wasp, mostly black with yellow markings on its body. The wings remained folded lengthwise. All members of the family Vespidae do this when not in flight. Now, I finally could decipher the mystery presented to me a year earlier in the same garden where I was doing my observations.

On that occasion, I noticed a mason wasp furiously searching a flower head, almost a carbon copy of the one in front of my eyes. It had the same kind of damage, but, at that point, I didn't know what or who had caused it. The mason wasp rummaged the entire center of the blossom, even burying half of her body in it with no apparent success. After observing these antics for a while I moved on without finding out what she was looking for. Now, I had the answer.

Another mason wasp found a year earlier

A mother mason wasp, like the one I was observing, builds her nest inside any available hole of the right size, slightly wider than her own body. It could be a hollow stem or some other cavity previously dug up by a beetle or other insect. Unlike their distant cousins, hornets and yellowjackets, they don't form large colonies. Each mother wasp minds her own business, builds a nest where it raises maybe a dozen babies and is not inclined to defend this small investment as ferociously as hornets tend to do. Thus, their presence can be easily tolerated in the garden.


Mason wasp tending her nest © 2010 Charles Schurch Lewallen

Interestingly the so called mason bees have similar nesting habits. These bees are excellent pollinators of fruit trees and other plants, so gardeners prize their services. Some gardeners provide housing for them by hanging a bundle of hollow twigs or drilling a number of holes in a piece of wood. These bee houses and bee hotels have been growing in popularity in recent years. You can purchase them from several venues usually accompanied by an instructions booklet.

Bee houses for mason bees

Mason wasps use caterpillars to feed their babies. They keep their prey alive, so the food supply remains fresh as long as needed. They do so by paralyzing but not killing their victims with their stingers, a delicate operation. This is why my wasp was so busy getting the right hold of the caterpillar, firm but not too harsh.

Most adult wasps are often seen on flowers because they feed mostly on nectar. This particular species has another reason for visiting flowers. This is where it finds its prey. Some caterpillars, especially some inchworms and owlet moths, show a preference for sunflower heads. They hide in the very heart of the flower and proceed to eat the growing seeds causing a bit of destruction. It is nice to see that this hunting wasp prey on them preventing further damage.

If you love pollinators you may be raising mason bees. Perhaps you find some of the holes in your bee houses occupied by mason wasps. You may be tempted to destroy the "invaders." Please, don't! You probably didn't know that they help the pollinators by keeping down the populations of plant eating insects. We need biological controls as much as we need pollinators to maintain a healthy garden.

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