What Made These Holes?

Leaf-cutter Bee damage on rose_DBG_LAH_7311Do your lilacs look like someone took a giant hole punch to their leaves? How about your rosebushes (left), green ash, or Virginia creeper? The first time I saw the precise circle of leaf missing from a leaf, I thought someone was playing a trick on me. It didn’t look at all like insect damage.

Grasshoppers and leaf-eating caterpillars tend to graze on the edges of the leaves, but the damage is irregular. Some leaves are barely touched while others are mere nubs, and the edges of the holes are wavy and random. These neatly cut circles were totally different.

It wasn’t until I became a Master Gardener that I learned what was munching in such a well-defined arc—leafcutter bees!

Leafcutter_bee_by_Bernhard_plank-001Since bees don’t eat plants, I hadn’t even considered that they would be responsible for the damage I was seeing. It turns out the leafcutter bees weren’t eating my lilacs. As solitary bees, these bees don’t gather together in a hive. Instead, each female burrows into a soft and pithy (and often rotting) plant stem (roses are one of their favorites) and hollows out a small chamber or tunnel in which to lay her eggs.

Inside the tunnel, the bee uses the leaves to make tiny compartments, much like a row of incubators. Each one is filled with nectar and pollen that will feed the developing larva inside. An egg is laid, the leaf is sealed, and the young bees develop in safety.

Leafcutter bee damage  @DBG 2008jun26 LAH 007In spite of the damage they do, leafcutter bees are considered good guys. They’re an important native pollinator in the western United States. Unless they occur in great numbers, their leaf cutting is more a nuisance than a serious problem. Their tunnels rarely damage the plant, being in the center of the stem, away from the phloem and xylem. Even their sting is much milder than a honeybee or yellowjacket.

In the rare case where a plant is being defoliated, a shroud of cheesecloth or other fabric will keep the bees away. Spraying holds no benefit—the bee has cut and gone long before a pesticide can kill it. And with honeybee populations in decline, our native bees are more important than ever.
Leafcutter bee photo by Bernhard Plank, wikicommons

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