Sunday, August 21, 2011

The Japanese beetle and the blue-winged wasp

The rose borders around the amphitheater in Brooklyn Bridge Park are relatively new -- I think they were planted last summer. But already Japanese beetles are abundant there. Japanese beetles love to eat roses. And after a meal of petals they meet and mate.

A Japanese beetle couple in the Brooklyn Bridge Park roses. 
Adult Japanese beetles are shiny metallic green, oval, convex, and about one-half inch long with bronze-colored wings. Five pairs of white hair tufts project from under the wing covers on each side of the body and another pair decorate the rear end of the abdomen.

The Japanese beetle, Popillia japonica. Pretty! 

The first report of Japanese beetles in the United States was from a plant nursery in New Jersey in 1916. The insects quickly spread to the wild and we have been trying to eradicate them ever since. Adult Japanese beetles eat flowers, vegetable crops, fruit trees, and ornamental plants. They are famous for eating the soft flesh of leaves from between the veins and leaving behind leaf skeletons. They eat soft rose petals completely. As if that were not bad enough, the larval stage lives in the soil and eats grass roots, damaging lawns. A Japanese beetle larva is a white worm-like "grub" that is about an inch long at maturity; it has three pairs of tiny legs and a round brown head.

After mating, a female Japanese beetle lays eggs in the soil. She lands on the ground and digs a burrow a few inches deep. She lays a few eggs and then she feeds and lays intermittently until she has ensconced a few dozen eggs in the soil. The eggs hatch into larvae by midsummer. The larvae burrow around underground, eating grass roots. When cold weather comes the larvae dig deep in the soil to spend the winter inactive far below the surface. In spring they burrow back up to root level and eat and eat and eat. After a brief pupal stage they emerge as adults and dig themselves out of the ground.

The Japanese beetle life cycle takes a year. Ten months of that are spent underground. Summer is their time to shine; they have just two adult summer months to fly in the sunlight and eat roses.

But Japanese beetles are not the only insects that have moved into the new park. I found the blue-winged wasp pictured below sipping nectar from goldenrod flowers in the park's water garden. The blue-winged wasp in about an inch long. Its head, antennae, and legs are black. Most of its abdomen is reddish brown. Two bright yellow spots on the upper surface of the abdomen make it easy to identify.

The blue-winged wasp, Scolia dubia
Female blue-winged wasps take breaks from sipping nectar to fly low over grass. They search for beetle grubs. They love Japanese beetles. The wasp burrows into the soil to find them. (Not surprisingly, they are sometimes called digger wasps.) The wasp will sting a captured grub to paralyze it. Then she digs deeper, constructs a little cell, lays an egg right on the unlucky grub, and departs. When the wasp egg hatches into a larva it will feast on the beetle grub its mother left for it. The wasp larva grows, eating grub, and eventually makes a cocoon right inside the grub's corpse where it spends the winter. Ghoulish, eh? The new wasp emerges as an adult in spring or summer, just about when the Japanese beetle grubs are beginning to fatten up.

This is almost enough to make me stop complaining about mosquito bites -- obviously some species have much bigger insect problems!

Blue iridescent reflections give the blue-winged wasp its common name. 


  1. Very interesting! The original hosts were the flower chafers of the genus Cotinis. It is a good thing that this wasp found the newcomer to its liking. Maybe it will help to keep the nasty invader in check.
    It is very interesting to check the geographic distributions and seasons for all three creatures in Although the data are far from complete, a pattern begins to emerge. The adult beetles are most abundant in June/July; while the adult wasps are most abundant in August/September.
    I find this very informative.

  2. Thanks, Mizz Bee. They did not give up on eating green june beetles, but developing a taste for Japanese beetles is a good turn -- I hope they find them delicious! The distribution maps are indeed informative and interesting.