Biological Control: Asian Wasps and the Emerald Ash Borer

The emerald ash borer (Agrilus planipennis) is a pest introduced to the US from Asia. The larvae are deadly to ash trees, which are a popular tree with residential developers because they grow quickly to give newly-built neighborhoods that appealing lush, tree-lined quality. This beetle has been slowly but surely expanding its range in the US. Because it is not native to North America, our ash trees have little natural resistance; insecticide success has been hit-or-miss in protecting the trees. As a result, Minnesota recently released Asian stingless wasps as a potential biological control.  The hope is the wasps, which lay their eggs in ash borer larvae, will slow the beetle’s expansion.

As an ecologist by training, I generally agree with the concept of biological control (using natural enemies to reduce or mitigate pest effects) as it applies to native pests – and therefore native enemies. I get concerned when critters are “imported” to control an invasive pest, for several reasons:

1. Multicolored Asian lady beetle (Harmonia axyridis) in the US: These were brought to the US from Asia to help control aphids and other soft-bodied insects. In Asia, these beetles hibernate in the cracks and crevices of cliff faces; however, many of the areas in which they were released in the US don’t have cliffs. As a result, the Asian lady beetle itself is now considered a nuisance pest species when they concentrate in and around buildings in the fall, looking for good hibernation spots. It is also displacing our native ladybugs in some areas of the US.

2. Cane toad (Bufo marinus) in Australia: The cane toad was brought to Queensland, Australia, in an attempt to control the greyback cane beetle (Dermolepida albohirtum). These beetles are native to Australia and love to eat sugarcane, much to farmers’ dismay. It sounds like the perfect scenario – the cane toad will eat the cane beetles, thus protecting the sugarcane crop without the use of pesticides. As it turns out, however, the cane toad will eat anything it can fit its mouth around – including insects, frogs, small reptiles, mammals, and birds. In fact, the only thing the cane toad doesn’t seem to be eating much of is the cane beetle! This is because the beetles spend most of their time at the top of plant, far out of the reach of the toad. The cane toad has now spread into the neighboring state of New South Wales and the Northern Territory, and it is thought to be an important factor in the decline of many native animals.

There are other examples of biological control gone wrong, but those two came to mind first.

So let’s say the Asian wasp is successful. It lays its eggs in the emerald ash borer larvae, killing the larvae and preventing or at least impeding the spread of the borer. Those wasp eggs hatch and grow into adult wasps, which then want to lay eggs of their own. If there aren’t enough emerald ash borers around, will these wasps switch to native beetles? What will we have to introduce in the future to control the Asian wasp?