In the biological control of pests, how do we make sure the control agents won’t go AWOL?

When a pest species is introduced, one of the major ways of trying to reduce or eliminate it is via biological control- that is, by introducing something else. But these introductions are risky because the biocontrol agent can have more adverse impact on the environment than the pest it was introduced to control. So, before any control agents are introduced they have to be carefully tested. Here’s is an example of the procedure for an Aussie pest in New Zealand.

Everybody here in Australia knows of the cane toad story –or if they don’t they ought to. Basically it’s a classic example of a biocontrol experiment gone wrong: beetle larvae ate roots of sugar cane, cane farmers didn’t like that, the Queensland government found someone who thought that an introduced American toad would eat all the beetles as they emerged from the ground, the toad was introduced, and it ate everything except the beetles. There’s now a big problem with the toad and a continuing problem with beetles.....Today, thank goodness, biological control of pests is done much more carefully.

In this case we consider how to go about introducing a biocontrol agent for an important defoliator of eucalypt plantations in New Zealand, where several Australian beetles have arrived through poor quarantine procedures in the past. These include paropsines, a kind of large pretty leaf beetle that voraciously munches eucalypts. Eucalypts arent native to New Zealand, but unfortunately the Kiwis have planted eucalypts extensively as timber trees and in the absence of native predators the Australian beetles are seriously damaging these plantations. The solution? Introduce an Australian predator to control the beetle.

In this case, the ‘predator’ is a red-headed wasp (Eadya paropsidis) that lays its egg in the exposed leaf-feeding larvae of the beetles. The wasp goes through its whole life cycle inside the beetle larva, hollowing it out.

Evidence from Australia is that this wasp is specific to paropsines on eucalypts and there are no native paropsines or eucalypts in New Zealand. However, there are native relatives of paropsines, plus other leaf beetles with exposed leaf-feeding larvae. There are also introduced leaf beetles working as biocontrol agents of weeds.

It is important to test the wasp against these potential hosts first, before release. But it is impractical to test the wasp against all such larvae. In this paper we review the exposed leaf feeding beetle larvae of New Zealand and draw up a short-list of species that need testing as potential hosts, based on evolutionary relationships, hosts and larval behaviour.

Of course the wasp may fail these tests and my New Zealand colleagues will have to start all over again.

Dr Chris Reid, Principal Research Scientist

More Information:

Withers, T.M, Allen, G.R., & Reid, C.A.M. (2015). Selecting potential non-target species for host range testing of Eadya paropsidis. New Zealand Plant Protection 68 (2015): 179-186.