Biocontrol of the New Zealand pea crab

What is biocontrol?

Biocontrol is a biological approach to controlling a pest organism. It uses other organisms, or active ingredients from organisms, as ways to control the pest. Biocontrol of pests provides an alternative to chemical control (such as insecticides) and physical control (such as manual removal of weeds).

For the New Zealand pea crab, Oliver and Jessica are hoping to develop biocontrol approaches that use chemicals from the crab itself and from the crab’s major host, the green-lipped mussel. They aim to use these chemicals to change the pea crab’s behaviour at two points in its life cycle – mating and settlement.

An unusual biocontrol situation

In New Zealand, biocontrol is often thought of as a way of keeping introduced pests in check, and it commonly involves bringing another organism from elsewhere into New Zealand.

The New Zealand pea crab, however, is a native of New Zealand that inhabits another native – the green-lipped mussel. The pea crab isn’t considered to be a pest in the wild. However, Oliver’s research has shown that it slows the growth of farmed mussels significantly, leading to losses for the green-lipped mussel industry. For this reason, any biocontrol of pea crabs would focus on deterring pea crabs from inhabiting mussels on the farm, rather than limiting their overall numbers. Learn more in the article Testing how pea crabs affect mussel farming.

Biocontrol strategy 1: a pheromone trap

Oliver’s research on the mating strategies of pea crabs might one day be used in biocontrol. He is trying to prove that female pea crabs release pheromones that are detected by male crabs. The pheromones seem to be important for successful mating, because they help males to work out which mussel shells contain females. The article Getting into and out of mussels explains how and why this happens.

Oliver hopes that a pheromone trap could be positioned within mussel farms. It would emit an identical pheromone to the one released by female pea crabs. This could confuse male crabs, making them think that the trap contained a female crab. Male crabs would enter the trap and be unable to escape.

Before a pheromone trap can be developed, it’s important that Oliver nails down exactly what pheromone the female pea crab is releasing. To do this, he’ll need to analyse the seawater surrounding a mussel shell that contains a receptive female crab. The seawater will contain many different kinds of molecules, including the pheromone, so Oliver’s task will be to separate the pheromone from all the other molecules in the seawater.

Biocontrol strategy 2: disrupting settlement

Jessica’s research on the pea crab life cycle might also be used in biocontrol. Jessica’s preliminary experiments suggest that pea crabs use chemicals that are released from mussels as a way of seeking out mussels to settle in. In the future, it might be possible to design a way to use those mussel chemicals (perhaps in a trap) to confuse pea crabs near mussel farms. The aim would be to deter the crabs from settling within mussels on the farm.

Know your enemy

To develop an effective biocontrol, it is essential to understand the biology of the problem organism in detail. In the case of a parasite such as the New Zealand pea crab, it’s also important to understand the host-parasite relationship between the pea crab and the green-lipped mussel. Without this detailed knowledge, it’s impossible to know which aspects of the organism might be vulnerable to a biocontrol agent.

Oliver and Jessica have studied the mating strategies and life cycle of the pea crab closely, but more research will be needed before an effective biocontrol strategy can be tested.