Biological Control of Clover Root Weevil
Clover root weevil was first found in the Waikato and Bay of plenty in 1996 over quite a large area, so eradication was out of the question. The weevil belongs to the genus Sitona, which is not native to New Zealand. One other species from that genus arrived here accidentally about 35 years ago (the lucerne weevil).
The new species was identified as Microcotonus aethiopoides from the Mediterranean region where they have a farming system that is not as dependent on white clover as in NZ, and there are natural predators that help keep the weevil populations in check.
AgResearch has drawn on its overseas connections and previous experience with parasitoids to find an appropriate predator for the clover root weevil that is debilitating clovers in many areas of the Nth Island. Laboratory work has shown great promise, and application has been made to ERMA for release of the parasitoid (a small, wasp-like insect).
The adult female weevil lays eggs on the soil, grubs hatch out and burrow into the soil and feed on the clover root, particularly the root nodules where symbiotic bacteria fix nitrogen. This process damages the plant and reduces the amount of nitrogen fixed. The adult is 5 7mm long, slender and plain brown. The larvae are nondescript white grubs that look a little bit like little maggots, and there can be as many as several hundred per sq m.
After the initial findings in Waikato and Bay of Plenty another small population was found near Auckland, and they have since spread northwards probably to the Far North by now and down as far as Hawkes Bay and Palmerston North. The populations are moving at roughly 50 km per year. They breed less quickly as they head south into cooler regions, but in the northern hemisphere they have been found as far north as the Arctic Circle and as far south as Egypt, so the New Zealand climate shouldnt hold them back. Craig Phillips, senior scientist for AgResearch at Lincoln, suspects they are probably already in the South Island although they havent been found there yet.
Insecticides might be useful when sowing clover in pasture for protecting seedlings, but after that insecticide applications are not economic and not particularly effective.
Globally only about 20% of parasitoids tested for biological control have been successful, but in NZ pastures there have been several successes and our pastoral system seems well suited to the approach. Because of this AgResearch drew on international links formed in earlier programmes to try to find out if there were any natural predators for clover root weevil in Europe and America, which might be of use here. They came across a survey team in Europe that was looking at white clover for other reasons, and they agreed to collect weevils from their white clover plots and send them to NZ.
The weevils natural enemies spend part of their life cycle inside the weevil, so Craig and colleagues dissected the adult weevils to see if there were any parasites present that would give clues as to potential biocontrol agents.
The initial test was successful, and they obtained funding from various sources to work both in France and Britain and also continued to collaborate with the white clover research program, which involved 10 countries and helped them get weevils from all around Europe. They found six predator species, and chose Microtomus echipoides because it was one of the most widespread and common biocontrol agents and they already knew quite a lot about it. It is the same species of parasitoid that was introduced to New Zealand about 30 years ago to control the lucerne-eating Sitona species that arrived here, and which has had a massive impact against the lucerne pest.
When the clover root weevil aws first found in 1996 they were very hopeful that this same parasitoid would attack it, but it didn't. A closer look revealed there are several strains of Microtomus echipoides that do not attack clover root weevil, so they had to find a strain that did. Not only that, they had to find a strain that reproduced asexually because any new strain that reproduced sexually could interbreed with the strain that was already here and potentially destroy the very valuable biocontrol system for lucerne.
They found one celibate strain from Ireland, and have been studying that. An important consideration is that the strain should not attack NZ native weevils, so they have been testing the environmental safety of it in the lab. This is a real concern the lucerne weevil parasitoid has actually been found to attack some native weevil species, but fortunately most of those species have adapted well to agriculture and are present in large numbers and they are being very careful.
The first phase of testing has been completed and they have applied to the Environmental Risk Management Authority to release the parasitoid in New Zealand.
Craig says that they are quite optimistic about the potential of the new strain as a biocontrol agent, but you can never be 100% certain of success or that it will not attack other desirable species.
They should know within the next three months whether the application for its release has been successful. If it fails they still have other potential biocontrol agents up their sleeves. There is also plant breeding work going on to try to find white clovers resistant to the clover root weevil.
The new species was identified as Microcotonus aethiopoides from the Mediterranean region where they have a farming system that is not as dependent on white clover as in NZ, and there are natural predators that help keep the weevil populations in check.
AgResearch has drawn on its overseas connections and previous experience with parasitoids to find an appropriate predator for the clover root weevil that is debilitating clovers in many areas of the Nth Island. Laboratory work has shown great promise, and application has been made to ERMA for release of the parasitoid (a small, wasp-like insect).
The adult female weevil lays eggs on the soil, grubs hatch out and burrow into the soil and feed on the clover root, particularly the root nodules where symbiotic bacteria fix nitrogen. This process damages the plant and reduces the amount of nitrogen fixed. The adult is 5 7mm long, slender and plain brown. The larvae are nondescript white grubs that look a little bit like little maggots, and there can be as many as several hundred per sq m.
After the initial findings in Waikato and Bay of Plenty another small population was found near Auckland, and they have since spread northwards probably to the Far North by now and down as far as Hawkes Bay and Palmerston North. The populations are moving at roughly 50 km per year. They breed less quickly as they head south into cooler regions, but in the northern hemisphere they have been found as far north as the Arctic Circle and as far south as Egypt, so the New Zealand climate shouldnt hold them back. Craig Phillips, senior scientist for AgResearch at Lincoln, suspects they are probably already in the South Island although they havent been found there yet.
Insecticides might be useful when sowing clover in pasture for protecting seedlings, but after that insecticide applications are not economic and not particularly effective.
Globally only about 20% of parasitoids tested for biological control have been successful, but in NZ pastures there have been several successes and our pastoral system seems well suited to the approach. Because of this AgResearch drew on international links formed in earlier programmes to try to find out if there were any natural predators for clover root weevil in Europe and America, which might be of use here. They came across a survey team in Europe that was looking at white clover for other reasons, and they agreed to collect weevils from their white clover plots and send them to NZ.
The weevils natural enemies spend part of their life cycle inside the weevil, so Craig and colleagues dissected the adult weevils to see if there were any parasites present that would give clues as to potential biocontrol agents.
The initial test was successful, and they obtained funding from various sources to work both in France and Britain and also continued to collaborate with the white clover research program, which involved 10 countries and helped them get weevils from all around Europe. They found six predator species, and chose Microtomus echipoides because it was one of the most widespread and common biocontrol agents and they already knew quite a lot about it. It is the same species of parasitoid that was introduced to New Zealand about 30 years ago to control the lucerne-eating Sitona species that arrived here, and which has had a massive impact against the lucerne pest.
When the clover root weevil aws first found in 1996 they were very hopeful that this same parasitoid would attack it, but it didn't. A closer look revealed there are several strains of Microtomus echipoides that do not attack clover root weevil, so they had to find a strain that did. Not only that, they had to find a strain that reproduced asexually because any new strain that reproduced sexually could interbreed with the strain that was already here and potentially destroy the very valuable biocontrol system for lucerne.
They found one celibate strain from Ireland, and have been studying that. An important consideration is that the strain should not attack NZ native weevils, so they have been testing the environmental safety of it in the lab. This is a real concern the lucerne weevil parasitoid has actually been found to attack some native weevil species, but fortunately most of those species have adapted well to agriculture and are present in large numbers and they are being very careful.
The first phase of testing has been completed and they have applied to the Environmental Risk Management Authority to release the parasitoid in New Zealand.
Craig says that they are quite optimistic about the potential of the new strain as a biocontrol agent, but you can never be 100% certain of success or that it will not attack other desirable species.
They should know within the next three months whether the application for its release has been successful. If it fails they still have other potential biocontrol agents up their sleeves. There is also plant breeding work going on to try to find white clovers resistant to the clover root weevil.