Nematode field trials

Wasp nests prove to be worm-proof

Landcare Research NZ, AgResearch NZ and Adelaide University

Entomopathogenic nematodes, microscopic worms that parasitise and kill insects, are increasingly being used to control a range of insect pests. During the recent wasp season, researchers from Adelaide University and Landcare Research in New Zealand have been collaborating to find out whether they will be effective against introduced wasps. Two types of nematode were tested, 'Heidi' (Steinernema carpocapsae) and 'Riwaka' (Heterorhabditis zealandica). Riwaka proved to be capable of killing wasp larvae which had been removed from their nest and kept in the laboratory, while Heidi had little effect.

Steinernema carpocapsae, a nematode
that parasitises insects

The next step was to determine whether these nematodes could kill wasps nests in the wild. To get the nematodes into the nest, they were mixed in with a sardine bait which is very attractive to foraging wasps. These baits were placed immediately outside the nest entrance, where they were quickly discovered by the workers. The baits were chewed up and carried into the nest by the workers, where they would then be fed to the larvae. The nests were observed over several days to see if the nematodes had any effect on the number of wasps leaving the nest to feed. The nests were then dug up and the larvae examined for signs of nematode infection. Despite the promising results from the laboratory, the nematodes did not appear to have any effect on whole nests. There was no change in the numbers of workers coming in and out of the nests, nor were any of the larvae inside infected.

These results suggested that either the nematodes were not infecting wasps inside the nest, or that any infected larvae were being swiftly removed before the infection could spread. To find out whether the nematodes were successfully infecting larvae, nests were dug up just a few hours after the introduction of the nematode baits. This would be too soon for any infected larvae to be showing symptoms, so they could not yet have been removed by the workers. The nests were taken apart, the workers were removed and the combs containing the larvae were kept in the laboratory for a few days. None of the larvae died from nematode infection, indicating that the nematodes had not been successfully introduced into the nest.

Pelorus Bridge Scenic Reserve,
where field trials were carried out

So why are these nematodes infectious in the laboratory but not in the field? One possible reason is that the nematodes are destroyed when the workers chew up the baits. The baits are initially chewed by the foraging workers and formed into small pieces that they can carry into the nest. Once inside the nest the bait is then likely to be chewed further by the nurse workers, before being chewed again as it is fed to the larvae. While the nematodes are small it is still possible for them to be damaged by this chewing action. Another possibility is that the temperatures inside the nests were too high for the nematodes to successfully infect. The laboratory trials were carried out at 28° C, but the temperature inside the nests may have been higher than this. Previous studies have shown that the types of nematodes used in these trials struggle to infect their insect hosts above 30° C, so if the nests are particularly warm the baits may be rendered ineffective. This problem might be overcome by selecting a strain of nematode that is more tolerant of high temperatures.

Last updated on March 6, 2002