Quick Dips

Penetrating Porinas' Disguises

Research at Victoria University has shown that New Zealand pastures are vulnerable to three different species of porina caterpillar, instead of one as previously thought.

Dr John Herbert has just completed his PhD in biochemistry, with a project that brought some important new findings on the porina pest. He used genetic analysis to demonstrate that three of the seven species of porina can turn pasture into mud holes by eating the tops of the grass.

Herbert is the first researcher to carry out genetic analysis of both the porina caterpillars and the moths; previously only the moths had been analysed in this fashion. The different species can have different reactions to insecticides and other control methods, so these findings open the way for the development of more specific and effective ways to save pastures from the caterpillars. The caterpillars of the different species all look quite similar, and so do the moths.

"The caterpillars are dark greeny-brown and quite distinct from other grass species," Herbert says. "They live in the ground, in silk-lined burrows with little piles of frass' -- eaten-up dirt, food and silk -- at the entrances, and come out at night to eat the grass. If you hold them in your hand they don't curl up like other species do."

"You can distinguish the male moths of the seven species by looking at them, once you're used to them. The moths are about 3cm to 5cm in wingspan, brownish and fairly nondescript, with some patterns on the wings. The female moths are harder to tell apart, although there are a couple of species where the females can be distinguished."

Herbert has produced a guide to help field workers identify the moths of each species, as well as a key to identify the species by genetic analysis in laboratories. The moths live for only about three days, and do not eat at all -- it's the caterpillars that do the damage.

The caterpillars of the different species are harder to tell apart visually, but again John Herbert had developed a key for genetic analysis. Identifying the species of porina present on an affected farm is important for control, he says.

"We've been treating all the species as the same, but there are indications that some insecticides, especially moult-inhibiting insecticides, are not working over the whole range of species. We need to be more careful when there is a number of species in the same pasture."

Where more than one species is responsible for pasture damage the timing of control applications may become critical, and differences in tolerance, resistance or behaviour between the species could affect chemical controls. Biological controls could also be affected. Herbert hopes that other researchers will now follow up his work, to investigate the best forms of control for each of the species that attacks pasture.