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Painted apple moths may be insect enemy No 1 in Auckland, but Rotorua scientists can't breed them fast enough.

For the past two years, thousands of painted apple moths have been reared at the Forest Research Institute to help rid Auckland of the pest.

About 1500 females a week are produced and sent to Auckland, where they are placed in sticky traps.

The females lure wild males, which get stuck in the trap, allowing scientists to estimate population size and range.

"It's the only way to find out what's going on with the population," Forest Research entomologist Nod Kay said.

At one stage, scientists were catching hundreds a week but lately the number has dropped to less than 10, indicating the population was on the decline.

Forest Research also produces males, which are sent to a laboratory in Canterbury for sterilisation.

Irradiated male pupae are released in Auckland, where they mate with wild females, destroying the female's chances of reproducing.

The programme is seen as a valuable part of the eradication programme because in some areas in Auckland vegetation is so dense that spraying does not get through.