Effects of exposure on chemical content and efficacy of male annihilation blocks used in the eradication of 'Bactrocera papayae' in north Queensland

A program to eradicate the introduced pest fruit fly species 'Bactrocera papayae' Drew and Hancock (papaya fruit fly) from north Queensland commenced in October 1995. The primary eradication strategy employed was male annihilation using fibre-board ('Cane-ite') blocks (5x5x1.3 cm) impregnated with approximately 18 mL of a 3:1 mixture of male lure (methyl eugenol) and toxicant (maldison ULV). Studies were undertaken to determine the effects of short and long term exposure on the chemical content and efficacy (attraction and toxicity) of these blocks. As the numbers of 'B. papayae' in the eradication area were very low, the efficacy of blocks was determined by their ability to attract and kill the endemic non-pest species 'Bactrocera cacuminata' (Hering) which also responds to methyl eugenol. Results showed that the loss of methyl eugenol and of efficacy followed similar exponential curves over a 52 week exposure period. After eight weeks exposure the efficacy of blocks was reduced by 50% in comparison to a new block and the methyl eugenol content was reduced by 73%. The maldison content of blocks did not change significantly over a 28 week period, although some small loss of maldison occurred after prolonged exposure up to 52 weeks. Blocks up to 52 weeks old continued to attract and kill up to 5% of the number of flies caught by new blocks. The significance of these findings with respect to eradication treatments for papaya fruit fly is discussed.