Plutella maculipennis, Curt., its natural and biological Control in England

Abstract

1. Plutella maculipennis, Curt., a widespread Lepidopterous pest of Brassica was introduced into New Zealand about sixty years ago. In most countries a high degree of natural control is maintained, but in New Zealand the moth population is permanently maintained at a high level. Investigation showed that in New Zealand there were no natural enemies of importance, while in other areas these are constantly associated with Plutella.2. The distribution of the moth throughout the world is exceedingly wide, but in England it only occasionally reaches pest proportions.3. Preliminary work indicated that two Campoplegines, Angitia cerophaga and A. fenestralis, constantly parasitized large numbers of the Plutella larvae. Initial efforts were devoted to collecting and breeding a large supply of these in England for introduction into New Zealand as controlling agents.4. From several thousand specimens of Plutella, fourteen species of parasites were recovered, of which eight were hyperparasites. The only parasites which were numerically important were the two parasites, Angitia cerophaga and A. fenestralis. Their efficiency is not impaired to any appreciable extent by hyperparasites. A key showing the diagnostic differences of the parasites and hyperparasites of Plutella is given.5. These two species of Angitia are commonly recorded as parasites of Plutella throughout the world, although under different names. A large quantity was bred in the laboratory and despatched to New Zealand after overcoming certain difficulties connected with transport. It appears that both species of parasite must overwinter in other hosts than Plutella.6. Certain experiments were performed in an endeavour to assess some of the effects of climate. The upper limiting temperature of Plutella is approximately 40°C. and the lower limit for breeding purposes about 10°C. All stages of the moth can survive short periods of cold greater than 10°C., but it is believed that hibernation is normally accomplished in the adult stage. As the immature stages of the moth live in a specialised microclimate of high humidity, changes in the moisture content of the general atmosphere have little effect. Rain, if appearing at certain critical times in the life-cycle, may be a controlling agent.7. The most favourable areas for multiplication of the moth appear to be in the sub-tropics and warmer temperate zones. It is suggested that Plutella originated in the Mediteranean region.8. Even in the cooler temperate countries the climate would allow an indefinite increase in the moth. It is believed that an effective check on this multiplication is supplied by parasites.9. As the climate of England and New Zealand are essentially similar there is reason to suppose that the introduction of parasites from the former country will lead to eventual control being obtained in New Zealand.