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Weevils controlling Patterson’s curse

Striking infestations of a biological control agent for Patterson’s curse, the crown weevil, have been observed across much of the NSW South West Slopes over the past few weeks. Grazing is recommended for earth mite and weed control in these situations

 

Where have they been reported?

Infestations of the biological control agent for Patterson’s curse, the crown weevil (Mogulones larvatus), have been observed in paddocks across much of the NSW South West Slopes over the past few weeks. Although the bio-control agent was first released some years ago, reports indicate the larvae numbers has never been so apparent. Larvae, mostly about 4-5 mm long, have been easily seen at the base of the weed’s rosette. In many cases the plants are struggling considerably.

Paterson's curse crown and root boring weevils, Mogulones spp., were first released by CSIRO in the early 1990’s. The crown weevil is most effective in high rainfall / low grazing pressure situations. The root weevil tolerates drier regions and since it feeds in the taproot below ground also performs better in more heavily grazed pasture.

Crown weevil habits and behaviour

The crown weevil has a single generation each year. Adult weevils become active from February to April after heavy rain stimulates Paterson’s curse germination. After feeding for a week on rosette leaves, females lay eggs into the leaf stalk of the rosette. The weevil larvae hatch and feed within the leaf stalk. As larvae mature they mine towards the crown and the top of the taproot, where 10-20 larvae can kill a rosette 15-20cm in diameter. After feeding, larvae leave the rosette to pupate in the soil.

The adults emerge in spring, feeding on the leaves and flowers of Paterson’s curse until the plant dies. During this time adults disperse to seek out new sources of Paterson’s curse. After feeding, adults move into the soil and leaf-litter and become dormant to escape the summer heat. The adults do not feed during summer and only the healthiest adults survive. Adult weevils remain dormant in the soil until autumn rain stimulates them to become active and start a new generation. The early activity of the crown weevil will limit it’s ability to survive in regions where late breaks to the season are common.

Our advice

The use of insecticides aimed at earth mites can significantly depress the numbers of weevil larvae and should not, therefore, be sprayed on paddocks in which weevil activity is being managed for control of Patterson’s curse. Under these circumstances, heavy grazing will both greatly weaken the Patterson’s curse and minimize mite carry-over, especially in cases involving redlegged earth mites if applied around the Timerite® date.

Description

The crown weevil is 3.5-4 mm long. It is dark brown and white with a characteristic patchy pattern. The root-boring weevil is slightly larger, 4-5 mm long, and has a light brown body covered with fine, pale lines. Both weevils have very hard bodies, a long snout (the rostrum), with mouthparts located at the tip, and elbowed / club-shaped antennae that can be folded against the snout. Further information about both insects can be found here.

 

* Sources of field reports of crown weevil

Peter Watt – Agronomist, Elders (NSW South West Slopes)

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