Weeds, pests & diseases No. 95 December 2011

An introduction to clover root in white clover crops

Key Points • This FAR Arable Extra has been written based primarily on our knowledge of clover root weevil in pastoral scenarios. • Clover root weevil (CRW) is a severe pest of mature white clover; the adults attack the foliage but larval feeding on the root system does the most significant damage. • Adult feeding can kill a large proportion of emerging white clover seedlings in newly-sown crops. • The larvae are creamy white, legless and can be found around the root system. Larval populations peak in late spring and again in autumn. • CRW adults are very mobile, being strong fliers with flights occurring December-March. They are also very good hitchhikers. • CRW adults are found almost all year round. Egg laying takes place December to November including over the winter. • A parasitic wasp has been released that attacks the adult CRW. • Application of a foliar insecticide during white clover establishment can be used to provide temporary control of adult CRW. • Seed coatings do not provide adequate protection against this pest and should not be relied on. • The use of insecticides can disrupt the biological control agent of CRW but it will eventually re-establish to attack adult CRW. • FAR has initiated a small programme of research to examine how this pest would affect white clover seed crops and if it does what control measures can be put in place.

Introduction Clover root weevil (CRW) or lepidus is the most serious clover pest in New Zealand. It is found in many parts of the world and was first discovered in the North Island in 1996. Currently (Dec 2011), CRW is well- established throughout the northern half of the South Island with isolated populations being found in mid and south Canterbury, Otago and Southland. CRW prefers white clover. The adults feed on clover foliage, but generally do not cause significant damage in established pastures. The larvae, hidden below ground feed on the roots, stolons and nodules of the clover plant.

Biology CRW adults are a type of , mahogany-brown, 4 - 6 mm long, with a short, blunt nose. Adults, which can live for several months are present for most of the year but most abundant in early summer through to autumn. Female CRW can lay several hundred eggs. They lay their eggs on the soil surface and when these hatch the larvae move into the soil seeking clover roots. The larvae are legless, creamy white grubs, from 1 - 6 mm long and with a brown head. They may be found by digging into the root zone around the clover plants. Larvae are present throughout the year but are more abundant from late autumn to spring. They are much smaller than grass-grub larvae and they lack the strong C-shape that characterise grass grub. In the South Island, there are one - two generations a year Insecticides with adult populations peaking in early and late summer. CRW larvae are difficult to control with soil insecticides. This Adults feed on clover leaves producing characteristic is because the larvae are relatively immobile and early instars C-shaped notching of leaf margins. These notches are are protected in the nodules. The adults are very mobile symmetrical and uniform, unlike the jagged or irregular and although they can be killed by some insecticides, clover damage caused by other pasture pests, such as slugs stands are rapidly reinvaded from surrounding areas. During and caterpillars. Dispersal by flight occurs from December white clover establishment, application of a foliar insecticide through to March. CRW is a very proficient hitch-hiker in can be used close to seedling emergence to control adults hay and on vehicles and it’s probably this method that has and provide a window in which clover plants may establish assisted the South Island spread of the weevil. before re-invasion by the weevil occurs. Seed coatings do not provide adequate protection against this pest and should not Impact be relied on. The use of insecticides will disrupt the biological While feeding by adult CRW causes distinctive semi-circular control agent of CRW but will eventually re-establish to attack notching on the edges of clover leaves may appear serious, the weevil. generally, it does not cause significant damage in established Clover and pasture management pastures. Adult feeding can however kill large numbers of Clover management to maintain healthy plants is important. white clover seedlings in newly sown pasture. A healthy plant can tolerate more larval feeding than an Although adult feeding damage is the most noticeable, larval unhealthy plant. Adequate fertiliser should be used to keep feeding is far more damaging. The young larvae feed on the clover growing in the presence of the weevil. Experience nitrogen-fixing root nodules and later the roots and stolons. in pastoral systems has shown that compensatory rates of This reduces the plant’s ability to fix atmospheric nitrogen nitrogen can be applied to infested crops in small amounts and reduces plant growth. A reduction in nitrogen fixation (20 - 40 kg N/application), applied as urea over the August can lead to an increased requirement for nitrogen fertiliser - October period. Ideally, the crop should not be stressed by to be applied. Moderate populations of larvae (approx 300/m2) other factors such as overgrazing over winter. have been shown to reduce clover dry matter (DM) For further information on CRW see: www.pestweb.co.nz/ production by 35% (1000 kg DM/ha) annually with greatest index.php or www.agresearch.co.nz/our-science/biocontrol- losses occurring in spring. biosecurity/pest-control/clover-root-weevil

Acknowledgements FAR wishes to thank Mark McNeill of AgResearch for compiling this information.

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Control A small parasitic wasp (Microctonus aethiopoides) has been released as a biological control agent against CRW. This parasitoid adult lays an egg inside the adult CRW. The resulting wasp larva grows inside the weevil and eventually kills it on emergence. Parasitised are also immediately rendered sterile thus preventing them from laying any eggs. The wasp has been released at numerous locations in the North and South Island. It has always established rapidly where it has been released and appears to be effective at reducing clover root weevil populations. The wasp is also spreading from those areas at about 15 - 20 km per year. This biocontrol agent will not eliminate CRW or the damage it causes, but it will reduce the impact the weevil has.