Major pest of clovers, both red and white cultivars but white clover often most severely attacked
Found throughout New Zealand
Adults feed on clover foliage leaving distinctive notches on leaf margins
Larvae are most the damaging stage destroying clover roots and root nodules
Infested pastures become clover deficient.
Biological control has significantly reduced impact of this pest
Biology
Clover root weevil (CRW) adults are mahogany-brown, 4-6 mm long, with a short, blunt nose. It can be difficult to tell adult CRW apart from other weevils that can also be found in pasture. Adult weevils are most abundant in autumn and spring. They lay their eggs on the pasture surface and when these hatch the larvae move into the soil seeking food and shelter. These are legless, creamy white grubs, 1-6 mm long with a brown head. They may be found by digging into the root zone under clover plants. Larvae are present throughout the year but are more abundant in late spring and late autumn. The larvae are generally much smaller than grass-grub larvae, which can be common in pastures, and they lack the distinctive C-shape that characterise grass grub
Clover root weevil has been present in New Zealand since at least 1994 but was not discovered until 1996. By 2006 it was present throughout the North Island and by 2015 throughout the South Island. The combination of a favourable environment, lack of competition for an abundant food resource, high reproductive capability of the weevil and a lack of natural enemies inevitably meant clover root weevil was going to become one of New Zealand\’s most damaging pests of clover.
There are two generations a year with adult populations peaking in early and late summer in the North Island and summer and autumn in the South Island. The adults can live for several months with the females laying up to several hundred eggs. Young larvae mainly feed on clover root nodules. As they mature they move onto the roots and stolons. Adults feed on clover leaves producing characteristic notching of leaf margins. Dispersal by flight occurs during the summer and autumn with the proportion of adults having flight or reproductive capability dependent on prevailing climatic and pasture conditions
While clover is abundant and summer rainfall high or irrigation used most weevils remain reproductive and stay in the pastures they lived in as larvae. When there is drought and clover is limited a large proportion of weevils stop being reproductive and instead develop flight muscles and disperse to new locations. Once they have located a suitable site the flight muscles atrophy and the weevils again become reproductive. Clover root weevil is a very proficient hitch-hiker in hay and on vehicles.
Impacts
Feeding by adult CRW causes distinctive semi-circular notching on the edges of clover leaves. These notches are symmetrical and uniform, unlike the jagged or irregular damage caused by other pasture pests, such as slugs and caterpillars. While this damage can appear serious, generally, it does not cause significant damage in established pastures. Adult feeding can however, kill large numbers of clover seedlings in newly sown pasture
Although adult feeding damage is most noticeable, feeding by larvae is more damaging. The young larvae feed on root nodules and later the roots. This reduces the plant’s ability to fix nitrogen from the atmosphere and reduces plant growth. A reduction in nitrogen fixed also impacts on the growth of other pasture plants and leads to an increased requirement for nitrogen fertiliser to be applied. Prior to the release of the biocontrol agent moderate populations of larvae (approx. 300/m2) have reduced clover production by 35% (1000kg DM/ha) annually with greatest losses occurring in spring. Without additional nitrogen it has been estimated a typical Waikato dairy farm would suffer a 16% reduction in net profit. An extra 180 kg N/ha was required on some farms to alleviate the impact of the weevil. During invasive periods larval numbers as high as 1800/m2 have been recorded. With the establishment of biological control numbers are now usually much less. Left unchecked MPI estimated clover root weevil could have cost New Zealand between $200m and $1b per annum.
A cost: benefit analysis undertaken in 2015, has shown that for Southland alone the impact of the parasitoid wasp in supressing CRW was worth $2.3m pa for beef and lamb farms and $7 M pa for dairy farms
Control
Insecticides
During pasture establishment application of a foliar insecticide can be used close to seedling emergence to control adults and provide a window in which clover plants may establish before re-invasion by the weevil occurs. Seed coatings do not provide adequate protection against this pest and should not be relied on
Clover root weevil is difficult to control with insecticides in established pastures. Because the larvae live in the soil and are relatively immobile, and because New Zealand soils are high in organic matter which binds up insecticide, getting an insecticide to them is difficult to achieve. The adults are very mobile and although they can be killed, pastures are rapidly reinvaded from surrounding areas.
Insecticides are harmful to the biocontrol agent
Clover and pasture management
Clover root weevil is dependent on clover for its survival. When pastures are severely damaged destroying the remaining clover with a selective herbicide and/or cultivation followed by a farrow period leaving the pasture clover free for 6-8 months can cause the weevil population to die off and provide a clean pasture in which clover can be re-established. Drilling clover seed into a damaged pasture without removing the older clover plants should not be attempted as the weevils prefer seedlings and will concentrate their feeding on them. Biological control will not prevent seedling damage.
Establishment of clover seedlings at sowing can be improved by utilising break crops rather than straight pasture to pasture rotations. A brassica or an all grass break crop will provide a clean paddock to which clover can be re-introduced. Clover management to maintain healthy plants is important. A healthy plant can tolerate more feeding pressure than an unhealthy plant. Adequate fertiliser should be used to keep clover growing in the presence of the weevil and to keep other pasture plants, which would normally benefit from clover nitrogen fixation, healthy as well. When clover is put under pressure by clover root weevil not only do the larvae consume nodules but the plants respond to the stress by reducing nodule production. In such instances compensatory nitrogen should be applied frequently to infested pastures in small amounts. Clover should not be put under stress by other factors, do not overgraze and avoid pugging of soil over winter.
A parasitoid wasp for biological control
In 2006 AgResearch introduced a small parasitic wasp (Microctonus aethiopoides) as a biological control agent against clover root weevil. This wasp was released at numerous locations in both islands and is now very widely distributed. This biocontrol agent will not eliminate clover root weevil or the damage it causes, but it has reduced the impact of the weevil and in most locations the impact of the weevil has been negated by the biocontrol agent. However, there will always be some years and some areas where the biocontrol agent temporarily losses effectiveness either because of climatic events, use of insecticides that kill the wasp or pasture renewal events. Biological control will re-establish.
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