How to identify clover root weevil

Clover root weevil (CRW) has four distinct stages in its life cycle: egg, larva, pupa, and adult. The eggs and pupae are extremely difficult to find, while adults can sometimes be seen.

The most obvious sign of CRW is the distinctive notching on clover leaves caused by adult feeding. If notching is observed, further evidence of CRW can be found by digging up some clover plants to look for larvae in the top 5 cm of soil near clover roots.

CRW larvae

Clover root weevil larvae

CRW larvae live in the soil, where they feed on clover roots and nodules. The larvae are small creamy white grubs, from 1-6 mm long, with a brown head capsule. They may be found by digging into the root zone under white clover plants.

CRW larvae are generally much smaller than grass grub larvae, which are also common in pastures, and lack the distinctive C-shape and bulbous tail that characterise grass grub.

Clover plants under stress from CRW tend to be small leafed and low growing.

Adult CRW

Clover root weevil adult

Weevils are a type of beetle. They have snout noses which range from short and blunt in some species to long and slender in others. The adult CRW is a mahogany-brown weevil, 4-6 mm long, with a short, blunt nose.

Unfortunately, it can be difficult to tell adult CRW apart from other similar weevils which are commonly found in New Zealand pastures. A few other weevil species commonly found in New Zealand pastures are shown below.

Clover root weevil Sitona lepidus	Lucerne weevil Sitona discoideus
Little fringed weevil Atrichonotus taeniatulus	Argentine stem weevil Listronotus bonariensis

The Argentine stem weevil (Listronaris bonariensis) is smaller (only 5 mm long) and greyer, and has a longer nose. It is widespread throughout New Zealand pastures, where the larvae tunnel into, and feed on, ryegrass tillers.

The lucerne weevil (Sitona discoideus) is closely related to CRW, and similar in appearance. However, the lucerne weevil is greyer and generally more slender, and has distinctive light-coloured stripes running down its thorax (the middle of the three body sections, between the head and abdomen). Lucerne weevil larvae feed on the roots of lucerne, but adults disperse widely in autumn and it is not uncommon to encounter them in ryegrass/clover pastures throughout the country.

The little fringed weevil (Atrichonotus taeniatulus), originally from South America, can occasionally reach large numbers in pastures. It is currently abundant in the Nelson region where it is causing notching on clover leaves similar to that from CRW. The adult weevil differs from CRW in being grey rather than brown, and in having a shorter, more oval-shaped abdomen (the hind-most body segment) compared to the long square abdomen of CRW.

Various New Zealand native weevils are often encountered in pastures, though they rarely occur in sufficient numbers to cause damage.

Damage to clover

Feeding by adult CRW causes distinctive semi-circular notching on the edges of clover leaves. These notches are symmetrical and uniform, unlike the jagged notching caused by some other pasture pests, such as slugs and clover flea. Note however that some other weevil pests like little fringed weevil may cause similar notching in clover, and occasionally may become abundant enough to cause noticeable damage.

Notching on white clover, caused by clover root weevil adult feeding

CRW larvae feed on the roots of clover, injuring the roots and damaging the nitrogen-fixing nodules. This reduces the vigour of the clover, and additional use of nitrogen fertilisers is needed to maintain pasture quality.

Damage to white clover roots caused by clover root weevil larval feeding