Signature scalloped-shaped leaf damage from adult weevils has been reported in canola and lucerne, but what’s causing it?
Where have they been reported?
Extensive foliar damage has been reported in an emerging lucerne paddock northeast of Wellington in the NSW Central West Slopes and Plains; almost every plant had some level of damage. Based on morphological characteristics, the adult weevils responsible for this damage are suspected to be the grey-banded leaf weevil (Ethemaia sellata).
In the same district, weevil damage has also been observed on 50% of plants in a canola paddock southwest of Wellington; the paddock had been in fallow the previous spring. In this case, adults of the sitona weevil (Sitona discoideus) are believed to be the culprits.
Distinguishing between adult weevils
This article is designed to provide some broad guidelines on methods to distinguish adult weevils.
Adult weevils are frequently associated with above ground plant damage, which is typically seen at this time of the season.
As some actively feed at night and shelter during the day, a pitfall trap can be useful to capture the pest responsible for observed damage. Pitfall traps can be a plastic cup buried flush in the soil containing a small amount of water/detergent mix in the bottom to help contain insects that fall into the trap. Traps should be left for at least 24 hours but preferably longer and are useful for catching insects that actively move across the ground.
Successful management of pests starts with the correct identification. Generally it is easier to distinguish between adult weevil species than larval forms.
The first feature to pay attention to is the shape of the weevil’s snout. Generally adult weevils can be divided into those that have an elongated, definite snout or a short, broad snout.
The second feature to note is size. Some weevils are notably larger (>7 mm) than others (3-5 mm).
In addition to snout shape and weevil size, there are some further distinctive features of many broadacre crop weevils that can be seen with a hand lens. For example the sitona weevil has three pale stripes on the thorax behind the head.
It is possible to narrow down the range of pest weevils according to the crops in which they are found. However, paddock history and green bridges should also be taken into consideration when using this method.
A Back Pocket Guide containing descriptions of 11 key crop weevil species found in Australian broadacre systems has been produced by cesar and GRDC. Keep the guide on your phone or tablet by downloading it from: www.grdc.com.au/GRDC-BPG-CropWeevils
There are several foliar insecticides registered for the control of sitona weevil and some of these may also provide adequate control of grey-banded leaf weevil. However spraying of weevils may not be economically viable. Adults should only be controlled if large numbers are causing extensive leaf loss.
As a general rule, do not use foliar sprays against weevil larvae because, for many of them, their subterranean feeding habits prevent exposure.
For control of certain weevil larvae consider seed dressings. Chlorpyrifos seed dressings are registered against spotted vegetable weevil and spine-tailed weevil (Steriphus caudatus) in cereals in some states.
Click here for more detail information about common weevil pests, including their occurrence, lifecycle, behaviour, damage symptoms and management strategies.
Sources of field reports of weevils
Jack Edwards – Agronomist, D&J Rural (Central West Slopes & Plains)