Unknown damage in canola

We have received numerous enquires from agronomists concerned about damage to canola crops where no potential pests can be found. Adrian Tyler (Tyler’s Hardware & Rural Supplies) reports finding severely damage canola plants sporadically distributed across most of the paddock. Healthy plants within the crop are at the 4-leaf stage, however the damaged plants have had most of the leaves chewed off, and some have just a stem remaining. The pattern of damage within the paddock is quite unusual. There are no obvious hotspots or any clear pattern to the damaged areas. Adrian says the only invertebrates that can be found are high numbers of earthworms.

Agronomist, Greg Toomey (Landmark), has found 1-2 ha areas of heavily damaged canola within a paddock near Elmore, in the Northern Country district of Victoria. The hotspots have occurred across the paddock and are on a range of soil types. Slugs or earwigs were initially suspected as possible causes although no pests have been found in high numbers despite a large amount of trash present in the paddock.

A canola crop has been found with stunted seedlings and some bare areas near Donald, in the Wimmera district of Victoria. Agronomist, Kerry Bormann (JSA Independent), says the damage resembles that of wireworms or false wireworms with affected plants found to have a weakened point in the stem just below the soil surface. A high number of true wireworm beetles (Family: Elateridae) were found flying around during one visit to the paddock, however nothing was present a few days earlier and/or later. The paddock has been regularly monitored and no other potential pests have been found during any visits.

In these instances, it is important to establish which pest(s) is causing damage prior to deciding on the most appropriate control option. There are several potential strategies that may be used to determine the cause of the damage. The type of plant damage seen is the first thing which can help. Is the damage below ground, above ground or at the soil surface? Is the damage caused by sucking pests (e.g. leaf cupping, silvering, shrivelling) or chewing pests (e.g. scalloped leaves, leaf holes, stems cut through)?

The next step is to thoroughly search the area, looking on the plants, at the bases, digging around the plants and checking within the soil and any trash or stubble that is lying around. If insects are not easily found during the day they could be sheltering somewhere and may be much easier to find in the evening or at night. Checking the affected area after dark can often result in surprising observations. If there is still nothing readily observable, then setting some pitfall traps can be used. Pitfall traps can be a small plastic cup buried flush in the soil. A small amount of liquid in the bottom will help to contain the insects that fall into the trap. Traps should be left for at least 24 hours but preferably longer and are generally useful for catching insects that actively move across the ground.