Insect damage is a serious production risk for canola – but manageable today with a range of tactics.

Integrated control measures are the most effective and sustainable. These include green bridge management, surveillance, biological control and a judicious use of insecticide given the risk of promoting insecticide resistance.

GRDC-supported research at the Department of Primary industries and Regional Development is taking a methodical approach to these issues for the diamondback moth (DBM), Plutella xylostella L. This sporadic but serious migratory pest of canola has a voracious appetite and a predisposition to developing resistance to insecticides rapidly.

With GRDC support, DBM surveillance work is determining what role DBM moth and larvae presence in summer green bridges which contain brassica species such as radish, turnip and volunteer canola have in driving the severity of in-crop infestations.

The crop environment also needs consideration, as natural DBM biological control agents that play an important role in DBM population control may be present. For example, outbreaks of an entomopathogenic fungus (Zoophthora radicans) can reduce DBM numbers by 90 per cent in some years with warm and humid conditions. Parasitic wasps (such as Diadegma semiclausum, Apanteles ippeus, Diadromus collaris and Oomyzus sokolowskii) as well as brown and green lacewings, spiders and other predacious bugs, also reduce DBM populations by feeding on eggs, larvae and pupae.

To reduce yield loss to DBM, growers should monitor canola crops using a sweep net from late winter onwards, noting DBM presence and abundance  as well as beneficial insects, and only spraying when threshold levels are exceeded (Table 1) to allow natural predators to aid in control.

If spraying is warranted, select and rotate products according to the industry’s Resistance Management Strategy for DBM, to preserve the effective life of theseinsecticides.

Table 1: Diamondback moth larvae thresholds to guide insecticide spray decisions at various canola growth stages. Source GRDC

* Moisture stress is not listed for these growth stages, but note that moisture-stressed crops are more susceptible to insect damage. A lower threshold may be used if extended dry periods are expected.

Insecticide resistance

As part of the GRDC-supported research, the South Australian Research and Development Institute has investigated levels of insecticide resistance in DBM from the five port zone regions of Western Australia. In total, 21 populations were collected between spring 2020 and autumn 2021 from canola crops, forage rape crops, brassica vegetable crops or wild brassicaceous plant species. Populations were screened using phenotypic insecticide bioassays to determine mortality responses to five commercially available insecticides (Table 2).

Among the DBM population tested, some degree of reduced sensitivity was detected which may indicate a reduction in field efficacy. For example, synthetic pyrethroids (e.g., alpha-cypermethrin) should be avoided in canola crops where DBM is a problem, as DBM has moderate to high resistance to these insecticides (Table 2). Also, these broad-spectrum products destroy natural enemies, enabling DBM populations to increase more quickly.

The expected registration of Group 28 chemistries (chlorantraniliprole and cyclaniliprole) will provide a new insecticide option, however, due to its registration and use for DBM management in other agricultural commodities, there is already evidence of reduced sensitivity to this insecticide group.

Table 2. Mortality response ranges for diamondback moth (DBM) populations sourced from five port zones in Western Australia, at a discriminating dose (DD) of each chemistry. The DD is expected to kill 99.9 per cent of a DBM population that is not resistant, so mortality responses less than this value indicate some development of resistance.