Two recent reports of native cockroaches damaging canola are unusual, but can be related to retained stubble and possibly a warm autumn.
Where have they been reported?
Within the past fortnight, reports of cockroaches possibly feeding on canola cotyledons have been received from near Normanville (Victoria’s Northern Country) and Hopetoun (Victorian Mallee).
In both cases the cotyledons had either been chewed or completed nipped off, leaving only a stem. There was no direct evidence that the cockroaches caused this damage, but in the Hopetoun crop, they were quite abundant and there were no other pests found.
In Normanville, vegetable beetles were also present and probably contributed to the damage.
Both locations had stubble coverage close to the drill rows that is likely to have been a major factor in their presence.
About native cockroaches
More than 500 species of cockroaches have been recognised in Australia, over 90% of which are endemic. They are extremely diverse in behaviour and occupy just about every type of habitat.
The identity of the pest species apparently causing the damage to canola is unknown.
In Queensland, wingless (native) cockroaches (Calolampra spp.) are recognised pests of seedling field crops. Cockroaches are probably less common in south-eastern Australia as they prefer warmer conditions.
In cropping environments within south-eastern Australia, native cockroaches have not previously been reported as a pest. The majority of species are detritivores that prefer to break down dead organic matter.
However, increasing stubble loads within newly seeded paddocks provide the moist, dark and sheltered habitat that these insects prefer, and damage to the emerging seedlings is possibly opportunistic.
The damage caused by cockroaches this year may be a reflection of the warmer than average autumn. As such, cockroaches are unlikely to become a major pest in crops even within high stubble loads.
Control is unlikely to be warranted.
Management of stubble is the key to reducing feeding damage. Insecticide treated seed may also reduce damage to emerging crop seedlings.
Sources of field reports of cockroaches
Bronwyn Hunt – Grower, Merriwa Pastoral Company (Victoria’s Northern Country)
John Stuchbery – Agronomist, AGRIvision Consultants (Victorian Mallee)