Finding holes and shredded leaves throughout your brassica crops, paddocks, and garden? The nasty critter responsible for the damage? The Plutella xylostella, or as you may know them— the Diamondback Moth (DBM).
Usually brown flecked with white, these moths do not look particularly intimidating at only 10 mm in length. However, they are prolific breeders. Each female can lay up to 150 eggs per season. The larvae are especially problematic as they usually attach to the underside of the leaf, slowly nibbling their way into the leaf sheath. In large numbers, they can strip the brassica in your garden or young brassica within a paddock of all its leaves in only three days.
The summer heat tends to slow their mating activity. But as summer days begin cooling, their activity in the crops increases, as do the number of eggs they lay. An effective tactic for dealing with DBM is through Integrated Pest Management (IPM). There are several predators who treat DBM as a Christmas feast, such as parasitoid wasps, lacewings, hoverflies, and ladybirds.
When the equilibrium between beneficials and pests is too wide, turning to a chemical option can help restore the balance. Taking a blanket approach of killing all insects in the field works against you. Fortunately, our modern chemistry is designed to target particular pests and there are a wide range of insecticides to target DBM that are also IPM friendly. Mixing an aphicide with some of the available actives on the market may be a good option, as well. A successful spray programme not only restores the equilibrium between predators and pests but helps extend your spray intervals.
When nature is working in harmony, it is a killing field for DBM as the predatory insects jockey for position. So, as you walk through your brassica crops, look at the bugs around you to identify the good from the bad before turning to a one-size fits all solution.
Predators at work
- Parasitoid Wasps: They use the bodies of lepidoptera (winged insects that include butterflies and moths) larvae as hosts for their eggs. Once hatched, they eat the host's internal organs, killing it in the process. Dissecting captured larvae reveals how common this is.
- Lacewing: They are general predators who attack soft-bodied pests that feed on brassicas. Both larvae and adult lacewings target DBM larvae by injecting venom and sucking out the fluids.
- Hoverflies: Voracious predators with a taste for soft-bodied insects, spearing prey with a hook-like mouth and feasting on their insides.
- Ladybirds: They use venom against their prey, like lacewings, but also eat eggs and larvae.
