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Diamondback Moth in 2018 Growing Season

By Maryna Van Stavere, Summer Student, Moose Jaw

July 2018

Diamondback Moth is one of the major early season pests of the brassica family. Its damage intensity varies depending on the density of its population and pressure from its natural enemies. It may overwinter in the prairies; however, it mainly migrates into Canada through May to July from the southern or western U.S. Diamondback moth, being a multivoltine species, produces as many as four generations per year. Early arrival of these pests results in a higher number of generations and therefore a higher risk of economic damage. Adult pests do not cause economic damage to the crop but they lay eggs on the leaves of canola. The larvae then hatch within five or six days and begin to feed on the leaves and later on flower buds and pods, causing significant damage to the future yield.

The adult moth is small, approximately 10-12 mm in length and greyish-brown with a 12 mm wingspan covered in long hairs. The moth’s defining physical characteristic can be observed when it is at rest, for as it holds its wings together, a pattern of three yellow diamond-shaped spots can be seen along the top of the moth’s body. Its larvae are yellow-green in colour, covered with short hairs and are five to 12 mm long. Upon physical contact, the caterpillars tend to descend off of the plants and dangle from a silken thread. The caterpillar’s defining characteristics are its tapered ends and forked posterior.  The larva pupae is initially light green however upon maturity a brown adult moth becomes visible through a delicate white cocoon. This stage usually lasts five to 15 days, with warmer conditions aiding in faster maturity.

The worst damage occurs in the second and third generations, during mid-July to early-August, where the older larvae feeds on canola flowers, pods and stems. Larvae feeds on the internal leaf tissue and upon maturity move onto the outside of the leaf, leaving tan-coloured blotches on the plant.  Feeding tends to last for 10 to 30 days, depending on outdoor temperatures. Feeding during the early flowering stage will delay plant maturity and cause uneven development of the crop canopy. Larvae will typically prefer to feed on the flower bud prior to feeding on the pods. Damaged pods may be subjected to premature shattering.

Scouting for diamondback moth should be done weekly from mid-July to early August. When scouting the field, monitor at least five one-square-metre sections of the crop. Early damage can be observed in the ridges and knolls of the field in a form of abnormal whitening. Vigorously shake the crop canopy or pull plants in each of the chosen sections and count the larvae on the plants and the ground, as well as under the leaves and in the plant debris. It is important to keep in mind that the environmental conditions will determine the amount of eggs laid and the chance of larvae’s survival. Heavy rainfall washes young larvae off the leaves to the ground with a chance of drowning the pest all together; cold and windy temperatures slow the eggs’ maturity and reduces the adults’ activity. Ongoing humid conditions may cause the outbreak of Entomophthorales, a fungal disease that occurs in the later growing season during high diamondback moth populations, limiting the development of larvae into adults.

The economic threshold for diamondback moth varies upon the stage of the crop; with it being 25 to 30 per cent leaf damage at the seedling stage, 100 to 150 larvae per square meter during the flowering stage and 200 to 300 larvae per square metre at the pod stage. 

There are currently three parasitoid species of parasitic wasps that aggressively prey upon diamondback moth. Cresson ( Diadegma insulare), Muesebeck (Microplitis plutellae) which prey upon the larvae, and Gravenhorst ( Diadromus subtilicorinis); which feeds upon the prepupal and pupal stages. Other natural enemies include flies, lacewings, pirate bugs, beetles, spiders and birds. Timing of foliar application is key in successfully reducing the pest’s population. Insecticide application targeting the larvae should be applied once the economic threshold is exceeded. Once an infestation is successfully controlled at the podding stage, a new infestation is unlikely to occur due to the later stage of the crop. Controlling volunteer canola and other weeds of the brassica family will rid of additional hosts for the diamondback moth adults to continue their life cycle. Keeping updated with provincial agricultural websites for ongoing forecasting of the pest activity will aid in determining the early numbers in the population.

For further information, please refer to the Ministry of Agriculture’s 2018 Guide to Crop Protection, Prairie Pest Monitoring Network Blog, the Agriculture Knowledge Centre at 1-866-457-2377 or by email at aginfo@gov.sk.ca.

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