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Diamondback Moth and Cabbage Moth Found in Canola (06/16/16)

Larvae of the diamondback moth and imported cabbage moth have been observed on canola fields near Langdon in Cavalier County.

Diamondback Moth and Cabbage Moth Found in Canola

Larvae of the ent.knodel.5 6diamondback moth and imported cabbage moth have been observed on canola fields near Langdon in Cavalier County. Both larvae feed on the leaves causing defoliation and holes in leaves. Feeding injury by diamondback moth larvae have a characteristic windowpane effect and small, irregular-shaped holes. Larvae of diamondback moth are lime green and about ½ inch long with a forked posterior end. When disturbed, larvae thrash backward violently and often drop from the plant, suspended on a strand of silk. Larvae of the imported cabbageworm also are lime green with a white line down the side and about 1 inch long and larger. It develops into a cabbage butterfly as an adult. Both larvae are cryptic ‘green’ and blend in with the canola leaves making them difficult to see.

It is important to note that these species of larvae are present in the canola crop; however, significant damage is caused by the subsequent generations that emerge later during flowering and pod development. For diamondback moth, larval feeding on flower buds and flowers causes flowers to abort, and can results in the most significant injury and subsequent yield loss, especially during drought (not current conditions in northeast area). There are usually not high enough numbers of imported cabbageworms to cause yield loss in canola in North Dakota. In Canada, they have observed 8-10 larvae per plant without needing to spray, because parasitism rates are usually high in imported cabbageworms.

For diamondback moth, scout fields as we get closer to flowering by pulling up canola plants from a square foot and beat them in a white bucket. Then, count the number of larvae dislodged from plants. Larvae often will dangle from canola plants on a silk thread. Repeat this procedure in at least five locations in the field to obtain an average of the number of larvae per square foot. The economic threshold is based on larval densities of:

  • Flowering: 10 to 15 larvae per square foot;
  • Pod stage: 20 to 30 larvae per square foot.

 Anitha Chirumamilla

Ag Extension Agent, Cavalier County

&

Janet Knodel

Extension Entomologist

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