Articles for August 2019

SCOUT FOR DIAMONDBACK MOTH AND BERTHA ARMYWORM IN CANOLA

The diamondback moth migrates into ND and usually arrives in late May or early June in North Dakota. The complete life cycle takes about 32 days from egg to adult. There are several generations during a single growing season, so all different life stages (eggs, larvae, pupae, adults) can be found in the field at the same time.

The adult is small, about ½-inch long, drab brown in color and, at rest, the forewings of the male moth form three diamonds – hence the name diamondback moth. Females lay up to 160 eggs during the night. Eggs hatch in five to six days into pale yellowish-green caterpillars with a forked posterior end. The newly emerged larvae burrow into the leaf and mine the leaf for several days to a week. Then, the larvae exit the leaf and feed externally for another 7 to 14 days. When disturbed, the larvae thrash backwards violently and often drop from the plant on a strand of silk. The larvae pupate for 5 to 15 days in a white net-like cocoon attached to the leaves, stems or pods.

Larvae feed on the leaves, buds, flowers, seed pods, the green outer layer of the stems, and occasionally, the developing seeds. As leaves wilt and drop in late July to early August, larvae will feed on the stem, pods, and developing seeds. Damaged pods will not fill completely and may shatter. Severely damaged pods appear whitish in contrast to the normal yellowing and browning of ripening undamaged pods.

Scout fields for diamondback moth larvae by beating plants to dislodge the larvae into white buckets. After beating plants, count larvae in the bucket or dangling from plants on a silk thread. Again, check several locations per field. For the early flowering stage, insecticide applications may be justified at larval densities of 10 to 15 larvae per square foot. The action threshold for canola at the pod stage is an average of 20 larvae per square foot.

The best pest management strategies to avoid yield losses from diamondback moth include early field scouting for larvae, and judicious use of insecticides only when fields are above thresholds. For more information, consult the NDSU Extension publication on Diamondback moth in Canola: Biology and Integrated Pest Management E1346 (revised).

Bertha armyworm: The adult moth is about 1½ inches long and mainly grayish-black with a silvery-whitish kidney-shaped spot and with a silvery-whitish fringe on each forewing. Moths emerge from the overwintering pupae in mid-late June and emergence continues through early August. These night fliers are particularly attracted to blooming canola fields for their nectar and egg laying sites. Eggs are laid on the lower side of leaves in clusters of 50-500 eggs in a honeycomb pattern and hatch in about one week. The emerging larvae (1/10th of an inch) are usually green in color. Mature larvae are about 1½ inch long and vary in color from green to brown to velvety black. Larvae often hide underneath leaf litter and clumps of soil during the day, which makes them difficult to see. Larvae develop for six weeks and then drop to the ground in mid to late August to pupate. There is only one generation per year.

As the canola plant drops its leaves, the mature larvae (>½ inch) begin to feed directly on the pods which causes economic yield losses and premature shattering. Feeding injury by the mature larvae also accounts for 80 to 90 percent of the plant material consumed during a larvae’s life. Mature larvae will even continue to feed on pods in the swath. Fortunately, populations are kept low during most years due to natural environmental factors like harsh winters and the presence of biological control agents (diseases and parasites).

Thresholds would be 18 to 22 larvae per square yard, if leaf feeding is the extent of the damage observed. The key to controlling bertha armyworm is:

 Early detection of young larvae about ½ inch long in canola fields by regular scouting.

 Determining if fields are above economic thresholds for larvae.

 Fields above the economic threshold level should be sprayed once the hatch is complete and just before larvae move to the pods. Apply a well-timed insecticide in late evening when larvae are actively feeding.

 Use high volumes of water for good coverage of the dense canola canopy.

For more information, consult the NDSU Extension publication on Bertha armyworm in Canola: Biology and Integrated Pest Management E1347 (revised).

Please see the NDSU Extension E1143 2019 ND Field Crop Insect Management Guide for insecticides registered in canola. Please remember that blooming canola is attractive to bees, so insecticides should be applied in the late evening (preferred by honey bee keepers) or early morning to minimize negative effects of an insecticide on bees.