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Crop advisor casebook: What’s causing the shot hole damage to these canola leaves?

A Crop Advisor's Solution from the June 5, 2018 issue of Grainews

Michael Brown.
photo: Supplied

Brent, a Saskatchewan producer, owns a 10,000-acre grain farm near Torquay, which is located about 16 kilometres north of the Canada-U.S. border. Brent called me in early July about his canola crop after he found insect-feeding damage on the plants’ leaves and oldest pods.

“I might have a bertha armyworm problem in my canola crop,” he said. “I’m worried the damaged pods will shatter.”

Because bertha armyworm is an important canola pest and poses a significant threat to farmers’ crops, I headed out to Brent’s farm that afternoon.

At the field, I found interveinal leaf feeding as well as pod feeding and scabbing. Since the canola was a straight cut variety, I was also worried about pod shattering.

To date, environmental conditions had been dry and warm. Also of note, in May, the region experienced strong southerly winds.

Bertha armyworm larvae are light green in colour with pale yellow stripes on the body’s sides. They can consume canola plant leaves, stems and pods. The larvae will often chew holes in leaves as well as pods, to consume the seeds. To determine if this field had a bertha armyworm infestation, we beat and shook the canola plants to dislodge larvae. However, we found no such larvae after diligently searching the ground and leaf litter.

Another pest we had to consider was the cabbage seedpod weevil. The larvae can cause scabbing on pods. An adult female weevil lays her eggs inside the pods — usually one egg per pod. The larva emerges and feeds on the canola seeds in the pod. At maturity, the larva chews a hole in the pod wall so it can exit the pod and drop to the soil for pupation. These exit holes on the pods are a telltale sign of weevil infestation. There were no exit holes on the pods in this crop.

When present in high populations, these larvae can eat the entire leaf, leaving the veins. This insect also causes feeding damage and scabbing on pods but doesn’t leave exit holes.
photo: Supplied

In addition, pods often appear misshapen with cabbage seedpod weevil infestation as the insects eat some seeds, leaving others to mature. Also, weevils usually don’t produce shot hole damage on leaves.

Flea beetles can be responsible for leaf-chewing damage; however, we couldn’t find any flea beetles or typical signs of feeding, such as notches in leaf edges or small, circular pits on the leaf surfaces.

I knew of another insect pest that feeds on leaves producing shot holes. When present in high populations, the larvae can eat the entire leaf, leaving the veins. This insect also causes feeding damage and scabbing on pods but doesn’t leave exit holes.

A quick sweep net sample removed any doubt of what insect pest had infested Brent’s canola crop.

Crop Advisor’s Solution: Strong southerly winds? This is a pest to watch

The conditions were perfect — warm, dry weather, strong southerly winds, and infested regions south of the border. And the signs were there, too: interveinal feeding damage on canola leaves producing shot holes, and feeding and scabbing on the oldest pods. This field had a diamondback moth larvae infestation.

The sweep net sample revealed pale yellow- to green-coloured larvae, about half an inch in length, and with forked posterior ends — diamondback moth larvae.

The moths overwinter in the United States and migrate north on southerly wind currents. Diamondback moths normally lay eggs on upper leaf surfaces, which hatch in four to eight days. Larvae can consume leaves, buds, flowers and pods.

Generally, damage due to larvae feeding on plant leaves is minor, with little effect on yield. It is bud, flower and pod feeding that causes yield losses.

That year, the hot, dry environment was favourable for increasing larvae populations, as these conditions keep parasitoid and fungal infections of diamondback moth larvae low. Also, there were no rain events that normally knock larvae off plants, drowning them or exposing them to predators.

After establishing a diamondback moth larvae presence in the field with a sweep net sample, we pulled out all canola plants within one square foot and banged them on the hood of Brent’s truck, allowing us to determine population size. In this case, the larva population exceeded the economic threshold. Thus, I recommended Brent spray an insecticide to control the population in this field.

That season, a Group 3A insecticide seed treatment, with the active ingredient deltamethrin, was applied for flea beetle control. Thus, I recommended Brent apply a Group 1B insecticide (chlorpyrifos) to control the diamondback moth larvae, as deltamethrin shouldn’t be applied more than once per season.

After Brent applied the insecticide, feeding and pod damage stopped, lessening our concern about pod shatter and yield loss. At harvest, because we saved the crop from more diamondback moth larvae damage, yield was also protected.

It’s important for producers to scout for diamondback moth larvae feeding damage, especially when strong winds are coming from the south.

Michael Brown works for Richardson Pioneer Ltd. in Estevan, Sask.

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