Insect update: Swede midge, diamondback, flea beetles, lygus

August 19, 2015 - Issue 22

Flea beetles on canola pod.

Flea beetles on canola pod.

Swede midge larvae were found in canola flowers in Alberta, near the Saskatchewan border south of Highway 16. Numbers are very low, but the insect is there. Nothing can be done about swede midge at this stage of the season, but Alberta will increase its monitoring for 2016 and entomologists across the Prairies will work on practical management practices. Swede midge has spread from northeast Saskatchewan, where the first Prairie population of swede midge was found in 2007. Damage has remained very low for the most part. More on swede midge symptoms.

Diamondback moth larvae could reach economic levels in very late canola fields. The latest generation of adults is flying around now and looking for nice green (late) canola fields to lay their eggs. Eggs will hatch in a week and about a week after that larvae are big enough to cause damage. It takes 200-300 larvae per square metre to cause economic damage at the pod stage, but this could happen in these late fields if adults are attracted to them in high concentrations.

Flea beetles have been observed in high numbers in some fields. Adults emerge from pupae in August and overwinter to feed on young canola seedlings the following spring. These same adults do feed now, but Julie Soroka and Larry Grenkow (Can. J. Plant Sci. 2012: 97-107) found that flea beetle feeding on canola in late-summer is rarely an economic concern. In the report, they wrote: “Flea beetle feeding that occurs when seeds in lower pods of canola are at the green stage or beyond is unlikely to affect seed yields regardless of the infestation rate of flea beetles. Even when seeds are translucent to green, numbers higher than 100 flea beetles per plant, and for some cultivars higher than 350 per plant, may be necessary to cause significant yield reductions.” Concentrations this high may be found in very small areas. If growers decide to spot spray, heed pre-harvest intervals.

Lygus. Lygus bugs are still at high numbers in some fields, but note that once pods get mature and leathery, lygus can no longer penetrate them.

Honourable mentions. Even with very dry conditions this season, La Crete, Alberta, is seeing bertha at threshold levels. Berthas tend to be very spotty, so check fields even though most areas showed low adult populations this year. Grasshoppers are also at threatening numbers in some fields.

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