Lygus: Scouting, thresholds and timing

August 1, 2018 – Issue 18

August 9, 2018 – Issue 19

Lygus bugs have piercing-sucking mouthparts and physically damage the plant by puncturing the tissue and sucking plant juices. They puncture seed pods and feed on the developing seeds causing them to turn brown and shrivel.

Scouting

Scout lygus at late flowering and podding stages using a standard insect sweep net of 38 cm (15″) diameter. Take ten 180° sweeps, and aim to sweep the flowers and pods while moving forward. Count the number of lygus in the net.

The Prairie Pest Monitoring Network notes that sampling becomes more representative IF repeated at multiple spots within a field. For lygus bug monitoring, sampling is most accurate when repeated at a total of 15 spots within the field. Samples can be taken along or near the field margins.

Since adults can move into canola from alfalfa, check lygus bug numbers in canola when nearby alfalfa crops are cut.

Count adults and late-instar nymphs. Nymphs are young lygus, and only the larger nymphs do enough damage to be included in sweep net counts. A key feature is the black dots on the back. Count nymphs with developing wingpads or dark shoulder blades. Don’t count them if they’re small and don’t have the any sign of a dark growth where wings will be developing soon.

These small nymphs don’t feed very aggressively, and they are vulnerable to heavy rains and wind that push them down in the canopy or to the ground where they drown or get eaten by other insects. If you are seeing a lot of earlier nymphs, check again every few days. It may take only a week for lygus to grow from early to late instar stages. If swathing is a week away and most lygus are small nymphs, these nymphs are unlikely to have time to grow into a threat before the crop is cut.

This close up of a third instar shows the wingpads developing on the back. Count these. Credit: Dan Johnson

Here’s a close up view of this early instar lygus. Don’t count these. Credit: Dan Johnson

Adult lygus. Credit: Dan Johnson

Thresholds

In very dry conditions: Threshold tables for lygus indicate that if canola is $12 per bushel and spray costs $8 per acre, the threshold at the early pod stage is 5 lygus adults or late instar nymphs per 10 sweeps (0.5 per sweep).

In moist and high-yield conditions: The economic threshold is quite a bit higher. At early pod stage, 50 lygus per 10 sweeps (5 per sweep) could cause a 2 bu./ac. reduction in yield — which could be a more suitable economic threshold in this situation. At late pod stage (the last week or so before cutting), pods are too tough to penetrate.

Wise and Lamb acknowledged the possible growing condition effect in their research paper used to set the lygus thresholds. In “The Canadian Entomologist 130” pages 825-836 in 1998, they wrote: “When precipitation is greater than 100 mm from the onset of bud formation to the end of flowering, the crop may partially compensate for plant bug damage.”

Timing

Pods are the focus. The most vulnerable crop stage for lygus feeding is after flowering and when seeds are enlarged on lower pods. (This would be around stage 5.2 for a dense stand and 5.1 for a thinner stand.) After sweep netting, look for sticky sap spots on pods, pedicels and stems before making the spray decision. Oozing on pods suggests active feeding. If sweep net counts exceed thresholds, but there is no evidence of pod damage, growers could decide to hold off on spraying. Alternatively, if evidence of feeding is present but sweep net counts are low, try sweep netting again in another area or another time of day. Counts can be dependent on weather conditions in the sweep zone.

Lygus thresholds for the bud and flower stages are not needed because economic loss is rare at these stages. Research shows that low level lygus feeding at flowering can actually stimulate yield. The only situation where spraying may be warranted at flowering stages is if numbers are so high that large areas of the crop are not coming into flower because the flowers are being eaten.

The threat generally ends when most pods become “leathery” and when seeds inside are no longer squishy. As outer pod tissue toughens up and seeds become firm to roll, lygus can no longer penetrate the pods.

Other considerations

Rainfall. Rain improves crop growth, lowering the risk from lygus. A heavy rain at podding will also knock lygus to the ground, where ground beetles and other beneficial insects can eat them.

Other pests present. If lygus are at or above thresholds and other pests, such as diamondback moth larvae, are also feeding on pods and are near thresholds, this combination improves the potential economic return from a spray.

Lygus and cabbage seedpod weevil. According to recent research by AAFC in Alberta, CSPW and lygus do not become pests in the same fields. Early planted fields with high CSPW have very low lygus populations. Thus, there is no economic return to spray for lygus at early flower in early seeded fields.

Further reading:

PPMN lygus bug monitoring protocols

Canola Watch