Lygus bugs – scouting, timing and thresholds

Lygus bugs have piercing-sucking mouthparts. They use these to puncture plant tissue and suck plant juices. They also inject salivary enzymes that, in some cases, cause more damage than their mouthparts. Lygus at the third instar or larger can damage buds, flowers and seeds. Lygus at the third instar or larger feed on the developing seeds, causing them to discolour, deflate 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. In thick crops, take a couple of steps and stand while sweeping vigorously. If you are not seeing a few pods in the net, sweep harder. Count the number of big lygus in the net. Sweep net technique.

The Prairie Pest Monitoring Network notes that sampling becomes more representative if repeated at multiple spots within a field. You don’t want to be making management decision based on a single spot. For lygus bug monitoring, sampling is most accurate when repeated at a total of 15 spots within the field. Observation of lygus have shown that lygus populations are relatively evenly distributed across canola fields at podding, and sweeping protocols equivalent to cabbage seedpod weevil (four spots per field, at two paired locations) is likely appropriate for most fields.

Recent studies have confirmed that lygus DO NOT migrate into canola from cut alfalfa hay in the Prairies, although this had historically been considered as a risk factor that might drive lygus numbers up. Lygus complete the first generation on weeds and native plants and the new adults migrate from there to crops in flower. These crops include canola, flax, faba bean and many others.

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 any sign of a dark growth where wings are developing. Also make sure not to confuse them with alfalfa plant bugs or aphids.

Small nymphs (without black dots) 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 other insects eat them. If you are seeing a lot of earlier nymphs, check again every few days. It may take as little as 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 are unlikely to be a threat before the crop is cut.

Lygus adult and nymph in a sweep net. Credit: Keith Gabert
This close up of a third instar shows the wingpads developing on the back. Count these. Credit: Dan Johnson
Sap from lygus nymph feeding site.
Here’s a close up view of this early instar lygus. Don’t count these. Credit: Dan Johnson
Lygus keltoni species – adult right, fifth instar left

Lygus life cycle

In southern regions of the Prairies, the majority of lygus that feed on canola at the pod stage are the offspring of adults that entered canola at the late bud and early flower stages and laid eggs within canola tissue. For northern areas, overwintered adults enter canola. The success of this overwintered generation dictates the level of damage or concern growers may have with lygus as their crop comes out of flower.

Long warm seasons may allow a third generation in the southernmost Prairies. However, sustained very warm temperatures (>30°C) can actually reduce egg production and shorten the lives of (L. lineolaris) adults.

Small nymphs can reach damaging size (third instar) in as little as seven to 10 days under warm dry conditions. Most of the potential damage occurs from the larger mouth parts of fourth and fifth instars and adults. Small mouthparts require lygus to focus on softer tissue. As seeds and pods harden, lygus will keep moving to progressively younger tissue. Historic thresholds focused on the crop period between the end of flowering and the time period that the seeds in the lower main stem remain soft. When only the youngest pods at the tips of canola racemes are left, the threat from lygus is diminished as these late pods may not contribute significantly to yield. This race to maturity requires growers to identify if a population of lygus will mature to beyond the third instar before canola seed matures beyond the soft green stage.

Thresholds

A threshold of 20-30 per 10 sweeps is suitable for good growing conditions. Agronomists in the Black Soil Zone of Alberta with good soil moisture have in the past considered 50-80 per 10 sweeps to be a more likely threshold. You might even go higher. Threshold tables developed from lygus research in the 1980s and 1990s were very specific about the estimated damage of each lygus bug in 10 sweeps relative to crop price and cost of control. That level of precision below one lygus per sweep is deemed inaccurate in light of new data from Agriculture and Agri-Food Canada lygus research in the recent years.

In dry conditions: A lower threshold may be suitable for dry conditions, but since drought also restricts canola yield potential, growers should be cautious about spraying under the established threshold of 20-30 per 10 sweeps. Using the lower end of the threshold (about 20 per 10 sweeps) may be appropriate for stressed canola with less ability to compensate for feeding. Research indicates that lygus numbers below 10 per 10 sweeps (one per sweep) can on occasion increase yield in good growing conditions – likely through plant compensation for a small amount of feeding stress. Numbers below 10 per 10 sweeps are unlikely to affect yield even under dry conditions, although this has not been researched. Spraying below 10 per 10 sweeps as indicated on the older threshold charts would not be advised, regardless of canola price.

In high moisture and high yield conditions: The ability of canola to compensate for some feeding damage from lygus is relatively high. Research from Wise and Lamb (1998) that generated the lygus threshold charts acknowledge cases of limited yield impact on canola given adequate moisture. In “The Canadian Entomologist 130” pages 825-836 in 1998, they wrote: “When precipitation is greater than 100mm 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 for crop protection efforts against lygus. The most vulnerable crop stage for lygus feeding is after flowering and when seeds are enlarging on lower pods. (This would be around stage 5.1 – firm green seed at bottom of main stem.) 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; avoid windy and rainy conditions.

Lygus thresholds for the bud and flower stages are not needed because economic loss is rare at these stages, and later lygus populations are not accurately predicted from early season sweep counts. Research shows that low level lygus feeding at flowering can actually stimulate yield. The rare 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 at bolting because the buds are damaged or blasted.

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

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.

Pod damage concern. Research observations indicate that discrete holes from lygus feeding don’t compromise pod integrity significantly. The risk of pod shatter due to insects is higher following feeding by cabbage seedpod weevil (CSPW) and their feeding and exit hole damage, diamondback pod stripping or even late season flea beetle pod feeding. Lygus are less likely to cause pod shatter than these other insects.

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 applying insecticides.

Lygus and cabbage seedpod weevil. According to recent research by AAFC in Alberta, cabbage seedpod weevil (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.

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