- 1 Important tips for best management
- 2 Overview of the lygus bug
- 3 Identification
- 4 Management | How do you manage lygus bugs?
- 5 Footnotes
A common pest in canola (and can be an issue in other crops), the lygus bug has piercing-sucking mouthparts which cause physical damage to the plant through their The juvenile or immature stage (ex. of an insect) which goes through incomplete metamorphosis. and adult stage by puncturing the tissue and sucking plant juices. There are economic thresholds to consider before potential insecticide applications for lygus bugs.
Important tips for best management
- Scout for lygus bugs under fair weather conditions (ex. sunny, low wind, above 15°C) and typically aiming between 10:00- 16:00) when the crop is at the bolting through late pod stages.
- Using a sweep net to collect samples and consult the An economic threshold is the level of infestation (ex. pest insect density) at which lost yield (ex. due to feeding/insect pest damage) exceeds the cost of the chemical and its application. charts, which consider application costs, canola prices, and plant development stage (end of flowering versus pod ripening) in determining the need for insecticidal control. Economic thresholds also help conserve and protect natural enemies
- The oldest (fourth and fifth A developmental stage within one life stage (ex. of an insect)) nymphs and adult lygus bugs are responsible for most of the feeding injury.
- Scout and manage cabbage seedpod weevil and lygus bug separately (and ensure to not confuse lygus bugs with alfalfa plant bugs).
Overview of the lygus bug
Lygus bugs refer to members of the genus Lygus within the plant bug family (Miridae). They are small, oval-shaped insects that feed on a range of crops and weeds. At least as many as four Lygus species are found on the Canadian prairies. They feed on the sap of new growth and reproductive tissues of plants. Host plants can include alfalfa, canola, lentils, potato, strawberries, vegetable crops, flax, hemp, faba bean, tree fruits, and weeds such as redroot pigweed, stinkweed, wild mustard and lamb’s-quarters. Lygus bugs are important pests of canola with both adult and The juvenile or immature stage (ex. of an insect) which goes through incomplete metamorphosis. (juvenile) stages causing economic damage 1,2,3,4 that can reach levels where insecticidal control is necessary.
Lygus bugs are always present in canola fields because they are native to the area. Yet widespread outbreaks (high populations) causing economic levels of damage are not common. In 1996, the first time canola was treated for a lygus bug infestation, about 4,000 hectares (10,000 acres) were sprayed in the Vulcan, Alberta area. In 1997, lygus bugs damaged canola in the Foothills, Newell, Vulcan, Willow Creek, Bonnyville and Peace River regions of Alberta and in the Meadow Lake area of Saskatchewan. Insecticide was applied to about 162,000 hectares (400,000 acres) for lygus bug control that year, yet crop damage was estimated to have exceeded $10 million. In 1998, more than 1,400,000 acres were sprayed to control lygus bugs in Alberta. Since then outbreaks tended to be isolated (ex. the Peace River region and isolated fields in south central Alberta in 2010), until 2012 when high populations were reported over a wider geographic area, including a large portion of Alberta and some fields Saskatchewan and Manitoba. Historically low populations of lygus bugs have generally been reported in western Canada from 2016 to 2019. Lygus have been reported in much higher numbers across this same region in 2021 5.
Life cycle of the lygus bug
- One generation in northern regions.
- Two generations in southern regions.
- Three generations in southern Manitoba.
In all regions, an extra generation is possible in years when an early spring and late fall occur. This is because lygus bugs emerge early in the spring, feed and begin to reproduce on early season host plants. This means an additional generation that can attack canola at bolting is possible.
Lygus bugs overwinter as adults by finding shelter under plant litter in shelterbelts, headlands, uncultivated areas and field margins. Field experiments conducted in southern and northern Alberta showed that in these insulated areas, temperatures can remain above minus five degrees Celsius, although air temperatures can dip below -30 degrees Celsius 8. Lygus bugs are sufficiently cold hardy and do not experience significant mortality until temperatures drop below -10 degrees Celsius 9. In northern Alberta, overwintered adults emerge in the early spring (e.g. early May) to seek out early growing vegetation such as budding willows, and then move onto rapidly growing winter annual weeds, such as stinkweed or flixweed. Overwintering adults can also be abundant in volunteer canola and early-seeded (spring) canola, especially if these crops are in budding or flower stage while other hosts are not yet available. After mating (in May through early July) females seek suitable host plants, such as budding alfalfa or canola, to lay their eggs on. Eggs are laid individually into the stems and leaves of host plants. The first nymphs appear around the end of May. In the south, the new generation adults first appear around the end of June. Nymphs (first to fifth instars) can emerge and develop on these earlier season weed hosts, but usually adults relocate to, or even emerge from overwintering in, alfalfa and clovers in mid-spring.
Once canola is at bolting to early flowering stages, lygus adults start to appear and, by mid-flowering, high numbers of lygus adults and nymphs can be present, with densities typically peaking during late flower to early pod stages. Late flower to early pod stages are the most critical canola stages for lygus bug damage to affect yield 10. Scout and assess whether populations are above economic thresholds from mid-flowering through pod-ripening stages. Under hot conditions small nymphs can reach damaging stages in seven to 10 days. In late summer, the new generation adults disperse from mature canola fields into other hosts and continue feeding until they migrate to overwintering sites (late in the season).
Growing regions where spring commences earlier, such as in southern Alberta, or years when spring is unusually warm and dry, can result in overwintered lygus emerging in mid-April and completing a generation in the spring on flixweed, hoary cress, stinkweed or alfalfa. In southern Manitoba an additional generation of lygus can arise in buckwheat into mid-September 7. Thus, in southern Alberta, lygus bug populations typically peak twice in the growing season, whereas lygus bugs in southern Manitoba can have three peaks 7.
Influence of environment
The damage caused by lygus bugs in canola is related to weather conditions at the time of the infestation. Weather, particularly temperature and precipitation, affects the development of both lygus bugs and the crop. If rainfall is abundant (more than 100 millimetres or four inches) from the time of bud formation to the end of flowering, and if growing conditions are adequate for the rest of the season, canola can compensate for much of the bud damage. Plants under moisture stress during this time may not be able to compensate for most of the feeding injury, so scouting to assess crop stage and conditions as well as population numbers is critical to ensure the appropriate threshold is not exceeded.
Adult lygus bugs are about three millimetres (0.1 inch) wide and six millimetres (0.2 inch) long. They have relatively long antennae and legs. They vary from pale green to reddish-brown to black and can range from a fairly uniform colour to a mottled appearance. Lygus bugs have a distinctive triangle or “V” shaped marking in the upper centre of their backs and membranous wing tips. Adults are active and fly readily when approached.
Ensure lygus bug adults are not confused with the bigger, longer alfalfa plant bug adults, which are similar in appearance (i.e. V- shape pattern on the back, green colour).
Lygus bug nymphs are light green and wingless. Young nymphs are often mistaken for aphids, which are similar in size and shape. However, lygus bug nymphs are much more active, are harder bodied and lack the cornicles (or “tail pipes”) of aphids. There are five A developmental stage within one life stage (ex. of an insect) stages of lygus nymphs, of which the youngest are bright green and very mobile and the older ones are darker coloured. Nymphs feed on new growth and reproductive parts of the plant. Third A developmental stage within one life stage (ex. of an insect) and older begin to develop wingpads (or wing buds) which are evident on the dorsal side of their bodies. Several black spots, usually five, become noticeable on the backs of nymphs as they moult or mature through instars before becoming adults.
Begin monitoring canola (for lygus bugs) from bolting through late podding stages (when seeds within the pods are firm). Collect samples in several locations in the field, including along or near the field margins. Taking more sweeps will provide a better assessment of pest populations. To collect an accurate sample, use a standard insect net of 38 centimetres (15 inches) in diameter, take ten 180° sweeps, and aim to sweep the buds, flowers and pods while moving forward. Count the number of lygus in the net. Scouting and sampling should be done under fair weather conditions (e.g. sunny, low wind, above 15 degrees Celsius) and typically between 10:00-16:00 to ensure lygus bugs are active within the canopy.
Use economic thresholds at the appropriate stage 10 which consider application costs and canola prices (as well as the canola development stage) to determine the need for a potential insecticide application or just continued scouting.
Identify and count third to fifth A developmental stage within one life stage (ex. of an insect) lygus nymphs (which have black spots on their backs) when sweep net sampling at late flower and early pod stages of canola development. Third A developmental stage within one life stage (ex. of an insect) or older nymphs and adults should be counted and compared to the economic thresholds at late flower stages because:
- Younger instars are more susceptible to rainfall and predation and may not reach adulthood. (However, keeping track of the population percentage below third A developmental stage within one life stage (ex. of an insect) can be useful in prediction the impending third A developmental stage within one life stage (ex. of an insect) population.)
- Third, fourth and fifth A developmental stage within one life stage (ex. of an insect) nymphs experiencing warm growing conditions can develop quickly (moving through an A developmental stage within one life stage (ex. of an insect) every two to three days).
- Both nymphs and adults are feed on (causing damage to) canola.
- Second, third, fourth and fifth A developmental stage within one life stage (ex. of an insect) lygus nymphs on mid-flowering canola will contribute to adult lygus bug populations at (the vulnerable) early pod stages.
In western Canada, four Lygus species have been observed destroying canola flower buds and seeds, and all four species are thought to be equally destructive:
- L. keltoni Schwartz and Foottit
- L. lineolaris (Palisot de Beauvois) (also call the tarnished plant bug)
- L. elisus Van Duzee (also call the pale legume bug)
- L. borealis (Kelton)
- In northern Alberta, L. keltoni and L. lineolaris species dominate.
- In southern Alberta, the L. elisus species dominated from 1999 to 2004, but L. keltoni and L. borealis were also present. However outbreaks in 2010 and 2011 were dominated by L. keltoni.
- L. lineolaris is very rare in the short grassland prairies of southern Alberta 6.
Type of crop damage
Adults and nymphs use a piercing-sucking mouthpart called a proboscis to feed on reproductive structures. All species of lygus bugs feed preferentially on buds, flowers, or developing seeds. Lygus are generalist herbivores attacking hundreds of plant species globally 13 .
Once a suitable plant structure is encountered, the proboscis is inserted into the tissue and various digestive enzymes are injected and then sucked back along with pre- digested plant nutrients 14.
Feeding punctures on the outside of pods and stems may ooze droplets of sap, causing an infested crop to become noticeably sticky. These droplets promote the entry of pathogens. Lygus feeding creates small, dark circular patches on the pod surface.
Feeding moves to other plants or younger tissue as canola seeds become firm, hard, and pod surfaces become difficult to pierce and feed upon.
Seeds that have been fed upon will collapse or shrink, darken and lose their quality and viability. Additional loss may occur if flowering is delayed by heavy feeding pressure or drought. In western Canada, lygus bugs typically damage up to seven per cent of the seed.
Adults and the oldest (fourth and fifth A developmental stage within one life stage (ex. of an insect)) nymphs are responsible for most of the feeding injury.
One instance in southern Manitoba (prior to 1998), canola yields were reduced by approximately 20% when lygus bugs (mainly L. lineolaris) were confined on Also referred to as Argentine canola, it is the species of canola currently commonly grown in Canada. between flowering to pod maturity 18.
Management | How do you manage lygus bugs?
Natural enemies and biological control
Lygus bugs, like other insects, have a suite of natural enemies that in most years help to keep populations from reaching pest levels, such as parasitic wasps and predatory insects.
A survey near Lethbridge in 2005-2006 found four species (Anaphes iole, a Telenomus species and two Polynema species) of parasitoid wasps (The insect order (the scientific classification more broad than Family) which includes sawflies, wasps, bees, and ants.: Mymaridae) that feed on lygus eggs 19. The average rate of lygus egg attack due to all of these parasitoids was nine per cent.
Some wasps also feed on lygus nymphs. Parasitoids of the genus Peristenus (The insect order (the scientific classification more broad than Family) which includes sawflies, wasps, bees, and ants.: Braconidae) caused 70 per cent The juvenile or immature stage (ex. of an insect) which goes through incomplete metamorphosis. mortality at peak times in some alfalfa sites as reported for P. braunae near Saskatoon 20. In southern Alberta, the dominant wasp species attacking spring and early summer first generation lygus nymphs is P. carcamoi, whereas the second generation is attacked mainly by P. broadbenti 21. In northern Alberta, P. braunae and the more common P. otaniae attack the first and only generation of Lygus nymphs occurring in alfalfa and clover hay mixes 22. In addition, parasitism rates by Peristenus from rearings of third to fifth A developmental stage within one life stage (ex. of an insect) lygus bugs ranged from 9.8 to 38.3 per cent in an alfalfa hay field in late June near Beaverlodge, Alberta.
Tachinid flies have also been reported to attack lygus bug adults in the Peace River region of Canada, although no specimens have been obtained to confirm the species 23.
Laboratory studies showed ladybird beetles (Coccinella trifasciata, C. septempunctata) consumed low numbers of third, fourth and fifth A developmental stage within one life stage (ex. of an insect) lygus bug nymphs whereas damselbugs (Nabis roseipennis, N. alternates), lacewing larvae (M. emuncta), and crab spiders (M. vatia) consumed greater numbers of lygus bug nymphs while also each demonstrating feeding preferences for different A developmental stage within one life stage (ex. of an insect) staged nymphs. This suggests that these general predators all play a role in suppressing lygus bugs in canola within the canopy. They may also have feeding niches with each predator, collectively suppressing lygus bug populations but preferring specific instars as prey. Other lab results found lacewing larva consumes seven lygus bug nymphs every 24 hours.
Therefore, it is important to conserve and protect these natural enemies by using economic thresholds to determine the need for insecticidal control.
A European wasp, Peristenus digoneutis, has been introduced into alfalfa fields in eastern North America where it parasitizes approximately 40 per cent of the tarnished plant bugs. One of the few parasitoids of lygus adults is a tachinid fly, Alophorella sp. Current research is in progress to evaluate the exotic P. digoneutis as a potential neoclassical biocontrol agent to assess the impact of this European species on lygus nymphs occurring in canola and to evaluate its impact on native communities of natural enemies.
Field management (cultural control)
Preseed and in-crop weed control and tillage reduces the availability of host weeds and volunteer canola host plants to early emerging overwintering adults and early hatching first generation nymphs. Control volunteer canola and other weed hosts that provide a food source or sites for egg laying. Weed hosts include flixweed, lamb’s quarters, wild mustard, stinkweed, redroot pigweed, kochia, Russian knapweed, and Russian thistle.
Lygus survivorship is lower in alfalfa compared to canola and they prefer to feed on canola compared to hairy, high glucosinolate mustard. Even so, lygus bug infestations still occur in mustard 20 and lygus bug populations in canola can exceed those in adjacent alfalfa fields, 24. There is an incorrect belief that lygus can reach pest status in canola after alfalfa hay is harvested nearby. However, a study over several site years in the parkland region (dominated by L. lineolaris in canola and L. borealis in alfalfa) including the three prairie provinces, found no such evidence, 24,13 though none of the sites experienced outbreak levels in the years of the study. Another study by Alberta Agriculture in 2014 -2016 found that the changes in Lygus spp. population numbers in neighbouring canola and alfalfa fields were largely independent of alfalfa harvest. The belief that lygus bugs move from alfalfa to canola following cutting of alfalfa is likely due to alfalfa harvest preceding natural population increases in canola fields rather than movement from alfalfa 25.
Hairy pods and relatively high concentrations of the glucosinolate sinalbin in Sinapis alba deterred L. lineolaris feeding, although these plant characteristics did not deter egg laying 26.
Economic thresholds should be considered for lygus bugs in canola at the end of the flowering and the early pod ripening stages. Once the pods have become tough or leathery and seeds have hardened, the cost of controlling lygus bugs may exceed the damage they will cause prior to harvest, so insecticide application is not warranted. Furthermore, the pre-harvest interval required after spraying may not allow application this late.
The commonly used An economic threshold is the level of infestation (ex. pest insect density) at which lost yield (ex. due to feeding/insect pest damage) exceeds the cost of the chemical and its application. tables 27 depict the number of lygus bugs (per 10 sweeps) at which the cost of controlling the pest equals the economic damage. However, they are based on research conducted on field studies between 1989-1993 in southern Manitoba using Westar (a canola A cultivar is a variant in a species developed through the intervention of humans (despite the term 'variety' often being incorrectly used to describe this). A cultivar can be open-pollinated type, hybrid, synthetic, composite, etc. grown in the 1980s and early 1990s) and using previous plant stand recommendations (of seven to 14 plants per square foot) 18. The thresholds were validated in 1999-2001 with the Q2 A cultivar is a variant in a species developed through the intervention of humans (despite the term 'variety' often being incorrectly used to describe this). A cultivar can be open-pollinated type, hybrid, synthetic, composite, etc. in Lethbridge, Beaverlodge and Dawson Creek. However, it was noted that as new canola cultivars are developed, plant tolerance to insect feeding damage (and therefore yield) may also change.
While the industry continues to utilize these An economic threshold is the level of infestation (ex. pest insect density) at which lost yield (ex. due to feeding/insect pest damage) exceeds the cost of the chemical and its application. tables 28,27, 29, they will likely soon be replaced with more recent research 30. The research – which is supported by several entomologists in Western Canada 31 and the 2021 Oilseed Crops section of the Western Committee on Crop Pests Guide to Integrated Control of Insect Pests of Crops 32 – has shown that a range of 2-3 lygus per sweep (or 20-30 lygus per 10 sweeps) is a more suitable recommendation for current practices and current canola cultivars, which appear to tolerate lygus damage better than older cultivars. Risk-averse growers can take action at the lower range (of 2 lygus per sweep or 20 lygus per 10 sweeps), while those willing to take on more risk and also increase protection of natural enemies by spraying in fewer instances, can take action only when lygus are over 3 per sweep or 30 lygus per 10 sweeps. Reducing the threshold because of increasing prices is not recommended for lygus or most pests because there is not a close strong linear relationship with most insect pests and yield throughout the range of pest abundance. For lygus, abundance below one per sweep may in fact stimulate yield.
Additional information on the updates to the economic thresholds for lygus in canola is available in this Canola Watch article.
Usually one application of a registered insecticide at the end of flowering (bloom 90% complete or more) or at pod formation will prevent most yield losses 33. Plot research has shown that insecticide applications applied at flowering stage do not control later emerging nymphs.
The exception to this is in the Peace River region where early, dry springs and unusually high densities of lygus bug adults can occasionally occur at bud stage. In this situation, high numbers of lygus bugs feeding on moisture-stressed canola at bud stage is suspected to result in a delay of flowering, so producers in that region should monitor in fields that fail to flower.
If soil moisture levels and rainfall are high at flowering, plants will likely be able to compensate for damage caused by lygus bug populations near or above economic thresholds, and control may not be necessary. Since plants under moisture stress during this plant development stage will usually be unable to compensate for most of the feeding injury, control may be required using economic thresholds.
In southern Alberta and Saskatchewan, lygus bug often co-occurs with the cabbage seedpod weevil, which attacks canola at the early flower stage.
A four-year study (initiated in 2012) assessing the impact of cabbage seedpod weevil control on lygus bugs found that spraying for weevils at the early flower stage (earlier than the optimum stage for lygus bug feeding), reduced lygus bug abundance at most of the sites, but yield responses were inconsistent. Therefore, scouting for and management of cabbage seedpod weevil and lygus bug should be considered separately.
Chemical control options
Check Manitoba, Saskatchewan or Alberta or other provincial crop protection guides for a current list of registered insecticides which can be used for lygus bug in canola and always follow product label directions. Once populations have reached An economic threshold is the level of infestation (ex. pest insect density) at which lost yield (ex. due to feeding/insect pest damage) exceeds the cost of the chemical and its application. levels, insecticide application is the only option for control of lygus bugs.
To reduce direct exposure to pollinators, apply insecticides very late in the evening or early morning when bees are not foraging and the crop has completed at least 90 per cent bloom. When treating for lygus, prevent spay drift from moving onto beehives, blooming weeds or surrounding fields. Notify beekeepers of your intention to treat the field 48 hours in advance of spraying. Try to select an insecticide that has a short residual activity to reduce the impact on pollinators.
Table 3. Insecticides registered for Lygus spp. in canola
|Product||Chemical Group||Chemical Group Number||Active Ingredient||Pre-harvest Interval||Application||LD50 Mammalian Toxicity1|
|Decis, Poleci 2.5 EC||Pyrethroid||3||deltamethrin||06/07/2022||Aerial or ground||395|
|Matador, Silencer||Pyrethroid||3||lambda-cyhalothrin||06/07/2022||Aerial or ground||278|
|LorsbanNT, Pyrinex, Nufos, Citadel, Warhawk480EC, Sharphos||Organophosphate||1B||chlopyrifos||20/07/2022||Aerial or ground||220-776|
|Voliam Xpress||Pyrethroid, Diamide||3,28||lambda-cyhalothrin, chlorantraniliprole||06/07/2022||Aerial or ground||98|
1 LD50 values represent the relative toxicity of a pesticide. They represent the dose (in mg/kg body weight) that will kill 50% of the test animals. Thus the lower the number the greater the toxicity. Values given are for oral female rat LD50.
Source: Crop Protection 2020. Alberta Agriculture and Forestry
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