Lygus bug

Table of contents

    Important Tips for Best Management

    Monitor fields adjacent to alfalfa fields, where overwintering lygus bug populations may be high. Scout for lygus using a sweep net from bolting through pod ripening stages. Sweep-net monitoring should be done under fair weather conditions (e.g., sunny, low wind, above 15°C and typically aiming between 10:00-16:00). Consult the economic threshold charts, which consider application costs, canola prices, and plant development stage (end of flowering versus pod ripening). Adults and the oldest (fourth and fifth instar) nymphs are responsible for most of the feeding injury. Properly identify lygus adults to differentiate from alfalfa plant bug adults. Conserve and protect natural enemies by using economic thresholds to determine the need for insecticidal control. Scout and manage cabbage seedpod weevil and lygus bug separately.

    About Lygus Bug

    Plant bugs belonging to the genus Lygus (Hahn) are small, oval-shaped insects that feed on a variety of crops and weeds. Commonly referred to as lygus bugs with as many as 4 different species found on the Canadian prairies, they feed on the sap of new growth and reproductive tissue. Host plants of lygus bugs include alfalfa, canola, lentils, potato, strawberries, vegetable crops, flax, hemp, fababean, 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 juvenile (nymph) stages causing economic damage [1],[2],[3],[4] that can reach levels where insecticidal control is necessary.

    Background

    Lygus Bug

    Lygus Adult

    Lygus bugs are always present in canola fields because they are native, yet widespread outbreaks of high populations causing economic levels of damage are not common. In 1996, about 4,000 ha (10,000 ac) were sprayed (the first time canola was treated for a lygus bug infestation) in the area of Vulcan, AB. In 1997, lygus bugs damaged canola in the Foothills, Newell, Vulcan, Willow Creek, Bonnyville and Peace River region of Alberta and in the Meadow Lake area of Saskatchewan. Insecticide was applied to about 162,000 ha (400,000 ac) 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 (e.g. 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 AB and some fields SK and MB.

    Life Cycle

    Lygus bugs have one to three generations per year, depending on the geographical region of the Prairies [9],[10]:

    • 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 related to the fact that Lygus emerge early in the spring, feed and begin to reproduce on early season hosts such as alfalfa. 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 -5 degrees C although air temperatures can dip below -30 degrees C. [11] Lygus bugs are sufficiently cold-hardy and do not experience significant mortality till temperatures drop below -10°C. (Cárcamo and Herle unpublished data). In northern Alberta, overwintered adults emerge in the early spring (e.g., early May) to seek out early growing vegetation such as willows breaking bud, and then move onto rapidly growing winter annual weeds like stinkweed or flixweed. Overwintering adults can also be abundant in volunteer canola, early spring seeded canola, especially if these crops are in bud or flower and other hosts are not yet available. After mating in May through early July females seek suitable host plants, such as budding alfalfa or canola, on which to lay their eggs. Eggs are laid individually into the stems and leaves of host plants. The first nymphs appear by about the end of May. In the south, the new generation adults first appear by about 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.

    ACTION: 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. Once canola is at bolting to early flowering stages, lygus adults start to arrive 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 damage to affect yield. [12]

    ACTION: Scout and assess whether populations are in excess of economic thresholds from mid flowering through pod ripening stages. Under hot conditions small nymphs can reach damaging stages in 7-10 days. In late summer, the new generation adults disperse from mature canola fields into later maturing hosts, such as alfalfa, and continue feeding until they migrate to overwintering sites. Growing regions where spring commences earlier in the calendar, 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 such cases, newly molted adults relocate from maturing weedy hosts or swathed alfalfa hay crops to canola at early to mid-flower stages, where they mate and lay eggs. This results in lygus adults and nymphs occurring in canola from late-flower to early pod stages in southern Alberta. In southern Manitoba an additional generation of lygus can arise in buckwheat into mid-September. [10] Thus, in southern Alberta lygus bugs typically peak twice through the growing season whereas lygus bugs in southern Manitoba can be three generational. [10]

    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 mm or 4") 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.

    Identify lygus bug

    Adult lygus bugs are about 3 mm (0.1") wide and 6 mm (0.2") long. They have relatively long antennae and legs. They vary from pale green to reddish brown to black and can be fairly uniform colour ranging 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.

    Immature lygus bugs (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 ("tail pipes") of aphids. Nymphs feed on new growth and reproductive parts of the plant. Several black spots, usually five, become noticeable on the backs of nymphs as they moult or mature through five instars (growth stages) before becoming adults. Wing buds are evident on the dorsal side of fourth and fifth instars.

    Scouting techniques

    Begin monitoring canola when it bolts and continue until seeds within the pods are firm. Use the Sequential Sampling chart [12] to determine the need for further, detailed scouting.

    If the total number is below the lower threshold line, no treatment is needed. If the total is below the upper threshold line, take more samples. If the total is on or above the upper threshold line, calculate the average number of lygus per 10-sweep sample and consult the economic threshold table.

    When: Start scouting fields at the late rosette stage.

    Where: Sampling several locations in the field and taking more sweeps will provide a better assessment of pest populations. Samples can be taken along or near the field margins.

    How: With a standard insect net of 38 cm (15") 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.

    Action Threshold: Consult the economic threshold charts, which consider application costs, canola prices, and plant development stage.

    Sweep-net monitoring should be done under fair weather conditions (e.g., sunny, low wind, above 15°C and typically aiming between 10:00-16:00) to ensure Lygus are active within the canopy.

    Properly identify lygus adults from Alfalfa plant bug adults, which are similar in appearance (i.e., V- shape pattern, green colour, but are bigger and especially longer in size compared to Lygus). There are five instar or juvenile stages of lygus nymphs – all are bright green and very mobile.

    Identify and count third to fifth instar lygus nymphs when sweep-net sampling at late flower and early pod stages of canola development. Third to fifth instar nymphs will develop black spots on their backs. Nymphs and adults should be counted and compared to the economic threshold tables at late flower stages because:

    • Third, fourth and fifth instar nymphs experiencing warm growing conditions can develop through an instar stage every 2-3 days.
    • Both nymphs and adults are sucking insects that feed on canola.
    • When sweep-net sampling at mid-flowering, second, third, fourth and fifth instar lygus nymphs will contribute to adult lygus bug populations at the vulnerable early pod stages.


    Damaging species and staging

    In western Canada, four species have been observed destroying canola flower buds and seeds, and all four species are thought to be equally destructive:

    • L. keltoni Schwartz,
    • L. lineolaris (Palisot de Beauvois), (tarnished plant bug),
    • L. elisus Van Dusee,
    • L. borealis (Kelton),

    Various species can compose a varying “complex” that appears to change by host plant, by region, or even seasonally [9],[13],[6],[14]:

    • In northern Alberta, the Lygus “complex” is dominated by L. keltoni and L. lineolaris.
    • In southern Alberta, pan trap catches from 1999 to 2004 indicated that the Lygus complex is dominated by L. elisus, but L. keltoni and L. borealis were also present. In 2010 and 2011 outbreaks in canola in southern Alberta were dominated by L. keltoni.
    • L. lineolaris is very rare in the short grassland prairies of southern Alberta. [9]


    Type of Crop Damage

    Adults and nymphs feed by sucking on reproductive structures using a piercing-sucking mouthpart called a proboscis. All species of lygus bugs feed preferentially on either buds, flowers, or developing seeds. Lygus are generalist herbivores attacking hundreds of plant species globally. [14]

    Once a suitable plant structure is encountered, the proboscis is inserted into the tissue and various digestive enzymes are injected then sucked back along with pre- digested plant nutrients. [22]

    Feeding on buds, flowers and young pods causes "blasting" (buds turn white and fail to develop), flowers fall without forming pods or pods drop without maturing. [1],[2],[4],[8] 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.

    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 7% of the seed.

    Adults and the oldest (fourth and fifth instar) nymphs are responsible for most of the feeding injury.

    In southern Manitoba, yields were reduced by approximately 20% when lygus bugs (mainly L. lineolaris) were confined on B. napus between flowering to pod maturity. [7]


    Minimize infestations

    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.

    Parasitic wasps:A survey near Lethbridge in 2005-2006 found four species of parasitoid wasps (Hymenoptera: Mymaridae) that feed on lygus eggs. [36] The average rate of lygus egg attack due to all of these parasitoids was 9%.

    • Anaphes iole,
    • Telenomus sp.
    • Polynema (2 species)

    Wasps also feed on nymphs:

    • Parasitoids of the genus Peristenus (Hymenoptera: Braconidae) caused 70% nymph mortality at peak times in some alfalfa sites as reported for P. braunae near Saskatoon. [27]
    • 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. [37]
    • 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. [37]
    • Parasitism rates by Peristenus from rearings of third to fifth instar Lygus ranged from 9.8% to 38.3% in an alfalfa hay field in late June near Beaverlodge, Alberta.

    Tachinid flies have been reported to attack Lygus adults in the Peace River region of Canada, although no specimens have been obtained to confirm the species (Otani, unpublished data).

    Predators:Laboratory studies showed ladybird beetles (Coccinella trifasciata, C. septempunctata) consumed low numbers of third, fourth and fifth instar Lygus nymphs whereas Damselbugs (Nabis roseipennis, N. alternates), Lacewing larvae (M. emuncta), and Crab spiders (M. vatia) consumed greater numbers of Lygus nymphs while also each demonstrating feeding preferences for different instar staged nymphs. This suggests that these general predators:

    • all play a role in suppressing lygus in canola within the canopy, and
    • may have “feeding niches” with each predator collectively suppressing lygus populations but preferring specific nymphal instar stages as prey.

    Lab results found a lacewing larva consumes seven lygus bug nymphs every 24 hours.

    Action: Conserve and protect these natural enemies by using economic thresholds to determine the need for insecticidal control.

    Biological control:

    A European wasp, Peristenus digoneutis, has been introduced into alfalfa fields in eastern North America where it parasitizes about 40% 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)

    Weed 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.

    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 [27] and lygus bug populations in canola can exceed that observed in adjacent alfalfa fields. [15]

    Monitor fields adjacent to alfalfa fields, where overwintering lygus bug populations may be high.

    There is a common 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, [14] though none of the sites experienced outbreak levels in the years of the study. Similar results were observed in southern Alberta at Vauxhall with the local lygus species.

    Resistance breeding

    Hairy pods and relatively high concentrations of the glucosinolate, sinalbin, in S. alba deterred L. lineolaris feeding, although these plant characteristics did not deter egg laying. [24] (Bodnaryk 1996).


    Reduce economic losses

    Economic Threshold

    The economic threshold for lygus bugs in canola covers the end of the flowering and the early pod ripening stages. Once the seeds have ripened to yellow or brown, 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 would not allow application so late.

    The economic threshold or number of lygus bugs per 10 sweeps at which the cost of controlling the pest equals the economic damage are based on the work performed in Manitoba. [7] The original economic threshold research was based on field studies between 1989-1993 in southern Manitoba using Westar variety and plant stands of 7 to14 plants per square foot. The thresholds were validated in 1999-2001 with Q2 variety in Lethbridge, Beaverlodge and Dawson Creek. Producers should be reminded that, as new canola varieties are developed, plant tolerance to insect feeding damage (and therefore yield) may also change.

    Recently, some producers have indicated that they feel current economic threshold tables for lygus in canola may be too low when applied to current canola production systems across the Prairies. In some cases they are increasing the researched economic threshold for lygus bugs in canola substantially in vigorously growing stands of new canola varieties. There is no data to support this so future research needs to reexamine the thresholds with respect to newer varieties of canola.

    Lygus bug thresholds in canola at end of flowering
    Application Cost End of Flowering (Canola crop stages 4.4 to 5.1)
    $/ha $/ac Economic injury level (Number of lygus bug/10 sweeps)
    22 8.00 11 8 7 5 5 4
    25 10.00 13 10 8 7 6 5
    27 12.00 16 12 10 8 7 6
    30 14.00 19 14 11 9 8 7
    32 16.00 22 16 13 11 9 8
    35 18.00 24 18 15 12 10 9
    Canola Price ($/bu) 6.00 8.00 10.00 12.00 14.00 16.00

    Source: MAFRI

    Lygus bug thresholds in canola at pod ripening
    Application Cost Pod Ripening (Canola crop stages 5.2)
    $/ha $/ac Economic injury level (Number of lygus bug/10 sweeps)
    22 8.00 15 12 9 8 7 6
    25 10.00 19 14 11 10 8 7
    27 12.00 23 17 14 11 10 9
    30 14.00 27 20 16 13 11 10
    32 16.00 30 23 18 15 13 11
    35 18.00 34 26 20 17 15 13
    Canola Price ($/bu) 6.00 8.00 10.00 12.00 14.00 16.00

    Source: MAFRI

    Usually, one application of a registered insecticide at the end of flowering (bloom 90% complete or more) (GS 69) or at pod formation (GS 71) will prevent most yield losses. [30] Plot research has shown that insecticide applications applied at flowering (GS 60 to 68) do not control later emerging nymphs. However, preliminary results from an ongoing study (2010-2013) performed on commercial farms suggests that a single spray for cabbage seedpod weevils at early flower may reduce lygus bug populations during the early pod stage. This suggests that immigration at flower stage is less prevalent in large fields. Delaying application may be prudent if nearby alfalfa crops are being cut. Early emerging small nymphs do not harm the canola crop. Use enough water to ensure the insecticide penetrates the canopy and provides adequate coverage.

    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 delay of flowering so producers in that region must monitor in fields that fail to flower as expected.

    If soil moisture levels and rainfall are high at flowering, plants likely will 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 time usually will be unable to compensate for most of the feeding injury, control may be required using the economic thresholds above.

    Dual infestations: In southern Alberta and Saskatchewan, lygus bug often co-occurs with the cabbage seedpod weevil, which attacks canola at the early flower stage.

    A 4-year research study was initiated in 2012 to assess the impact of cabbage seedpod weevil control on lygus bugs. The two-year summary of data collected from 2010 and 2011 found that spraying for weevils at the early flower stage (earlier than the optimum stage for lygus), reduced lygus abundance at most of the sites, but yield responses were inconsistent. Growers should continue to scout and manage cabbage seedpod weevil and lygus bug separately until the research is completed, allowing a more reliable assessment of the effectiveness of this strategy.

    Chemical control options for lygus bugs in canola

    Check provincial crop protection guides for an up to date list of registered insecticides and follow label directions. Insecticide application is the only option for control of lygus bugs once populations have reached economic threshold levels.

    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% 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. In general, use an insecticide that has a short residual activity to reduce the impact on pollinators.

    Chemical Control Options For Lygus Bugs In Canola

    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 LD50.

    http://www.gov.mb.ca/agriculture/crops/cropproduction/gaa01d01.html
    http://www.agriculture.gov.sk.ca/Guide_to_Crop_Protection
    http://www1.agric.gov.ab.ca/$Department/deptdocs.nsf/All/agdex32


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