Flea Beetles

Last Update: Wednesday, December 10, 2014 5:04:47 PM

Table of contents

    Important Tips for Best Management

    • Seed into a warm, moist seedbed to promote good stand establishment and rapid seedling growth.
    • Use an insecticidal seed treatment.
    • Most injury occurs at the cotyledon and early true leaf stage, after the fourth leaf stage there will be minimal impact on yield. [1]
    • Scout fields in the spring. Assess damage to cotyledons and the first true leaves of seedlings daily. Continue to scout until the seedlings are past susceptibility, especially when temperatures exceed 14°C.
    • The action threshold for applying foliar insecticide is typically at 25% defoliation to reduce yield loss, if flea beetles are still present and actively feeding. For crops with lower plant populations this threshold will be lower.
    • Predators, parasites and diseases can be important in regulating insect populations. However, flea beetles emerge in large numbers during a short period of time so tend to overwhelm the limited number of parasites and predators that attack them.
    • Cultural control methods can help reduce flea beetle damage:
      • Less favourable microclimate in direct seeded crops may reduce injury.
      • Plants grown from large seed tend to outgrow flea beetle damage more effectively and have better stand establishment, higher shoot weights and higher seed yield than plants grown from small seed.
      • Seed canola as early as practical to maximize seed yield.
      • Increasing seeding rates can help reduce the impact of flea beetle damage.
      • For a given seeding rate, some research has demonstrated less flea beetle damage per plant for canola grown in 12" row spacing vs. narrower spacing. [31]
      • When flea beetle populations are very high, no cultural controls will effectively reduce their attack.


    About Flea Beetles

     

    Background

    Flea beetles (Coleoptera: Chrysomelidae) feed on plants of the mustard family (Brassicaceae) grown across Canada and throughout the Northern Great Plains of North America (including North Dakota, South Dakota, Montana, and northwestern Minnesota, Manitoba, Saskatchewan, Alberta, the Peace River region of British Columbia, Ontario, Quebec and New Brunswick). Eight flea beetle species are known to attack canola, mustard and rapeseed.

    Crucifer, Striped and Hop Flea Beetles

    Three main species attack canola in western Canada:

    • Crucifer flea beetle,Phyllotreta cruciferae(Goeze)
    • Striped flea beetle,Phyllotreta striolata(F.)
    • Hop flea beetle,Psylliodes punctulataMelsh.


    The crucifer flea beetle and the striped flea beetle were both introduced from Eurasia, and are significant pests. The crucifer flea beetle is the most prevalent species in Canada. The hop flea beetle is a native species that occurs in low numbers throughout the prairies. [2],[3]

    The economic impact of flea beetles on crop production varies with population densities. Yield losses of about 10% are common where flea beetles are abundant even when the crop is protected with insecticides. [4],[5]  A 1% per acre yield reduction results in a total crop loss of about $25 million to $35 million. Annual crop losses in North America from flea beetles probably exceed $300 million. [6]

    Life Cycle

    Flea Beetle Life Cycle

    Flea beetles have 1 generation per year in western Canada, although adults appear twice during the growing season. In the spring, overwintered adults emerge and feed on canola seedlings. In the fall, it is the adult offspring of those overwintering adults that are observed feeding on canola leaves, stems and seed pods.

    Flea beetles overwinter as adults near the surface of the leaf litter, grass and debris beneath hedges, shelterbelts, poplar groves and in association with canola stubble and volunteer cruciferous plants. Within the leaf litter flea beetle densities may be as high as 140 to 250 beetles/m2.

    Adult flea beetle in spring leaf litter

    Flea beetles become active with the first extended period of warm weather in April and May, 5 to 11 days after leaf litter begins to thaw in the spring. Depending on temperature, it may take an additional three weeks before all the overwintering adults emerge.

    Emergence:  Hops => Striped=> Crucifer

    Hop flea beetle adults emerge about 10 days earlier than those of striped flea beetle, with the striped flea beetle emerging 1 to 4 weeks earlier than the crucifer flea beetle. [7]  Peak emergence of the crucifer flea beetle occurs when ground temperatures reach 15° C. [8]

    Under cool conditions, flea beetles walk or hop into the adjacent cruciferous crops or weeds. They feed on volunteer canola and mustard, or on weeds such as wild mustard, flixweed, lamb's quarters, stinkweed or peppergrass. Then they will move to newly emerged canola seedlings. When air temperatures exceed 14°C and winds are calm they will fly, dispersing throughout the field and invading other fields, attacking seedlings as they emerge. [9],[10]

    Flea Beetle Larvae

    After feeding has commenced, beetles mate repeatedly. Egg laying begins in late May and continues for about 30 days. [6],[8]  Some egg laying may continue until early August. Females deposit about 100 smooth, yellow, elongated, oval eggs 0.38 to 0.46 mm (0.01 to 0.02") by 0.18 to 0.25 mm (0.006 to 0.009") wide, either singly or in groups of three or four in the soil adjacent to the host plant's roots. [8] Eggs must be in contact with moist soil, or they desiccate within a few hours. The eggs take about 12 days to hatch. After egg laying the overwintering adults die off.

    Flea beetle larvae are grub-like with off-white bodies and a brown head and anal plate. Larvae moult twice during 25 to 34 days, completing three larval stages usually mid-June to late July. Full-grown larvae are about 3 to 4 mm (0.1 to 0.2") long.

    Larvae feed on the root hairs and taproots of seedlings. In a few cases, larvae have been observed burrowing into the plant near the juncture of the root and stem. When larval development is complete, larvae pupate in small earthen cells

    Flea beetle pupae are usually present in the field by early to mid-July. They are entirely white except for the eyes, which darken as the pupal stage progresses to completion. The body appendages are free and distinguishable. The pupal stage lasts for about seven to nine days.

    Adult flea beetle feeding on pods and stems

    Adult emergence begins after mid-July and continues until October. [11] The beetles feed on the leaves, stems and pods of cruciferous plants. Development from egg to adult takes about 7 weeks. In late August and September, adults move into leaf litter and debris to overwinter.

    Influence of Environment

    Sunny, warm, dry weather increases feeding activity. Cool, damp weather slows flea beetle activity, but they tend to survive fairly well in spite of the reduced activity level.  If the weather warms up, growers should continue to scout for damaging levels of flea beetles.


    Identify flea beetles

    Flea beetles attacking canola, mustard and rapeseed are small, elliptical or oval-shaped and less than 2.5 mm (0.1") long. When disturbed they use their powerful hind legs to jump away like a flea - hence, the name flea beetle.

    Scouting techniques

    Adult flea beetle on cotyledons ALT

    Scout fields in the fall, observing flea beetle densities. This is the first sign of potential problems next spring. If flea beetles are abundant, use a seed treatment with an insecticide level that provides extended control.

    Heavy infestations can severely damage cotyledons, the first leaves, petioles and stems. Crop thinning and delayed crop development caused by flea beetle feeding are most severe the first two weeks after seedling emergence. At the 3- or 4-leaf stage, the plants are established and can outgrow the feeding damage. At this time, adult flea beetle numbers often begin to decline. The crop can usually compensate provided large portions of the crop are not totally destroyed.

    Feeding damage is most severe when flea beetles attack the growing point limiting the plants' ability to compensate. Heavy attacks can destroy the entire crop, forcing growers to reseed or leave the field fallow.  Seedling death can be rapid when conditions are warm, dry and windy. During cold inclement weather and periods of slow plant growth flea beetles can feed on seedlings at or near the soil surface, severing the stem or destroying the growing point and eventually the seedling.

    Severe flea beetle feeding damageCan be confused with: Seedling loss from plant disease can be mistaken for flea beetle damage. Dig damaged plants from the soil and examine carefully for evidence of disease on the shoots and roots.

    Delayed maturity from early flea beetle damage can expose the crop to adverse temperatures during flowering or to frost before the plants have matured. Uneven maturity at harvest reduces seed quality or yield. Delaying harvest to allow immature pods to ripen contributes to yield loss when over-ripe pods shatter during harvest. Harvesting too early can produce a crop with many immature seeds containing high chlorophyll levels, affecting seed quality and yield. Most of this damage can be prevented if canola is protected from flea beetle injury during the two to three weeks following emergence.

    Flea beetles that emerge after mid-July can also affect yield. Heavy feeding during pod development and filling causes injury to pods, leading to premature pod drying, shrivelled seeds, pod shattering, increased chlorophyll content, and encourages fungal growth within the pods during damp weather. Injury to the pods is usually concentrated on the youngest pods and on late seeded crops. Yield is not usually affected when the crop has matured to at least the stage when seeds in the lower pods have turned from translucent to green.


    Minimize infestations

    Natural enemies and biological control

    Predators, parasites and diseases can be important in regulating insect populations. To date the effect of biological control agents seems to be limited but several insects have been observed attacking adult flea beetles.

    The flower beetle, Collops vittatus (Say) [18], big-eyed bug, Geocorus bullatus (Say)  [19], damsel bugs (Nabis spp.) [20], field crickets (Gryllus pennsylvanicus Berm.) [21] and lacewings (Chrysopa spp.) play a minor role in controlling flea beetles.

    The native braconid wasp (Microctonus vittatae Muesebeck) parasitizes flea beetle adults and is widespread across the prairies. [22] However, parasitism by the wasp usually averages less than 5% of the flea beetle population. [23],[24]

    Townesilitis bicolor (Wesmael), a braconid wasp that occurs in central Europe, was found to parasitize up to 50% of the crucifer-feeding flea beetles there. [25] This wasp was imported and released in Manitoba in multiple introductions from 1978 to 1983 but did not become established for unknown reasons.

    Isolates of entomopathogenic nematodes that are promising candidates for the biological control of P. striolata under field conditions have been recently found in China. [26]

    Unfortunately, flea beetles emerge in large numbers during a relatively short period of time and tend to overwhelm the parasites and predators.

    Field Management (cultural control)

    Agronomic practices that promote good stand establishment and rapid seedling growth will reduce the impact of flea beetles on canola yield. Emergence and growth can be improved by planting seed lots with higher seed weights and germination above 95% in the standard germination test or pre-chill test. [27],[28]

    Seed Size: Seedlings grown from large seed (1.8-2.0 mm for B. rapa and 2.0-2.2 mm for B. napus seed) are more vigorous and tolerant of flea beetle damage than seedlings grown from small seed (1.4-1.6 mm for B. rapa and 1.6-1.8 mm for B. napus seed). [29],[30] Tolerance in B. napus is due to a higher initial seedling weight and faster growth rate. Plants grown from large seed have less flea beetle damage, better stand establishment, higher shoot weights and higher seed yield than plants grown from small seed.

    When damage exceeded 50%, large heavy seeds had the best stand establishment, best shoot growth and highest yield.  Compared with small seeds, yields improved by 24% with medium-sized seeds and by 52% with large seeds. 

    Tillage: Flea beetles prefer bright relatively warm conditions. Direct seeding provides a microclimate that is less ideal for flea beetles than tillage before seeding. Planting into stubble may provide a microclimate that is cooler, moister and less favourable to flea beetles.  Injury may be reduced due to the cooler microenvironment created by stubble shading on surface soil. Cooler temperatures at the soil surface slow flea beetle activity reducing damage. However, crop development can also be slowed by cooler temperatures, so frequent crop scouting is still essential to ensure damage thresholds are not exceeded.

    In Alberta, zero till canola production can reduce flea beetle damage to levels equivalent to those found with the use of insecticidal seed treatments. [31]  This seeding method produces large plants early and may reduce the grower's dependence on seed treatments and foliar insecticides, except under conditions of intense flea beetle pressure.

    Sumerfallow weed control: Do not cultivate summerfallow fields with cruciferous weeds and volunteer canola until canola crops are at the 4-leaf stage. Leaving a trap strip of volunteer canola near overwintering sites can be an effective control strategy if the trap strip is sprayed before beetles move into the canola crop, although this strategy can be difficult to implement successfully. Control cruciferous weeds and volunteer canola in cereal fields to starve out early spring populations

    Seeding Date: Producers are urged to seed canola as early as practical to maximize seed yield. Early seeding will maximize plant size before flea beetle emergence and the larger plants tolerate more injury. [32]  In southern Alberta, canola planted in April has fewer flea beetles and suffers less damage than canola planted in May, while the opposite is true in central and northern Alberta. [33]

    Seeding Rate: Increasing seeding rates can help reduce the impact of flea beetle damage, because spreading the feeding damage over more plants per unit area means that feeding damage per plant is reduced and seedlings can recover more readily from flea beetle injury.

    Research has shown damage to individual plants to be lower with a 10 kg/ha (8.9 lb/ac) seeding rate than with a 5.0 or 7.5 kg/ha (4.5 or 6.7 lb/ac) rate. [30] Higher seeding rates and plant densities are believed to dilute and reduce damage to individual plants. However, the benefits of higher seeding rates for flea beetle control need to be tempered by the costs, which include increased production costs and pesticide load to the environment, and the possibility of increased lodging and susceptibility to diseases.

    Row Spacing: At a given seeding rate, row spacing of 20 to 30 cm (8 to 12") have been shown to result in less flea beetle damage per plant than 10 cm (4") row spacing. [31] Although the reasons are not yet clear, it appears that flea beetles are more attracted to the reduced visual contrast between vegetation and soil that occurs at narrow row spacing.

    Crop Rotation: Crop rotation is not an effective means of controlling flea beetles. Adults overwinter inside and outside of the cropped areas and are capable of long-range migration. While intercropping with non-host plants has reduced flea beetle damage in brassicaceous vegetables, the benefits of field-scale intercropping of canola and wheat [34] or canola and field peas [35] currently are not sufficient to warrant their use as flea beetle management options.

    Seed treatment blends: Blending a proportion of seed without insecticide together with treated seed for planting is a practice that has been of interest to some growers and researchers from the standpoint of reducing cost and insecticide load on the environment.

    Under moderate levels of flea beetle damage, research has demonstrated that as much as one-third of planted seed could be untreated with insecticide without subsequent seed yield reduction. [48] However, this practice currently is not recommended because of the highly variable and unpredictable nature of flea beetle damage, and the potential for the use of farm saved seed for the "untreated" proportion which has other negative agronomic and legal risks. This strategy could be explored further in the future in conjunction with other integrated management strategies or with improved forecasting.

    When flea beetle populations are very high, no cultural controls will effectively reduce their attack.

    Resistance breeding

    Hairy canolaWhile no canola variety has resistance to flea beetle, researchers are working on developing resistance as several studies have shown that flea beetles prefer certain crucifer hosts. [36],[37],[38]

    The lower levels of flea beetle feeding seen on yellow mustard seedlings compared to canola seedlings is likely due to an aversion to the levels of glucosinalbin in mustard seedlings and to seedling tolerance to feeding injury. [39],[40] Trichomes (leaf hairs) impart resistance to flea beetles in certain crucifers. [41],[42]

    Researchers have found canola germplasm with increased numbers of hairs on young true leaves displayed resistance to flea beetles in field trials [43],[44], in some cases surpassing the protection level provided by neonicotinoid seed treatments.  These hairs may have discouraged flea beetles from walking up hairy stems. Advanced canola lines with further elevated numbers, lengths, and complexities of plant leaf hairs are currently in trials and may achieve levels of resistance to flea beetles at present seen only with insecticide use. 

    Courtesy: University of Saskatchewan Media Production Unit


    Reduce economic losses

    Economic Threshold

    Flea Beetles causing defoliationUse the economic threshold to decide whether a foliar spray for flea beetle control will be economical. Since seed treatments are standard practice, the use of economic thresholds for flea beetle control in canola only apply when foliar sprays are being considered.

    Canola seedlings can withstand 50% leaf loss. Flea beetles can damage plants very quickly, however, so the economic threshold for flea beetle feeding on canola is when there is 25% defoliation and flea beetles are present and continuing to feed. Applying controls at 25% defoliation will reduce the risk of flea beetles reaching a level where yield loss and plant development are substantially reduced.

    Action Threshold: 25% defoliation

    This economic threshold is based on an optimal plant stand of 75 to 150 plants per square metre (7 to 14 plants per square foot). Growers should take plant stand into consideration when making economic threshold decisions. Thinner plant stands would require a foliar insecticide application at a lower defoliation percentage than an optimal plant stand, because in this situation maintaining plant health is more critical to maximize production per plant in order to compensate for reduced plant numbers. Any plant mortality is also more likely to directly impact yield when plant density is already lower than optimal.

    Flea Beetles causing defoliationWhen assessing whether a foliar insecticide spray is needed, growing conditions, soil moisture, overall crop health, yield potential and commodity prices need to be balanced with flea beetle numbers and damage. Control decisions should be based on current defoliation and the potential for crop growth to exceed feeding by flea beetles going forward. If soil moisture and growing conditions support rapid canola growth and seedlings are vigorous, the crop may be able to stand more feeding than slow growing drought stressed canola. However, when flea beetle feeding is combined with poor plant growth during hot, dry weather, canola can tolerate less feeding than if plants are growing under more ideal growing conditions.

    Although it is the damage to the youngest tissue that is of immediate concern, the damage to the two youngest expanded tissues should be assessed to determine if feeding levels are decreasing or increasing.

    When scouting fields for flea beetle damage, it is important to understand that flea beetles generally invade canola fields from the field edges. Flea beetle damage and the number of flea beetles may be higher at the field edge than farther into the field. An insecticide application to field perimeters may control invading beetles if they have not spread throughout the field. On hot and calm days, flea beetles are capable of moving longer distances and may populate the field more uniformly.

    Flea beetle numbers can be very high in ripening canola in the fall. If crops are very late in maturity, extensive feeding on pods can lower yields and increase green seed content. However, seed yields are usually not affected when crop maturity is more advanced than growth stage 5.2, when the seeds in lower pods have turned from translucent to green. [45]

    Insecticide control options

    Canola, mustard and rapeseed crops can be protected from flea beetle attack through insecticide application as a seed treatment or post-emergence foliar sprays. Granular, in-furrow insecticides were once popular, but have been withdrawn from the market due to environmental concerns. 

    Because flea beetles emerge early in the spring and can migrate into seedling canola fields rapidly, insecticide-coated seed treatments are the most effective means of flea beetle control. [3],[13],[46]

    Since 2001, all seed treatments registered for control of flea beetles in Canada contain a neonicotinoid insecticide. This insecticide class has systemic activity in plants and inhibits nervous conduction in insects by blocking the nicotinoid acetylcholine receptor. [47]

    Coating seeds with an insecticide in combination with one or more fungicides prior to planting is a widespread practice. Currently over 90% of the canola grown in western Canada is treated with an insecticide seed treatment. [48] Seed treatments protect canola seedlings from flea beetles as soon as the plants emerge. Some seed treatments currently available for control of flea beetles in canola come with the insecticide at either a lower or high rate, but most companies offering pretreated seed tend to favour the higher recommended rates of application to provide maximum protection. The higher rates are more expensive than the lower rates, but can extend the period of protection from flea beetles, especially under conditions that lead to slow crop emergence and development (e.g. early and/or direct seeding into cool soil temperatures).  

    In a greenhouse trial, when crucifer flea beetles were exposed to seedlings of canola grown from neonicotinoid-treated seed, they fed less and had higher mortality than striped flea beetles in the same trial. [51] The differential toxicity of seed treatments to the two species may result in a shift in their geographic distribution and economic importance. Also, for growers in regions where striped flea beetles are naturally more prevalent, the importance of frequent crop scouting to assess the need for foliar insecticide rescue treatments may be increased.

    If high flea beetle populations occur and the seed treatments are not protecting the seedlings adequately, use a foliar spray.  Application of foliar insecticide may be required when feeding damage encompasses 25% of the leaf surface [49],[50]. If damage around field edges is high but flea beetles have not dispersed throughout the field, limiting insecticide application to the field perimeter may be sufficient to control the insect.

    Growers who routinely mix a foliar insecticide with their herbicides should check that flea beetle damage is actually at the threshold.  Spraying before damage reaches economic threshold can increase the risk of flea beetles developing resistance to insecticide, and can have a negative impact on beneficial insects.

    Daily field scouting for flea beetles is important, particularly on hot, calm days. If plants are missing, determine if the cause is flea beetles, cutworms or seedling blight. A post-emergent insecticide application may be required to protect seedlings that are exposed to severe or prolonged periods of intense attack. If heavy flea beetle damage nears threshold levels and high numbers of flea beetles are noticed in the field, apply foliar sprays as soon as possible, since flea beetles can cause substantial damage quickly.

    Where damage starts at the field margins, only a small portion of the field may require treatment. Apply sprays when it is sunny and warm, and the beetles are active and exposed on plants and soil. Additional foliar sprays may sometimes be needed since flea beetles may continue to move into fields at the susceptible stage after residual from the first foliar spray has become ineffective. Scout fields daily to determine if the insects have moved into the untreated areas and are damaging plants. 

    Insecticide products for control of flea beetles
    ProductChemical
    Group Name
    Chemical
    Group Number
    Active
    ingredient
    Preharvest
    Interval
    ApplicationLD50 (Mammalian
    Toxicity)1
    Seed Treatments
    Helix; Helix Xtra Neonicotinoids 4A thiamethoxam Seed treatment Seed treatment >5000
    Gaucho Canola System; Neonicotinoids 4A imidacloprid Seed treatment Seed treatment 450
    Gaucho Platinum
    Prosper Neonicotinoids 4A clothianidin Seed treatment Seed treatment >5000
    Foliar sprays
    Decis Pyrethroids 3A deltamethrin 7 Aerial or ground 395
    Ripcord; UP-Cyde Pyrethroids 3A cypermethrin 30 Aerial or ground (ripcord ground only) 242-542
    Matador; Silencer Pyrethroid 3A lambda-cyhalothrin 7 Aerial or ground 64-110
    Pounce; Ambush Pyrethroid 3A permethrin Aerial or ground 1276
    Sevin XLR Carbamates 1A Carbaryl Seedling application only Aerial or ground 649
    Furadan Carbamates 1A carbofuran 60 Aerial or ground 7.34
    Malathion Organophosphates 1B malathion 7 Aerial or ground 4302-5500

    1 LD50 values represent the relative toxicity of 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.

    Crop Protection Guides

    http://www.agriculture.gov.sk.ca/Guide_to_Crop_Protection

    http://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/agdex32

    References

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    [2] Chittendon, F.H. 1923. Notes on the distribution and habits of North AmericanPhyllotreta(Coleop.) Proc. Entomol. Soc. Wash. 25:131-139.

    [3] Wylie, H.G. and C. Loan.1984. Five Nearctic and one introduced euphorine species (Hymenoptera: Braconidae) that parasitize adults of crucifer-infesting flea beetles (Coleoptera: Chrysomelidae). Can. Entomol. 116:235-246.

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    [14] Lamb, R.J. 1984.Effects of flea beetles,Phyllotretaspp. (Coleoptera: Chrysomelidae), on the survival, growth, seed yield and quality of canola, rape and yellow mustard. Can. Entomol. 116:269-280.

    [15] Westdal, P.H., W. Romanow, and W.L. Askew. 1979. Control of flea beetles on rape. pp 27-30 in Technical and Scientific Papers. 1979 Manitoba Agronomists' Annual Conference. Winnipeg, MB. Dec. 12-13, 1979.

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    [18] Gerber, G. and C. Osgood. 1975.Collops vittatus(Coleoptera:Melyridae): a predator of flea beetle adults in rapeseed (Phyllotreta cruciferae). Manitoba Entomol. 9:61.

    [19] Burgess, L. 1977b.Geocoris bullatus, an occasional predator on flea beetles (Hemiptera: Lygaeidae). Can. Entomol. 109:1519-1520.

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    [21] Burgess, L. and C.F. Hinks. 1987. Predation on adults of the crucifer flea beetle,Phyllotreta cruciferae(Goeze), by the northern field cricket,Gryllus pennsylvanicusBermeister (Orthoptera: Gryllidae). Can. Entomol. 119:495-496.

    [22] Soroka unpublished.

    [23] Wylie, H.G. 1981. Effects of collection method on estimates of parasitism and sex ratio of flea beetles (Coleoptera: Chrysomelidae) that infest rape crops in Manitoba. Can. Entomol. 113:665-671.

    [24] Wylie, H.G. 1982. An effect of parasitism byMicroctonus vitattae(Hymenoptera: Braconidae) on emergence ofPhyllotreta cruciferaeandPhyllotreta striolataColeoptera: Chrysomelidae) from overwintering sites.  Can. Entomol. 114: 727-732.

    [25] Wylie, H.G. 1988. Release in Manitoba ofTownesilitus bicolor(Hym: Braconidae), an European parasite ofPhyllotretaspp. (Coleoptera: Chrysomelidae). Entomophaga 33:25-32.

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