Diamondback Moth

Last Update: Monday, March 17, 2014 1:58:44 PM

Table of contents

    Important Tips for Best Management

    • Control brassicaceous weeds including volunteer canola.
    • Monitor provincial agricultural websites for early warning notices.
    • Early arrival = multi-generations = higher risk of economic damage.
    • Scout fields in July and August. Monitor crops at least twice per week.
    • Removing plants in an area measuring 0.1 m2 (about 12" square), beat them onto a clean surface, and count the number of larvae dislodged from the plants.
    • Consider insecticide application when 20 to 30 larvae/0.1 m2 are present at the advanced pod stage. This works out to approximately two to three larvae/plant if plant population is close to 100 plants/m2).
    • Minimize impact on beneficial insects by using economic thresholds to ensure insecticide application is made only when warranted.


    About Diamondback Moth

    Diamondback moth (Plutella xylostella L.), was introduced into North America from Europe about 150 years ago. It now occurs throughout North America wherever its host plants are grown.  Populations of the diamondback moth routinely infest crops of canola and mustard in Canada. In most years the insect causes minor economic damage, but in some years populations reach outbreak densities and substantial crop losses can occur.

    Background

    Although the diamondback moth occurs each year throughout the Canadian prairies and north central U.S., the severity of the infestation varies considerably from year to year and location to location.

    The insect may overwinter in the prairies, but not frequently or in large numbers, and instead migrates northward from infested regions in the southern or western U.S.A. or northern Mexico on wind currents. The composition and timing of influxes has immense bearing on the damage caused by diamondback moth: the species is multivoltine, capable of producing as many as four generations per year in the prairies, and early arrival times can therefore result in greater population build-up than later arrivals.

    Diamondback moth is restricted in its host range to plants of the family Brassicaceae [1],[2]:

    • canola
    • mustard
    • cabbage
    • cauliflower
    • broccoli
    • kale
    • many other brassicaceous crops and weeds.


    On a worldwide scale, diamondback moth has been considered responsible for over $1 billion (U.S.) annually in economic losses [3]. In western Canada, crop losses vary considerably from year to year. In some years the population densities and economic importance of diamondback moth can be negligible, but in other years the pest can cause many millions of dollars in lost revenue [4],[5].

    Life Cycle

    Diamondback life cycle

    In some years the moths can reach parts of the main canola growing areas in Canada from the U.S. early enough thatmany of the canola crops will not have emerged yet. In these situations cruciferous weeds and volunteer canola become important alternate hosts for egg laying.

    ACTION:  Control brassicaceous weeds including volunteer canola.

    Normally, the diamondback moth takes about 32 days to develop from egg to adult. However, the time to complete a generation may vary from 21 to 51 days depending on weather and food conditions. There may be several generations per growing season. Generations usually overlap and all four life stages including egg, larva, pupa and adult may be present in the field at the same time.

    Adult females lay an average of 160 eggs during their life span of about 16 days. Egg laying occurs at night. The greatest number of eggs is laid the first night after emergence and egg laying continues for about 10 days.

    Diamondback moth eggs are laid mainly on upper leaf surfaces [15] and hatch in four to eight days. Eggs are oval, yellowish-white and tiny. They are glued to the upper and lower leaf surfaces singly or in groups of two or three, usually along the veins or where the leaf surface is uneven.

    Eggs on leaf in typical pattern

    Diamondback Moth larvae

    Photo by Roy Ellis

    Diamondback moth pupa

    First-instar larvae bore through the leaf epidermis and feed internally on cells of the leaf mesophyll [16]. The three subsequent larval instars are surface feeders, and consume leaves, buds, flowers, or pods. An average duration of 4.0, 3.6, 3.4, and 4.2 days were required for completion of the first to fourth larval instars, respectively, and 7.8 to 9.8 days were required for pupation under field conditions in Ontario [16]. The larval stage lasts about 10 to 21 days, depending upon temperature and the availability of food.

    ACTION: Consider insecticide application when 20 to 30 larvae/0.1 m2 are present at the advanced pod stage. This works out to approximately two to three larvae/plant if plant population is close to 100 plants/m2.

    Larvae pupate in delicate, white, open-mesh cocoons attached to the leaves, stems or seedpods of the host plant. Initially, the pupae are light green but as they mature they become brown as the adult moth becomes visible through the cocoon. The pupal stage lasts from five to 15 days depending on environmental conditions, and then adult moths of the next generation emerge.

    Influence of Environment

    An infestation of diamondback moth cannot be predicted based on the previous year's population because very few, if any, pupae survive the long, cold Canadian winters.  In one study in central Alberta using trap cages placed in a field during early spring, diamondback moth was found to overwinter [11]. The occurrence of a heavy, insulating snowfall early the previous fall was thought to help explain these observations. Other attempts to overwinter diamondback moth were unsuccessful regardless of location (Saskatoon, SK and Vegreville, AB), soil organic residue levels, or the insect life stage used [12].

    Instead, the severity of the infestation in any given year depends on two factors-overwintering populations in the U.S. and strong south spring winds to transport the moths north into Manitoba, central Saskatchewan and eastern Alberta.

    Research found that high densities of diamondback moth on the prairies can be traced back to strong airflow from the southern U.S.A. [12],[13], and in some cases, possibly California and the Pacific northwest of the U.S.A. [13].

    A wind trajectory-modelling project is now implemented annually during the growing season and integrates a network of sentinel sites with pheromone traps. This project provides an early-warning system and provides agronomists and farmers with advance notice for the potential arrival of diamondback moth populations into canola production areas of the prairies [14].

    ACTION: Monitor Canola Council of Canada and provincial agricultural websites for early warning notices.

    Environmental factors can have a profound impact on diamondback moth populations. Cool, windy weather reduces adult activity and females often die before they lay all their eggs. Heavy rainfall can drown small larvae and reduce numbers by more than half. Humid conditions within the crop following a rainfall can promote the spread of fatal fungal diseases throughout the diamondback moth population.

    Cold, windy weather; heavy rainfall = reduced infestations


    Identify Diamondback Moth

    The adult moth is approximately 8 to 9 mm (0.3 to 0.4") long with a wingspan of 12 to 15 mm (0.5 to 0.6") (Figure 25). At rest, the moth folds its wings over the abdomen in a tent-like manner. The folded wings flare upwards and outward at the tips. The wing tips are fringed with long hairs. In the male, the forewing margins have a series of creamy-coloured wavy markings. When the wings are folded while the moth is at rest, these markings come together to form three light coloured diamonds, hence the name diamondback.

    The larvae are pale yellowish-green to green caterpillars covered with fine, scattered, erect hairs. The posterior end of the caterpillar is forked.  At maturity the larvae are cigar-shaped and about 12 mm (0.5") long. The diamondback moth larva is easily identified by its peculiar reaction to being disturbed. It will wriggle backward violently and may drop from the plant, suspended by a silken thread. After several seconds, the larva will climb back onto the leaf and continue feeding.

    Diamondback Moth

    Photo by Roy Ellis

    Larva eating leaf

    Photo by Lloyd Dosdall

    Sweep net collection of canola insects

    Photo by Lloyd Dosdall

    Scouting techniques

    Consult with crop specialists and entomologists for the size and timing of the moth flight, based on trajectory modeling and pheromone trapping.  Some provinces provide live updates of moth flights and numbers. The presence and relative abundance of the diamondback moth can be determined by using pheromone traps. These traps cannot predict the potential for crop damage, but trap counts can provide an early warning of a possible infestation. If these trap counts correlate well with the wind trajectory models it provides an even stronger indication of regions at greatest risk. In the field, crop scouts may observe moths fluttering up as the crop canopy is disturbed. However, it is the feeding of the larvae that will lead to yield losses, and environmental conditions will ultimately determine how many eggs are laid and whether the larvae emerge and survive, leading to economical feeding damage.

    http://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/prm13796

    http://www.gov.mb.ca/agriculture/crops/insects/db/

    Sweep net sampling can determine the presence and general abundance of the species in the field, but does not provide a good estimate of larval density because no correlative studies have been conducted to relate sweep net captures with density in the field. Nevertheless, high population counts in sweep sampling can prompt producers to perform more accurate counts of densities per unit area.

    The most accurate method of estimating diamondback moth population density in canola is to perform counts of diamondback moth larvaein several locations throughout the fieldand determine the average population per unit area.

    When: Scout fields in July and August. Monitor crops at least twice per week.

    Where:  At least five locations per field.

    How: Remove plants in an area measuring 0.1 m2 (about 12" square), beat them onto a clean surface, and count the number of larvae dislodged from the plants.

    Action Threshold:  20 to 30 larvae/0.1 m2 (about 12” square) at the advanced pod stage. This works out to approximately two to three larvae/plant if plant population is close to 100 plants/m2).

    Damaging stage

    Crop damage is caused by the larval stage. Diamondback moth larvae feed on any green tissue of canola and mustard plants but prefer leaves. The amount of damage varies greatly, depending on plant growth stage, larval density and larval size.

    Type of Crop Damage

    Damage caused by early infestations can sometimes be seen as early as the first true-leaf stages of the crop.  In most instances, however, moth invasions of western Canadian canola and mustard crops occur later, when crops are in the rosette stage. An arrival time of moths in mid-May can enable the species to complete three or four generations by the time crops are in the pod development stages, and most vulnerable to attack [17]

    Early arrival = multi-generations = higher risk of economic damage.

    When larvae are small, damage is evident as small irregular holes or "shot holes" in the leaves. If larvae are numerous, they may eat the entire leaf, leaving only the veins. Larval feeding damage to canola leaves is usually considered to have a minor effect on yield, but larval feeding on buds and flowers can be more damaging, especially when plants are under abiotic stress (e.g., drought) and cannot compensate by producing new buds and flowers. Feeding damage during the early flowering stage can be extensive. Extensive feeding on the flowers will delay plant maturity, cause the crop to develop unevenly and significantly reduce seed yields.

    Diamondback Moth feeding damage to canola buds

    Photo by Lloyd Dosdall

    Canola severely damaged by diamondback moth feeding

    Photo by Lloyd Dosdall

    When plants are fully podded and leaves begin to wilt or die in late July or early August, larvae will remove the surface tissue from the stems and pods. The seeds within a damaged pod will not fill completely and pods may shatter, resulting in yield loss. Larvae may also chew into pods and eat the developing seeds. In prairie fields under severe attack by diamondback moth, yield losses can be extreme.

     Crop damage is usually first evident on plants growing on ridges and knolls in the field. In severe cases, damage shows from a distance as abnormal whitening. 

    Crop response: B. napus, B. rapa, and S. alba can respond to larval feeding by increasing their root biomass, presumably as a strategy to enable them to compensate for foliage loss, through the uptake of greater quantities of soil water and nutrients [18]. However, the magnitude of root mass increase is varied by variety and species:

    • B. napus cv. Q2 produced approximately two-fold more root biomass when infested with 10 larvae per plant than when non-infested, but 
    • B. napus cv. Conquest root mass increased 1.5-fold more when infested, and
    • B. napus cv. Liberty produced root mass quantities that were similar among infested and noninfested plants.
    • Root biomass of B. juncea did not vary for infested and noninfested plants.


    The greatest increase in B. napus root biomass occurred to plants treated with intermediate levels of fertilizer (i.e., the rate recommended for canola production) rather than low or high fertilizer applications [19].

     

    Minimize infestations

    Natural enemies and biological control

    Natural enemies of diamondback moth are important for biological control, and comprise both parasitoids and predators.

    Parasitoids: The three main parasitoids of diamondback moth in the prairies of western Canada belong to two families of Hymenoptera. The wasps:

    • Diadegma insulare (Cresson) and Diadromus subtilicornis (Gravenhorst) are members of the Ichneumonidae, and
    • Microplitis plutellae (Muesbeck) is in the Braconidae [29],[30].


    D. insulare and M. plutellae attack larvae of diamondback moth, and D. subtilicornis parasitizes pupae [7],[30].

    D. insulare is considered the most important for biological control. For instance, in Alberta D. insulare parasitized 45% of diamondback moth individuals in 1992; by comparison, M. plutellae and D. subtilicornis were each responsible for approximately 15% parasitism [29]. Similarly, in Saskatchewan D. insulare accounted for 30% of diamondback moth parasitism, with about 15% parasitism by D. subtilicornis and M. plutellae combined [29].

    Adult diadegma (D. insulare), a beneficial insect.

    Photo by Lloyd Dosdall

    D. insulare is native to the Neotropics [30],[31], does not survive prairie winters [32] and is believed to migrate northward in spring with their diamondback moth hosts. D. insulare can parasitize all four larval instars of diamondback moth and the parasitoid emerges as a final instar when its hosts reach their prepupal stage. Adults require a nutrient source for survival, and as a result they spend more time in habitats where flowering plants are abundant so they can feed [33]. Flowering plants like alyssum, Lobularia maritime (L.) Desv. (Brassicaceae), can provide food sources for adults of D. insulare, and the parasitoid is attracted to plant tissue damaged by diamondback moth larvae [34]. This appears to explain why field populations of D. insulare can be clustered, often in patches correlated with high populations of diamondback moth [35],[22].

    M. plutellae has a widespread distribution in North America [29],[30], overwinters in western Canada, and can be present early in the season to parasitize hosts [39]. However, M. plutellae is generally less abundant than D. insulare in canola fields.

    D. subtilicornis is a solitary pupal endoparasitoid of the diamondback moth, but very little is known about its biology in western Canada. Females lay eggs in prepupae or in pupae, but much more frequently in newly formed versus older pupae [40]. Females of D. subtilicornis can use their mouthparts to enlarge wounds made by the ovipositor during egg-laying, and may then feed on fluid oozing from the wound [40].

    Predators:  The contribution of predators to biological control of diamondback moth in the prairies is unknown. Canola and mustard crops harbour a diverse fauna of predators, comprised mainly of various species of:

    • carabid beetles (Coleoptera: Carabidae),
    • lady beetles (Coleoptera: Coccinellidae),
    • syrphid flies (Diptera: Syrphidae),
    • lacewings (Neuroptera: Chrysopidae),
    • rove beetles (Coleoptera: Staphylinidae), and
    • several families of spiders (Araneae).


    With the exception of the Carabidae [41],[42],[43], little research on this fauna has been undertaken. For example, research [44] documented 59 species of carabids in canola from a single site in central Alberta over three years of collections. However, no studies have focused specifically on predation of diamondback moth in the prairies.

    In view of the importance of predatory insects for reducing populations of diamondback moth in some other cropping systems [45], further study is warranted to resolve this important gap in knowledge of mortality factors that can impact diamondback moth populations in prairie agro-ecosystems.

    ACTION: To minimize impact on beneficial insects, use economic thresholds to ensure insecticide application is made only when warranted.

    Fungi:  Entomophthorales fungi cause natural disease outbreaks in diamondback populations. These outbreaks usually occur late in the growing season when populations are high. The rate of infection of diamondback moth larvae can be high enough to limit the development of additional generations late in the season.

    Despite the abundance of potential biological control agents, the only effective way of controlling a diamondback moth outbreak once the population exceeds the economic threshold is to apply an insecticide.

    Biological control

    Although foliar applications with formulations of bioinsecticide containing the delta-endotoxin of the bacterium Bacillus thuringiensis (B. t.) subspecies kurstaki are proven to be effective and less damaging to non-target organisms than chemical insecticides [3],[27], B. t. is not registered for application in canola in Canada. 

    Field Management (cultural control)

    Preseed weed control:  Preseed weed control and tillage reduces the availability of cruciferous weeds and volunteer canola host plants, preventing the successful establishment of first generation larvae where moths arrive before canola emergence.

    ACTION: Control brassicaceous weeds and volunteer canola early.

    Soil fertility:  Research has found that for egg laying, females selected plants that received quantities of soil fertilizer that corresponded to the level recommended for canola production, compared to unfertilized plants or plants that received three and five times the recommended levels of fertilizer [19]. Similarly, larval survival was greatest and larval development was fastest on plants grown at intermediate levels of soil fertility [19].  Sulfur appears to have the greatest influence on diamondback moth [20],[21],[22], with females laying significantly more eggs on sulfur-fertilized plants than on plants grown in soil deficient in sulfur.

    ACTION: Develop a fertility program based on yield potential rather than as a management tool for diamondback moth.

    Resistance breeding

    Species of Brassicaceae differ in their susceptibilities as hosts for diamondback moth. Egg-laying females preferred S. alba and B. rapa, and development times of larvae and pupae were most rapid on B. juncea and S. alba. [18]

    Development of diamondback moth can also be influenced by varieties within species. Although survival of diamondback moth did not vary for individuals reared from egg to pupa on the B. napus varieties Q2, Liberty, and Conquest, females laid significantly more eggs on Liberty than on Q2 or Conquest [18]. Developmental time of females from larva to prepupa tended to be faster on Liberty and Conquest than on Q2, but female body weight was greater for individuals reared as larvae on Q2 than on Liberty [18].

    Resistance work revolves around incorporating one or more Bacillus thuringiensis genes into canola.  Canola transgenic for expression of the cry1Ac gene has been developed and provided protection from diamondback moth infestation [28], but no such transgenic crops are registered. Field trials in southern USA proved its effectiveness and there is interest in getting this type of resistance registered in Australia because diamondback moth is a huge problem in their canola crops.

    Since diamondback moth is only a sporadic pest in Canada, and effective control options are available to deal with occasional outbreaks, the incorporation of transgenic B.t. resistance into Canadian hybrid canola has not been viewed as an urgent need.


    Reduce economic losses

    Economic Threshold

    The economic threshold for diamondback moth in canola at the advanced pod stage is 20 to 30 larvae/0.1 m2 [26]. This works out to about two to three larvae per plant for plant populations close to 100 plants/m2.

    Action threshold: 20 to 30 larvae/0.1 m2  at the advanced pod stage.

    An economic threshold for canola or mustard in the early flowering stage has not been established. However, at this plant growth stage insecticide applications are likely required at larval densities of 10 to 15 larvae/0.1 m2 (one or two larvae/plant at 100 plants/m2 [26].

    For seedlings, control is recommended when 25 to 33% of the cotyledons or true leaves are defoliated. 

    These economic threshold recommendations are nominal, without basis in replicated experimental studies.

    Chemical control options

    Early field monitoring and the application of insecticides can prevent damage, if larval numbers exceed the economic threshold. After an infestation is controlled at the podding stage, a new infestation is not likely to become established because of the rapid advance of the crop toward maturity.

    Check provincial crop protection guides for registered insecticides. A single, well-timed application of an insecticide with either aerial or ground equipment is usually effective in controlling larval populations. Make insecticide applications when larval populations are high because the effectiveness is reduced against adults or pupae. Always apply insecticides with enough water to ensure adequate coverage. Use high water volumes and label rates when the crop canopy is dense. If the leaves are beginning to turn yellow and dry up, damage will become more serious as larvae move to feed on pods. If this is the case, consider control at the lower end of the economic threshold range. For most effective control, apply at dusk or at night when diamondback moths are most active.

    Injury to honeybees and other pollinating insects can be minimized by not spraying flowering crops. When it is necessary to apply an insecticide to a flowering crop, contact local beekeepers prior to application, use the safest product available and apply it during the evening. 

    Insecticides registered for diamondback moth larvae control in canola: Check your provincial crop protection guide for registered control products and up to date registrations.

    Product Chemical Group Name Chemical Group Number Active Ingredient Preharvest Interval Application LD50 (mammalian toxicity)*
    Decis 5EC Pyrethroids 3A Deltamethrin 7 days Aerial or ground 395
    Matador; Silencer Pyrethroids 3A Lamda-cyhalothrin 7 days Aerial or ground 64 - 110
    Malathion 500 Organophosphates 1B malathion 7 Aerial or ground 4302
    Malathion 85E Organophosphates 1B malathion 7 Aerial or ground 5500
    Lorsban; Pyrinex; Nufos; Citadel Organophosphates 1B chlorpyrifos 21 Aerial or ground 205 - 418

    * 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.agriculture.gov.sk.ca/Guide_to_Crop_Protection

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


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