Abstract

Recent evidence that a population of Plutella xylostella (L.) overwintered successfully in western Canada prompted studies to evaluate overwintering survival of diamondback moth under field conditions in Alberta and Saskatchewan. Successful overwintering was not demonstrated at either site for any life stage under a variety of tillage and organic matter treatments, using either laboratory-reared or field-acclimated specimens of diamondback moth. Diamondback moth infestations in western Canada evidently originate primarily from southern U.S.A. or Mexico when strong winds carry adults northward in spring. To provide early warning predictions of infestations, air parcel trajectories into western Canada were investigated for monitoring long-range movement of P. xylostella early in the season. Using wind fields generated by the Canadian Meteorological Centre’s Global Environmental Multiscale model, three-dimensional air parcel trajectories were calculated using time-forward (prognostic) and time-backward (diagnostic) modes for several sites in North America. The model predicted strong northerly airflow in May 2001 which coincided with the occurrence of massive infestations of diamondback moth in canola in western Canada. The model can therefore serve as an important new tool for monitoring the dispersal of this pest to western Canadian canola crops.

Key Words

overwintering, air flow trajectories, canola

Introduction

Populations of diamondback moth routinely infest canola (Brassica napus L. and Brassica rapa L.) in western Canada. In most years, P. xylostella causes minor economic damage, but in some years populations reach outbreak densities and substantial crop losses occur. For example, in 1995 more than 1.25 million ha were sprayed with insecticide to control diamondback moth populations at an estimated cost to producers of $45 to 52 million (Can.) (WCCP 1995). An outbreak on an even greater geographic scale occurred in 2001, with approximately 1.8 million ha treated with insecticide in western Canada (WCCP 2001).

The capability of diamondback moth to overwinter in Canada has been the subject of some controversy. Harcourt (1957) found that in eastern Ontario, P. xylostella survived in the field only until mid-December. Butts (1979) observed complete mortality of diamondback moth in field cages in Ontario, but predicted that the species should be able to overwinter there successfully. Putnam (1978) assumed that diamondback moth did not overwinter in Saskatchewan, but found that one of its parasitoids, Microplitis plutellae Muesebeck (Hymenoptera: Braconidae) survived under snow cover in the field. Although western Canadian populations were believed to originate from annual migrations from the south (Putnam and Burgess 1977; Philip and Mengersen 1989), Dosdall (1994) reported evidence for overwintering of P. xylostella in central Alberta during 1991–1992. Overwintering by diamondback moth in western Canada has important implications for its pest status in canola; consequently several studies were undertaken to investigate overwintering success under field conditions in Alberta and Saskatchewan. In addition, wind trajectories to western Canada from southern U.S.A. were analysed to determine the coincidence of favourable wind patterns with the appearance of P. xylostella in canola crops.

Materials and Methods

To investigate conditions that favoured overwintering of diamondback moth in western Canada, replicated field trials were conducted from October to July in 1993–1994 and 1994–1995 at Vegreville, AB (112°02’N; 53°05’W). The experiments were randomised complete block designs with five replications conducted on plots seeded in the preceding season to B. napus. The study comprised four treatments: 1) untilled canola stubble; 2) untilled canola stubble covered with 1.7 kg per m² of dried, threshed canola plant matter; 3) tilled canola stubble and 4) tilled canola stubble covered with 1.7 kg per m² of dried, threshed canola matter. In mid-October 1993 and 1994, one insect cage was placed onto each experimental plot; the cages each enclosed an area of 1 m². The cages were described in Dosdall et al. (1996) and were dug into the soil to prevent escape of insects from within the traps or their entry from the outside. Approximately 425 adults, 200 pupae, 300 larvae and 500 eggs of P. xylostella were placed into each cage. The diamondback moth specimens were F₃ progeny of ca. 100 adults collected in the field in June each year (1993 and 1994) and reared on canola in greenhouse chambers. The field cages were examined three times per week for the presence of living diamondback moths from 1 May to 31 July in 1994 and 1995.

The experiments were repeated in 1995–1996 and 1996–1997 at Vegreville and in 1996–1997 at Saskatoon, SK (106°38’N; 52°07’W). The same experimental design, treatments and diamondback moth numbers were used as described above except that the insects placed into the cages were acclimated, not derived from laboratory colonies. Acclimation of the diamondback moth specimens was accomplished by placing five large screened cages (3.5 m x 3.5 m at the base and 2 m high) onto tilled soil in the spring of 1995 and 1996, digging the cages into the soil and then seeding canola within each cage. Six weeks later, ca. 40 field-collected adults of P. xylostella were added to the cages and progeny from these individuals were removed and placed into the overwintering cages in mid-September of 1995 and 1996. The field cages were examined three times weekly for the presence of living diamondback moth specimens from 1 May to 31 July in 1996 and 1997.

A third overwintering study was conducted from mid-October 1997 to late July 1998 at the Vegreville, AB site. The study used the same experimental design, treatments, diamondback moth numbers and acclimated specimens as described above, but sheets of styrofoam (90 x 90 cm, and 5 cm in thickness) were secured over the diamondback moth specimens within the 1 m² field cages to simulate snow cover. The styrofoam sheets were placed in the field cages on 20 October 1997 and secured with ropes and metal stakes to the soil surface. The sheets were removed on 20 April 1998, corresponding to the approximate time of snow melt. The cages were examined three times weekly for living diamondback moth specimens from 1 May to 31 July 1998.

To investigate dispersal of diamondback moth to western Canada from source populations to the south, air parcel trajectories were analysed to follow air movement from southern North America to western Canada during 1999–2001. The trajectories were constructed from wind fields at discrete intervals and solved numerically (D’Amours & Pagé 2001). The trajectories utilised wind fields of the Global Environmental Multiscale (GEM) model, which had a horizontal resolution of 24 km and 28 vertical levels over North America. The trajectories have been used successfully as a diagnostic tool for documenting long-range transport of air pollutants (Olson et al. 1978), tracking volcanic ash clouds for aircraft advisories (Servranckx et al. 1999) and for other applications. In our study, the model was run at three levels corresponding to approximately near surface (950 hPa) and 1500 (850 hPa) and 3000 (750 hPa) m above sea level and followed parcels of air on curves denoting their successive positions in time. By following forward trajectories for the air parcels through time, it was then possible to diagnose advection from possible source locations.

Results

No living specimens of any life stage of diamondback moth were collected in the field cages on any of the tillage or organic cover treatment types at Vegreville, AB from 1 May to 31 July 1994 and 1995. Similarly, no overwintered, living specimens were collected at Vegreville or Saskatoon, SK from May to July 1996 and 1997 in the field studies using acclimated specimens of diamondback moth. No living specimens of P. xylostella were recovered from 1 May to 31 July 1998 in field cages after removal of the styrofoam sheets used to simulate snow cover.

From 1 May to 30 June 1999 and 2000, there was no evidence of sustained advection from southern North America to western Canada. However, in 2001 prolonged advection occurred for approximately 10 days in early May involving airflow from Texas, Louisiana, Georgia, and Florida into eastern Saskatchewan, Manitoba, and Ontario. A map of airflow into western Canada is presented in Figure 1 for a selected time during 2–7 May 2001; the map is reasonably representative of the direction of movement of air parcels on several occasions during this period.

Data indicate that air movement at all three levels (near surface, 1500 and 3000 m) was directed northward from the Texas-Mexico border near Brownsville, TX from 2–3 May 2001, but thereafter air parcels that had originated near the surface on 2 May continued on a northward trajectory, but airflow originating at 1500 and 3000 m on 2 May veered northeastward. The airflow that originated at 1500 and 3000 m on 2 May travelled through Oklahoma, Kansas, Missouri, Illinois, Indiana and Ohio before traversing the eastern seaboard of the U.S. and moving over the Atlantic Ocean. The air parcel originating at 1500 m on 2 May eventually veered northward after crossing the U.S. eastern seaboard to traverse Nova Scotia and Prince Edward Island in eastern Canada. Air parcels that originated near the surface on 2 May at Brownsville, TX climbed to 2000 m on 4 May while traversing the states of Oklahoma and Kansas and then on 5 May were elevated to 4000 m and crossed Nebraska and South Dakota. From 5–6 May, this air stream passed across North Dakota and Saskatchewan at a height of approximately 4000 m. By 7 May, the air parcels had crossed all regions of canola production in western Canada, and moved across Hudson Bay (Figure 1).

Diamondback moth is routinely abundant on cole crops in the Lower Rio Grande Valley of southern Texas (Santa Ana 1999) and in spring 2001 enormous numbers of P. xylostella larvae were observed in northwestern Texas feeding on brassicaceous weeds associated with wheat crops (Porter & Leser 2001). A report issued by the Lubbock Research and Extension Center, Lubbock, TX stated that on 30 April 2001 larvae were pupating and would be flying in four to five days (Porter & Leser 2001).

In early May 2001, large numbers of diamondback moth adults were observed in canola crops in Manitoba, Saskatchewan and Alberta (WCCP 2001).

Figure 1. Canadian Meteorological Centre time-forward trajectories starting at Brownsville, TX at 00 UTC, 2 May 2001 and ending at 00 UTC, 7 May 2001. Air parcels were tracked from initial pressure levels of 950, 850 and 750 hPa (approximately near surface, 1500 and 3000 m above ground). The approximate altitude of the air parcels is shown on the vertical cross-section (bottom), and the horizontal position is shown on the top diagram.

Discussion

Successful overwintering of diamondback moth, as reported by Dosdall (1994), is evidently a rare phenomenon in western Canada because we found no evidence for overwintering survival of P. xylostella in six site-years of study in either Alberta or Saskatchewan using laboratory-reared and field-acclimated specimens, with or without simulated snow cover. Overwintering of diamondback moth therefore could not presently contribute substantially to economic damage caused by this species in canola crops in western Canada. Nevertheless, current models of climate change predict that with increased greenhouse gas emissions, conditions in western Canada will become warmer and drier in future years (Yonetani & Gordon 2001) and, should this occur, the pest status of P. xylostella in canola in western Canada is certain to increase.

Although adults of P. xylostella disperse poorly under their own power, generally travelling less than 1 km in their lifetime (Shirai & Nakamura 1994), long distance passive dispersal by this species has been documented both in Europe and in Asia. French (1967) reported a sudden, vast increase in diamondback moth populations in northeastern England and Scotland in 1958, where numbers increased over a few days from near zero to approximately 70–140 million adults per ha. According to airflow trajectories, the moths originated in Scandinavia and migrated approximately 3700 km. The invasion of a weather ship in the Atlantic, some 800 km from the coast of Scotland, by thousands of moths in 1958 could only be explained by long range migration on prevailing winds from Norway (French & White 1960). In Asia, diamondback moth specimens have been collected in the Pacific Ocean far from their nearest possible source locations in China and Japan (Chu 1986).

In Canada, invasions by insects arising from advections from the south have been reported previously. For example, long-range migration from Texas to Saskatchewan has been demonstrated for the sunflower moth, Hoemosoma electellum (Hulst) (Lepidoptera: Pyralidae) (Rogers et al. 1986). Although similar long-range migration has been proposed for diamondback moth, little direct evidence exists from previous studies to document this phenomenon. Smith and Sears (1982) proposed that diamondback moth populations in southern Ontario originated from southeastern United States because successful overwintering was not observed in Ontario and pheromone trap captures of P. xylostella correlated well with periods of airflow from the south. Putnam and Burgess (1977) and Philip and Mengersen (1989) stated that advection from the south was responsible for diamondback moth infestations in canola in western Canada, but provided no corroborative evidence. Our study provides evidence to support the assumptions of these authors. Overwintered diamondback moth populations could not explain the massive outbreaks of this species in canola in western Canada in 1995 and 2001, or the sudden occurrence of enormous population densities of adults observed throughout Alberta, Saskatchewan, and Manitoba in early May 2001. We have documented advection from southern Texas during 2–7 May 2001 that occurred concurrently with great increases of diamondback moth adults in canola fields throughout western Canada. The source location was reported to harbour large numbers of diamondback moths in April 2001 (Porter & Leser 2001).

In the southern United States, cole crops and brassicaceous weeds would form the principal reservoirs of diamondback moth populations that could then be carried northward when strong winds develop. Peak densities of P. xylostella usually occur in this region from late April to early May (Reid & Bare 1952) at precisely the time of the northward advections observed in 2001.

Advection from southern U.S.A. to Saskatchewan and Manitoba in the first week of May 2001 cannot, in isolation, explain the enormous increases of P. xylostella populations observed in 2001 over some 11 million ha of canola cropland ranging geographically from the Peace River region of northwestern Alberta to fields in southeastern Manitoba near the Canada-U.S.A. border. The diamondback moth outbreak of 2001 in western Canada was probably initiated by a number of disjunct advection events, originating from more than one southern source location.

The outbreak of diamondback moth in canola in western Canada during 2001 was terminated by attack of its natural enemies. The principal agents responsible for biocontrol of P. xylostella were parasitism by Diadegma insulare (Cresson) (Ichneumonidae), Microplitis plutellae Muesebeck (Braconidae) and Diadromus subtilicornis (Gravenhorst) (Ichneumonidae) (Mason & Dosdall, unpublished data), and epizootics of fungal pathogens (Keddie & Dosdall, unpublished data).

The origins of diamondback moth parasitoid populations are uncertain: they may either overwinter in western Canada, migrate northward passively with the same or similar air currents that bring their hosts, or they may actively disperse northward after diamondback moth has invaded canola in western Canada. The latter hypothesis is unlikely because parasitoid infestations occurred too early in the season for wasps to have migrated such extensive distances independently. Overwintering of parasitoids is a more likely possibility and this may explain attack by at least one parasitoid species. Putnam (1978) concluded that M. plutellae could overwinter in western Canada and was important in regulating diamondback moth populations early in the season. However, not all parasitoids of diamondback moth may overwinter in western Canada. Diadegma insulare has greatest importance for reducing P. xylostella populations late in the season (Putnam 1978) and because it occurs as far south as Venezuela (Carlson 1979), it may not overwinter in western Canada, but migrate northward along with its host.

Information on the site(s) of origin of diamondback moth populations invading western Canada’s canola crops and their genetic background(s), has important implications for devising pest management strategies. Applications of broad-spectrum chemical insecticides are used routinely in canola to control diamondback moth infestations when densities exceed threshold levels (Philip & Mengersen 1989), so information on the origin of western Canadian populations can be important for determining appropriate chemical control measures. Insecticide resistance is common in this species following continual application of the same product over time (Talekar & Shelton 1993), so historical information on the insecticide treatment regime used on the migrants at their site of origin can be a key factor for determining the most appropriate control recommendations for use in canola in western Canada. By using airflow trajectories, it should be possible to determine this important background information and implement a sustainable management strategy for diamondback moth infestations in canola in western Canada.

Acknowledgements

We gratefully acknowledge funding for the project provided by Agriculture and Agri-Food Canada and the Alberta Research Council. Special thanks are extended the Canadian Meteorological Centre of Environment Canada, Dorval, PQ, for the trajectory model used for monitoring diamondback moth dispersal to western Canada and to R. Servranckx for helpful comments on the manuscript.

References