The Diamondback Moth in Canola and Mustard: Current Pest Status and Future Prospects.
Populations of the diamondback moth, Plutella xylostella (L.), routinely infest crops of canola (Brassica napus L. and Brassica rapa L.) and mustard (Brassica juncea (L.) Czern. and Sinapis alba L.) in the western Canadian prairies. In most years the insect causes minor economic damage, but in some years populations reach outbreak densities and substantial crop losses occur. 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. Its principal natural enemy, the hymenopteran parasitoid Diadegma insulare (Cresson), is not known to overwinter in the prairies and is believed to be carried northward along with its hosts. Consequently, whether invasions occur by one or both species has considerable effect on field densities of diamondback moth later in the season. Larvae may feed on all above-ground plant structures, but are particularly damaging when leaves senesce late in the season and they feed on pericarp of canola pods, preventing ripening, and so reducing yield. Brassicaceous weeds can provide important bridge hosts for diamondback moth, and among cultivated prairie crops, white mustard (S. alba) is most preferred followed by B. rapa, B. juncea, and B. napus. Soil fertility affects susceptibility of canola to infestation by diamondback moth. Females prefer to oviposit on plants fertilized at the rate recommended for canola production rather than on plants fertilized at higher or lower levels. Canola high in sulfur is particularly attractive, and field distributions of the insect were correlated with the highest levels of sulfur content in canola leaves. Nutrient levels in the leaves of plants fed upon by diamondback moth larvae also affect the D. insulare parasitoid which develops most rapidly in larvae that feed on leaves highly fertilized. The main control tactic for this pest currently involves chemical insecticide application when parasitoid populations are not abundant, but good potential exists for exploiting other control strategies, such as a bioinsecticide derived from the bacterium, Bacillus thuringiensis Berliner, or implementing agronomic practices that enhance the effectiveness of diamondback moth parasitoids and predators.
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