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The most obvious sign of attack is the presence of shiny black beetles crawling around the flowers of the host plant. Holes in the buds indicate where adults have fed on or laid their eggs in the buds. Severe damage to buds can cause the buds to drop leaving podless stalks (Williams and Free, 1978). Feeding in the flowers is restricted to the pollen-bearing stamens and few visible symptoms are apparent.
Hokkanen et al. (1986) described the use of trap crops to help protect cauliflowers and spring rape in southern Finland. Some trials showed that almost complete protection was possible, but this depended on being able to produce a trap crop in flower before the main crop, which could be difficult to time accurately. For cauliflower protection, the most attractive trap crops were Chinese cabbage, broccoli and rape. The trap crops were sprayed with insecticide, usually deltamethrin, when the pollen beetle numbers reached such a level that emigration onto the main crop was likely. During peak activity, spraying about twice a week was necessary. Crop losses were reduced from the 20-40% observed without a trap to 3-15% with the traps. For spring sown rape, earlier flowering varieties or winter rape was used as a trap in experimental plots. If the trap flowered at the right time before the main crop, savings of 50-95% in pesticide use were possible. There is currently renewed interest in the potential for exploiting pest preferences for host plant and growth stage to develop trap crop strategies which concentrate the pest onto early flowering rape and away from the damage-susceptible growth stage of oilseed rape (e.g. Nerad and Vasak, 2000; Frearson et al., 2005; Cook et al., 2006).
Research aimed at breeding cultivars of oilseed rape resistant to M. aeneus, by investigating pest responses to the glucosinolate content of different cultivars or other host plants have so far not shown a clear relationship between pest incidence and glucosinolate profile (e.g. Milford et al., 1989; Hopkins et al., 1998).
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:
M. aeneus is widespread and abundant throughout Europe where it is a major pest of Brassica crops, particularly oilseed rape (Brassica napus) and turnip rape (Brassica rapa) (Alford et al., 2003). Although it feeds from the flowers of many different plant species, it breeds only on cruciferous species and spreads rapidly to new rape-growing areas. The feeding activities of large numbers of new generation M. aeneus emerging in mid-summer can be of nuisance value to gardeners and ornamental plant growers. They can reach pest status on some flowering crops such as strawberries (Cross and Easterbrook, 1998) and runner beans at this time.
Adult feeding damage to buds can cause them to abort, thus reducing yields of seeds (Williams and Free, 1978; Nilsson, 1987, 1988). However, because of plant compensation, yields are affected only above 60% pod loss (Williams and Free, 1979). Larvae also feed on pollen (Cook et al., 2004) and nectar. Winter rape is affected less than spring rape because it usually starts to flower before the beetles are active, but the latter is much more heavily attacked. In Denmark, Hansen (2004) has reported that 80% yield reduction can occur. He has calculated that the economic damage threshold of 5% of yield can be exceeded by 0.1-3 M. aeneus per plant, depending on rainfall.
Flower visiting insects, including M. aeneus, are probably important pollinating agents (Williams, 1985).