Impact Type: Direct exposure to pollutant
A number of sudies (Dohmen et al. 1984, Mansfield & Freer-Smith 1981) have shown that plants that had been fumigated with SO2 and NO2 were more prone to aphid attack, and that aphid productivity was higher as a result of changes in the amino acid composition of the plants.
Work by Warrington (1987, et al. 1987), on peas (Pisum sativum L.) and the pea aphid (Acyrthosiphon pisum), showed that raised SO2 levels increased mean relative growth rate of the aphids and caused an increase the level of damage and reducing the yield from crop. Raised SO2 levels has the effect of increasing the reproductive rate and population growth rate of aphids to a rate at which natural predators are unable to keep them under control (Whittaker 1994, 2001). Similar paterns have been shown for other crop species (Whittaker 1994, 2001).
Some studies also suggest that aphids grow faster under conditions of high NO2 and SO2 (Whittaker 1994, 2001). Holopainen et al. (1995) have suggested that SO2 exposure has a greater effect on aphid performance than exposure to O3 and NO2 either separately or in mixtures.
Also moderately elevated SO2 concentrations can increase aphid numbers which in turn could increase stress and reduce growth of the plant. Even short exposure (4 hours) to elevated levels of SO2 can have as great an effect as long exposures (McNeill & Whittaker 1990).
Masters and McNeill (1996) suggest that it is difficult to predict how susceptible a particular variety of host plant will be to insect pests when stressed by pollution.
In healthy plant SO2 and other air pollutants are generally taken up diffusion through the stomata (Kersteins 1996), in addition to this the damage caused to the plant by insect feeding can itself significantly affect this uptake of pollutants (Whittaker 2001).
|Critical Load/ Level|
No estimate available