Some of these effects may have been partly attributable to the choice afforded by adjacent cultivars. The preferences of leatherjackets could not be associated with the experimental design and are likely to operate in 'no-choice' situations.Abstract

In plots of perennial ryegrass and white clover, regular treatment with chlorpyrifos and mcthiocarb more than trebled the clover content of the sward, compared with untreated plots, in just over 2 years from sowing. Chlorpyrifos alone accounted for most of the effect, which was attributed mainly to the control of Sitona lepidus. Combined treatment increased total herbage dry matter yield by 424 t ha ' during this period but had little effect on grass-only plots. Melhiocarb appeared to have contributed by assisting earlier establishment of clover, but otherwise had little effect until slugs, which were initially uncommon, became more abundant in the later stages ofthe experiment. The effect of slug control was most noticeable in a cultivar of low cyanogenie potential. This factor did not infiuence the effect of chlorpyrifos.

In two experiments in established swards of perennial ryegrass and white clover, plots were treated with pesticides to control slugs, leatherjackets, weevil larvae and lucerne flea as selectively as possible. Other plots were treated to control ail of these pests.Chlorpyrifos application in early summer (to control weevil larvae) and methiocarb pellets (to control slugs) also gave partial control of leatherjackets. Other effects on non-target species were considered to be unimportant, as pest status could not be attributed to the spKcies. A high level of control of lucerne flea by methiocarb pellets, noted previously, was confirmed.At both sites combined treatment against insect pests and slugs produced large increases in clover content and yield over a 3-year period, in comparison with untreated plots, although clover tended to decline In alt plots. At one site, where the clover content of untreated plots reached 33% at one stage, specialist clover herbivores such as weevils and lucerne flea were more common. Treatments directed against weevils and slugs gave the highest clover yields, and leatherjacket control may also have contributed, particularly when the population reached 0-9 m ha' in the final year. At the other site, where the clover content was low (ultimately 01-1-6%), leatherjacket populations reached 2-1 m ha"' and slug numbers were also relatively high. Control of these species contributed most to the relative increase in clover content and yield.

In laboratory experiments to clarify earlier field observations, Sitona lepidus larvae caused significant damage to established clover plants at infestations of approximately 1 -2 per plant. When eggs were placed on the soil, cultivars of differing cyanogenic capacity were damaged equally. The plants did not recover when larval feeding ended.Leatherjackets, slugs and 'lucerne flea' fed most on the least cyanogenic cultivars, even where no choice was offered. Plants ultimately recovered from damage by leatherjackets. Deroceras reticulatum caused more damage than did Arion fasciatus. The proportion of leaves damaged by lucerne fiea was highest in the least cyanogenic eultivar but also tended to increase in the most cyanogenic cultivar, confirming a field observation. At high population densities lucerne flea was capable of killing some seedlings but was not tested against established plants.

No abstract