Effects of polyphagous invertebrate predators on cereal pests

Abstract

The objectives of this study were to examine the impact of polyphagous predatory invertebrates, notably Carabidae, Staphylinidae and Araneae, on cereal pests and to quantify this with respect to yield and quality. This was achieved by manipulating the density of these predatory groups under field conditions using exclusion techniques in wheat crops. The impact of the polyphagous predators was also compared within conventional and integrated farming systems utilising the experimental set-up at one of the LINK Integrated Farming Systems Project (IFS) sites. This large-scale project was started in 1992 to compare conventional and integrated farming through a five-course rotation of cereals and break crops. Beneficial insects were encouraged for pest control within the integrated regime and this study allowed this to be examined.

A reduction in the number of polyphagous predators by up to 80% led to a 31% increase in the number of aphids per tiller. A greater impact was found with higher aphid populations and when infestation occurred earlier in the crop's development. Carabidae and Staphylinidae were more active and abundant in May and June and would be expected to exert a greater effect on earlier infestations. In contrast Araneae were most active in July and may be more useful for controlling late aphid infestations, as found in the two years of this study. Polyphagous predators did not reduce the number of tillers infested and therefore dod not affect when the recommended spray threshold was reached. Strong correlations were found between the activity/density and density of Carabidae, Staphylinidae and Araneae and cereal aphids in 1995 and to a lesser extent in 1996.

The maximum yield reduction where polyphagous predator activity and abundance was reduced was 0.37 t ha-1. Aphids infesting the crop during grain filling had a negative effect on grain weight, hectolitre weight and protein, although these effects varied with cultivar. For some cultivars and situations, aphids had a positive effect on thousand grain weight. However, when an insecticide was applied in 1995, at GS71, there was no increase in yield or quality compared to unsprayed areas.

Polyphagous predators were found to predate on orange wheat blossom midge (OWBM) before oviposition occurred in 1996 and they reduced the number of larvae per ear by an average of 1.5. Predation was greater following feeding when larvae vacated the grain and dropped to the soil to diapause. Where predators were reduced, the soil populations of OWBM larvae were increased by 30% and 180% in 1995 and 1996 respectively. Mis-timed or inappropriate insecticide treatments for midge may reduce this level of predation and allow midge populations to increase in the soil.

Later drilled crops had higher aphid populations because they were infested at a relatively late growth stage and the crops remained susceptible for longer. For midge, sowing date determined the crop's suitability for oviposition. The earlier sown crops were more vulnerable in 1995 whereas the opposite occurred in 1996. Higher larval populations in the ear also resulted in a greater return to the soil.

Numbers of Staphylinidae and Araneae were sometimes greater in the IFS plots. The species composition and abundance varied considerably between fields. Low predator numbers had no effect on aphids, because when numbers were reduced further by the enclosures, no increase in aphid numbers was detected. Greater measures would have to be taken to encourage beneficial invertebrates in an integrated farming system, but the results from this study indicate that this may be financially worthwhile.