Abundance, activity and distribution of soil-surface arthropod predators in arable farmland

Abstract

Intensive agricultural techniques, such as the high input of pesticides has caused increasing pest problems in arable agroecosystems. In New Zealand, biodiversity and sustainability in agroecosystems have become important issues along with increasing concerns about consumer food safety and the environment. An appreciation of the composition, distribution and predation activity of predatory arthropod populations will contribute to a greater understanding of their biological control potential in arable agroecosystems. Encouraging and conserving populations of natural enemies for biological control purposes will contribute towards a reduced-input, more sustainable agriculture.

The abundance and ecological diversity of beneficial arthropods were compared between 16 organic and 17 conventional carrot (Daucus carota L.) fields in the North and South Island of New Zealand using a suction sampler. Organic fields had significantly higher numbers of Hymenoptera: Parasitica, Staphylinidae (Coleoptera) and Neuroptera (Hemerobiidae) compared with conventional fields. Organic fields also had a significantly more diverse predatory and parasitic community than did conventional fields, as measured by the "Sequential Comparison Index". The implications of these results for the potential of biological control of pests in conventional and organic crops are discussed.

The composition, distribution, activity and potential of predatory arthropods within a local agroecosystem was investigated. Pitfall trapping and suction sampling were carried out to determine the composition and spatial distribution of soil-surface predators in the crop and boundaries of a commercial carrot field over the growing season (January-May). High numbers of soil-surface fauna such as Araneae (spiders), Staphylinidae (rove beetles), Acari (predatory mites) and Opiliones (harvestmen) were found in mid field and boundary habitats. In pitfall traps soil-surface predatory arthropods comprised 73% of all the beneficial arthropods captured. In suction samples, soil-surface predatory arthropods comprised 61 % of the beneficial arthropods captured. Reasons for, and limitations of, the sampling methods are discussed.

Prey facsimiles (eggs of the Indian meal moth (Plodia interpunctella Hubner) to mimic carrot rust fly eggs) were used to ascertain the potential of soil-surface arthropods as predators of invertebrate eggs. Predation ranged from 0-100% in baited dishes in mid field and boundary habitats; however it was significantly higher in carrot crop boundaries than in mid field habitats. Egg predation rates and soil-surface arthropod numbers declined significantly from summer to autumn. This is discussed with reference to pest activity. Egg predation was positively correlated with the numbers of rove beetles and predatory mites found in pitfall traps and suction samples. The potential of these arthropods as predators of invertebrate eggs is considered.

Time-lapse video recording was carried out on dishes baited with brown blow fly (Calliphora stygia F.) and Indian meal moth eggs to provide direct evidence of the predation of eggs by soil-surface predatory arthropods. Harvestmen, rove beetles and predatory mites were identified as predators of blow fly egg baits and rove beetles and predatory mites were predators of the moth eggs. Egg cluster size was shown to affect encounter/predation of blow fly eggs by harvestmen. This is discussed with respect to a predator's ability to locate naturally oviposited eggs. Although this work was not carried out in carrot crops, it provided important evidence of the predation of eggs by soil-surface predatory arthropods.

An important part of soil-surface predator ecology is determining the importance of landscape features within the cropping habitat. The importance of three common field boundary types as reservoirs of soil-surface predators was determined. Artificially-placed eggs (brown blow fly and Indian meal moth) were placed in dishes at ground level in the verge of boundaries to determine which were used by those predatory arthropods that were capable of removing invertebrate eggs. Gorse boundaries had significantly higher predation levels (blow fly) (x̅=87%) than in post and wire fences (x̅=76%) and macrocarpa (Cupressus macrocarpa Gordon) hedges (x̅=69%). Egg predation was also compared between the verge (comprised of a variety of grass and weed species), centre (bare of vegetation) and adjacent crop habitat (outside) of all boundary types. Overall egg predation (blow fly) was significantly higher in the verge (x̅=89%) of boundaries compared with in the centre (x̅=80%) and outside (x̅=31.5%) positions in all boundary types. Predation rates of Indian meal moth eggs followed a similar pattern to that of brown blow fly eggs. However, overall egg predation rates of the moth eggs were approximately 45% lower than those of fly eggs. Predation levels are discussed with reference to prey size. Overall, these results show the very high rate of predator activity in typical field boundaries on the Canterbury Plain, New Zealand. These high rates are brought about by a largely non-native invertebrate fauna in non-native field margin vegetation. However, unlike published data on some of the carabids in European field boundaries, which shows clear emigration into the adjacent crop in the spring, there is little "evidence in the current work that the field boundary fauna interact with the adjacent cropped area.

Grass bank refuges were established, similar to those described in U.K. studies, to determine if soil-surface predator populations could be enhanced in arable farmland. In the second year after establishment, grass bank refuges contained significantly higher densities (up to 720 m²) of carabid beetles, rove beetles, spiders and centipedes than did adjacent 'conventional' field boundaries (up to 14 m²). These results are discussed in the context of enhancing soil-surface predators and their predation rates within a cropping system.

High numbers of soil-surface predatory arthropods, particularly rove beetles, spiders, harvestmen and predatory mites were found in arable crops. The importance of soil-surface predatory arthropod populations in arable farmland is discussed with particular reference to their biological control potential of invertebrate eggs. It is suggested that the reduction of intensive farming practices and conservation of uncultivated areas of farmland may increase the importance of conservation biological control in New Zealand.