The degradation of sensitive agro-ecosystems coupled with the loss of biodiversity, and the enhancement of crop production, crop yield, as well as the reduction of crop production cost, have long been at odds, since chemical pesticides and crop enhancement products are among the major causes for biodiversity loss. That is, until the incorporation of Integrated Pest Management (IPM) methods into agricultural practices. IPM strives to optimise agricultural yields, minimise synthetic inputs, suppress pest populations to acceptable economic thresholds and boost ecosystem services by using biologically based pest management methods. This is achieved by promoting beneficial organisms and crop health. Beneficial organisms may promote crop health directly, via pollination, and indirectly via both biotic and abiotic means. Biotic benefits include the cultivation of habitats for natural enemies of crop pests and abiotic benefits are achieved by ecosystem engineers such as earthworms which aerate the soil.

Mulching is one such IPM method and was originally designed to benefit crop health via abiotic factors. Mulch consists of either woodchips, compost, straw, or black plastic, amongst others, which are applied to the soil surface surrounding the crops. The presence of mulch positively influences the crops by balancing soil humidity, temperature and structure. Mulches also increase the organic matter in soil which improves the crop’s affinity for pest resistance due to improved soil microbial activity, soil nutrient balance and decreased nitrogen content. The lesser known positive influence of mulching may be seen in its effect on arthropod communities. There are indications that mulching may influence ovipositional preferences, host plant discrimination and location as demonstrated by the pest insects’ ability to recognise crops as potential hosts, whereas the decreased soil temperature may affect the completion of the life cycle of pest insects.

Mulching is one such IPM method and was originally designed to benefit crop health via abiotic factors. Mulch consists of either woodchips, compost, straw, or black plastic, amongst others, which are applied to the soil surface surrounding the crops. The presence of mulch positively influences the crops by balancing soil humidity, temperature and structure. Mulches also increase the organic matter in soil which improves the crop’s affinity for pest resistance due to improved soil microbial activity, soil nutrient balance and decreased nitrogen content. The lesser known positive influence of mulching may be seen in its effect on arthropod communities. There are indications that mulching may influence ovipositional preferences, host plant discrimination and location as demonstrated by the pest insects’ ability to recognise crops as potential hosts, whereas the decreased soil temperature may affect the completion of the life cycle of pest insects.

pest_figure1
FIGURE 1. Vineyard with mulch on the vine row.
FIGURE 2. Average number of insects found in mulched plots versus control plots (clean cultivation) using pitfall traps during 2010. Plant feeders include potential pest species, generalists are omnivorous, while predators include predators and parasitic wasps. FIGURE 3. Parasitic wasp (Anagyrus sp.), a natural enemy of the vine mealybug.

There are many positive benefits that have been observed in mulched vineyards and orchards, such as an increase in natural enemies, though there have been a few negative effects too, which show that mulches have the potential to affect the arthropod composition in crop systems. This study therefore aimed to examine preliminary effects of various mulches on general soil-arthropod assemblages in South African vineyards in order to make recommendations for future research.

Four vineyards were sampled over one growing season in 2010 using pitfall traps, comparing plots with mulch layers (Fig. 1) and plots without mulch layers (clean cultivation). Mulches that were applied at the sites included woodchips, compost (comprising a dense layer of grape pomace) and straw. It was found that mulched sites had less pests and a greater arthropod diversity than non-mulched sites. Mulches could provide an ideal habitat for many insect species, improving the diversity of arthropods and thereby resulting in a more stable system. Springtails, which are common prey for many insect predators, as well as ants were also in higher abundance in non-mulched sites (control), which explains why predator populations were higher in non-mulched sites (Fig. 2). Ants have been shown to prefer bare soil in previous studies, as this provides optimal temperature conditions for their nests underground. Egg parasitoids, which are associated with predator roles, were also higher in mulched sites (Fig. 3). Mealybug parasitoids were found in the pitfall traps, which may show the role that mulches play as predator refugia and forage grounds on vineyard floors, though no significant difference was found between treatment (mulch) and control sites (non-mulch). The results further showed that there was a high degree of similarity between mulched sites, and that un-mulched sites also indicated a similar arthropod assemblage structure, which is a strong indicator that mulches do affect insect populations.

Pre-mulching sampling, compared to post-mulching sampling, would give a good idea of how mulching affects the arthropod assemblage and multi-season sampling would give an indication of seasonal influences. Although pitfall traps were adequate in this preliminary study, it was not ideal as an indicator of vine mealybug presence (a major vine pest) and the use of sticky traps and arboreal sampling would be recommended. A predator/prey interaction study would be useful to fully highlight mulch effects and benefits and this study suggests vine mealybug/ant/parasitoid tritrophic interaction as a model for future studies.

These preliminary results suggest that mulches do indeed affect arthropod assemblages; with higher arthropod diversity including more omnivorous species (generalist feeders) and fewer pest species and that they could therefore be useful for IPM.

Acknowledgements

This research formed part of an honours project conducted by Anna Baauw (Department of Conservation Ecology and Entomology, Stellenbosch University). The research was funded by Winetech and THRIP (Grant-specific unique reference number {UID} 71909).

References

Addison, P., Baauw, A.H. & Groenewald, G.A., 2013. An initial investigation of the effects of mulch layers on soil-dwelling arthropod assemblages in vineyards. South African Journal of Enology & Viticulture 34: 266 – 271.

– For more information, contact Pia Addison at pia@sun.ac.za.

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