Spiders as Biological Control Agents: The Guardians of Balance in Ecosystems
May, 2017Spiders may trigger aversion and fear, responses ingrained in our evolutionary history.[1] However, these arthropods are more than just creepy crawlies, balancing insect populations in various ecosystems. With over 45,000 known species, they reign as one of the most common predators in terrestrial habitats, with unique lifestyles and foraging behaviors.[2; 3] Arachnids have an important role as regulators of insect populations, and have been used as biological control agents, an eco-friendly method to reduce pest densities.[4] While spiders can influence the population dynamics of other arthropods, they might not be the best natural enemy to use for every occasion where a pest attacks an agroecosystem.
A generalist predator keeping insects in check
Spiders are mainly insectivorous, feeding on all stages of development, and are present in high numbers in habitats such as forests, grasslands, and shrublands. This might indicate that they are the main predators of insects.[2; 5] Their versatile palate also includes collembolans, other spiders, and occasionally non-arthropod preys. To supplement their diet, sometimes spiders will feed on plant materials.[2] As generalist predators, they also impact beneficial insects, like honeybees and butterflies.[3]
According to estimations, spiders consume 400 to 800 million tons of insects and other invertebrates annually. They exert a positive predation pressure on insect populations, some of which are pests, especially in forest and grassland areas.[2; 3; 5] This pressure occurs alongside other natural enemies of insects, like ants, predaceous bugs, and birds.[2]
The effect of spider communities on insect populations results from both mortality inflicted and indirect, nonlethal effects such as the reduction of insect feeding activity, a defensive strategy to diminish their risk of predation.[2]
Pest management allies
In China, there are examples of augmented spider populations in field crops as a strategy for pest management.[6] Nevertheless, some have argued that they cannot be an effective pest controller since they are polyphagous predators.[7] While their feeding behavior might exclude them as a good choice to eradicate a specific pest outbreak in fields, they are very good at reducing overall prey abundance.[3] Several studies demonstrated their effectiveness in reducing prey densities. These have determined that spiders in a maize crop diminished populations of leafhoppers, thrips, and aphids; proved that spiders are effective predators of herbivorous insects in apple orchards; and shown that herbivorous insect populations significantly increase when spiders are absent.[6]
As biocontrol agents, they have been used mainly in two groups of crop ecosystems: orchards (especially apples) and rice paddies. Spiders effectively reduced insect damage to harvests in apple orchards in Europe, Israel, Australia, and Canada. The pest management strategy in orchards has focused on spider conservation by reducing pesticide use.[6; 7] Also, in Japan, spider populations are maintained and enhanced by releasing fruit flies into the fields before pest insects become abundant.[6]
In conclusion, spiders can be effective predators of herbivorous insect pests. Besides the mortality produced, they also indirectly affect insect populations, reducing their feeding activity. Furthermore, they often catch more prey than required for consumption. While they might not be the best predator to apply on the field in case of a specific pest outbreak, they can be used as biological agents to maintain overall pest densities at low levels.
References
- Owano, N. (2015). Human fear of spiders draws scientific focus. Phys.org. https://phys.org/news/2015-04-human-spiders-scientific-focus.html
- Nyffeler, M. and Birkhofer, K. (2017). An estimated 400 – 800 million tons of prey are annually killed by the global spider community. The Science of Nature, 104: 30. https://doi.org/10.1007/s00114-017-1440-1
- Fosdick, D. (2016). Spiders are among most effective predators of plant pests. Phys.org. https://phys.org/news/2016-06-spiders-effective-predators-pests.html
- van Lenteren, J.C. (ed.) (2007). Internet Book of Biological Control. 4th Edition, www.IOBC-Global.org, Wageningen, The Netherlands.
- ScienceDaily (2017). Spiders eat astronomical numbers of insects. ScienceDaily. https://www.sciencedaily.com/releases/2017/03/170314111251.htm
- Maloney, D., Drummond, F.A. and Alford, R. (2003). Spider predation in agroecosystems: Can spiders effectively control pest populations? MAFES Technical Bulletin, 190
- Chatterjee, S., Isaia, M., and Venturino, E. (2009). Spiders as biological controllers in the agroecosystem. Journal of Theoretical Biology, 258 (3):352-62. https://doi.org/10.1016/j.jtbi.2008.11.029
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