Spiders invoke a sense of aversion and fear - responses that seem innate to human beings as part of our evolutionary history (1). But these arthropods are essential in maintaining the balance of insect populations in different ecosystems. Over 45,000 species of spiders have been described, and they are one of the most common predators in terrestrial habitats, with differing lifestyles and foraging behaviors (2; 3). Arachnids have an important role as regulators of insect populations and, indeed, have been used in biological control, which is an environmentally safe method of pest management where an organism is used to reduce the population density of another organism (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.
Spiders are mainly insectivorous, feeding on all stages of development, and are present in high numbers in habitats such as forests, grasslands, and shrublands, which might indicate that they are the main predators of insects (2; 5). They also feed on collembolans, sometimes on other spiders, and rarely on a nonarthropod prey. To supplement their arthropod diet, sometimes spiders will feed on plant materials (2). It should be noted that since they are generalist predators, they feed on many beneficial insects, which include honeybees and butterflies (3).
Studies have estimated that spiders eat between 400 and 800 million tons of insects, collembolans, and other invertebrates per year. A large percentage of these negatively affect humans, so spiders 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, for example (2).
The effect that spider communities have on insect populations is a result of both mortality inflicted and also of indirect (nonlethal) effects, such as a reduction of insect feeding activity as a method to diminish the risk of predation (2).
In China, spider populations in field crops have been augmented as a strategy for pest management for a long time (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, spiders are very good at reducing overall prey abundance (3). Several studies have demonstrated their capacity to significantly reduce prey densities. These have determined, for example, 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, finally, 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 apple), and rice paddies. Their effectiveness in reducing insect damage to harvest has been demonstrated 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 have indirect effects on insect populations, reducing their feeding activity. Besides this, they often catch more prey than what is required for consumption. While they might not be the best predator to apply in the field in case of a specific pest outbreak, they have been used as biological agents due to their potential of keeping overall pest densities at low levels.
References
(1) Owano, N., 2015. Human fear of spiders draws scientific focus. Retrieved from https://phys.org/news/2015-04-human-spiders-scientific-focus.html
(2) 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
(3) Fosdick, 2016. Spiders are among most effective predators of plant pests. Retrieved from https://phys.org/news/2016-06-spiders-effective-predators-pests.html
(4) van Lenteren, J.C. (ed.), 2007. Internet Book of Biological Control. 4th Edition, www.IOBC-Global.org, Wageningen, The Netherlands.
(5) ScienceDaily, 2017. Spiders eat astronomical numbers of insects: Spiders feed on an estimated 400 to 800 million tons of insects and other pests annually; in comparison, all humans consume about 400 million tons in meat and fish. Retrieved from https://www.sciencedaily.com/releases/2017/03/170314111251.htm
(6) Maloney, D., Drummond, F.A., and Alford, R., 2003. Spider predation in agroecosystems: Can spiders effectively control pest populations? MAFES Technical Bulletin, 190
(7) Chatterjee, S., Isaia, M., and Venturino, E., 2009. Spiders as biological controllers in the agroecosystem. Journal of Theoretical Biology, 258 (3)