Martin Nyffeler

Spiders in natural pest control: a review

In spite of the large number of studies about the ecology of spiders carried out in the last two decades in different types of ecosystems all over the world, the significance of these animals as natural control agents is still largely unknown. In this paper the literature about that subject is reviewed. Totally 300 scientific papers, published between 1920 and 1984, are cited here.

Feeding ecology and predatory importance of wolf spiders (Pardosa spp.) (Araneae, Lycosidae) in winter wheat fields

Predation by wolf spiders (Pardosa spp.) was studied for two years in two winter wheat fields in eastern Switzerland, one field being investigated each year. The following wolf spiders predominated in the investigated wheat fields: Pardosa agrestis (Westr.), Pardosa amentata (Clerck), and Pardosa palustris (L.). P. agrestis constituted ≥75 % of all Pardosa spp. observed in the field.

Insectivorous activities of spiders in United States field crops

A survey of average spider numbers in U.S. field crops (considering a geographic range from the east to the west coast) gave an overall mean density of ≅ 1 plant‐dwelling spider/m2(± 0.18 SEM). This value is more than 100 times lower than Turnbulls famous ‘overall mean value’ (= 130.8/m2) computed from 37 published censuses of spider numbers in a wide variety of environments from all over the world. Crop fields are disturbed systems whose spider numbers are drastically reduced by agricultural practices such as pesticide use, cultivation, harvest, etc. Small sized spider individuals (including large percentages of immatures) numerically dominate the faunas of U.S. field crops, and these feed primarily on tiny prey organisms (< 4 mm in length). Small web‐building spiders are almost strictly insectivore (insects constituting > 99% of total prey). In contrast, the ‘hunters’ (nonweb‐building spiders) that actively search the plant surface for prey, exhibit a mixed strategy of araneophagic and insectivorous foraging patterns (insects constituting ≅ 90% of total prey). The aggressive ‘active searchers’ are highly polyphagous (compared to the small web‐weavers), but can narrow their feeding niche significantly when a suitable prey species reaches high numbers relative to other prey groups. The numerically dominant spider predators in U.S. field crops feed heavily on small plant‐sucking insects such as plant bugs (Miridae), fleahoppers (Miridae), leafhoppers (Cicadellidae), treehoppers (Membracidae), planthoppers (Delphacidae), and aphids (Aphididae), which is of interest from a biocontrol point of view. A typical agroecosystem spider (i.e., striped lynx spider) may capture an average of ≅ 1 prey per rainfree day in the field (by multiplying this value with the overall mean spider density of 1/m2, it follows that the average prey kill may be in the order of magnitude of ≅ 1 prey/m2/day). At prey densities of 100–300/m2 (as recorded in literature), spiders kill perhaps ≅ 0.3‐1 % of the potential prey per day. Extensive field and laboratory observations, experiments, and computer models conducted by research groups in different parts of the U.S. indicate that the collective predation impact of spiders may contribute significantly to lower pest levels in some crop fields.

Bat Predation by Spiders

In this paper more than 50 incidences of bats being captured by spiders are reviewed. Bat-catching spiders have been reported from virtually every continent with the exception of Antarctica (∼90% of the incidences occurring in the warmer areas of the globe between latitude 30° N and 30° S). Most reports refer to the Neotropics (42% of observed incidences), Asia (28.8%), and Australia-Papua New Guinea (13.5%). Bat-catching spiders belong to the mygalomorph family Theraphosidae and the araneomorph families Nephilidae, Araneidae, and Sparassidae. In addition to this, an attack attempt by a large araneomorph hunting spider of the family Pisauridae on an immature bat was witnessed. Eighty-eight percent of the reported incidences of bat catches were attributable to web-building spiders and 12% to hunting spiders. Large tropical orb-weavers of the genera Nephila and Eriophora in particular have been observed catching bats in their huge, strong orb-webs (of up to 1.5 m diameter). The majority of identifiable captured bats were small aerial insectivorous bats, belonging to the families Vespertilionidae (64%) and Emballonuridae (22%) and usually being among the most common bat species in their respective geographic area. While in some instances bats entangled in spider webs may have died of exhaustion, starvation, dehydration, and/or hyperthermia (i.e., non-predation death), there were numerous other instances where spiders were seen actively attacking, killing, and eating the captured bats (i.e., predation). This evidence suggests that spider predation on flying vertebrates is more widespread than previously assumed.

Spiders and harvestmen as gastropod predators

1. Reports are reviewed of gastropod feeding (malacophagy) by spiders and harvestmen. Although the standard textbooks on arachnids recognise the importance of gastropods as prey of harvestmen, none apparently refers to malacophagy by spiders. A review of several hundred papers on spider feeding habits revealed that species from several families kill and devour slugs and snails in the laboratory and/or field.

Spiders as predators of arthropod eggs

Examples of spiders preying upon the eggs of Araneae and Insecta are given from descriptions found in the literature from different regions of the world. In many cases, spiderlings or female spiders were feeding on the eggs of conspecifics (cannibalism). Evidence of conspecific predation on eggs has been reported for the families Dipluridae, Uloboridae, Scytodidae, Loxoscelidae, Pholcidae, Theridiidae, Agelenidae, Lycosidae, Gnaphosidae, Clubionidae, Thomisidae and Salticidae. Spiders feeding on the eggs of other species (interspecific predation by Salticidae upon eggs of other Salticidae and Uloboridae; Theridiidae upon the eggs of orb‐weaving spiders) has also been noted.

Fish predation by semi-aquatic spiders: A global pattern

More than 80 incidences of fish predation by semi-aquatic spiders – observed at the fringes of shallow freshwater streams, rivers, lakes, ponds, swamps, and fens – are reviewed. We provide evidence that fish predation by semi-aquatic spiders is geographically widespread, occurring on all continents except Antarctica. Fish predation by spiders appears to be more common in warmer areas between 40° S and 40° N. The fish captured by spiders, usually ranging from 2–6 cm in length, are among the most common fish taxa occurring in their respective geographic area (e.g., mosquitofish [Gambusia spp.] in the southeastern USA, fish of the order Characiformes in the Neotropics, killifish [Aphyosemion spp.] in Central and West Africa, as well as Australian native fish of the genera Galaxias, Melanotaenia, and Pseudomugil). Naturally occurring fish predation has been witnessed in more than a dozen spider species from the superfamily Lycosoidea (families Pisauridae, Trechaleidae, and Lycosidae), in two species of the superfamily Ctenoidea (family Ctenidae), and in one species of the superfamily Corinnoidea (family Liocranidae). The majority of reports on fish predation by spiders referred to pisaurid spiders of the genera Dolomedes and Nilus (>75% of observed incidences). There is laboratory evidence that spiders from several more families (e.g., the water spider Argyroneta aquatica [Cybaeidae], the intertidal spider Desis marina [Desidae], and the ‘swimming’ huntsman spider Heteropoda natans [Sparassidae]) predate fish as well. Our finding of such a large diversity of spider families being engaged in fish predation is novel. Semi-aquatic spiders captured fish whose body length exceeded the spiders’ body length (the captured fish being, on average, 2.2 times as long as the spiders). Evidence suggests that fish prey might be an occasional prey item of substantial nutritional importance.

The southern black widow spider, Latrodectus mactans (Araneae, Theridiidae), as a predator of the red imported fire ant, Solenopsis invicta (Hymenoptera, Formicidae), in Texas cotton fields

In cotton fields of east Texas, the southern black widow spider, Latrodectus mactans (F.), was observed to capture the red imported fire ant, Solenopsis invicta Buren, which constituted 75% of the prey of L. mactans. Remains of workers, queens, and males of S. invicta were found in the spider webs. L. mactans were observed to feed on ants captured in the webs indicating that S. invicta were used as a food source. Juveniles as young as third instar were observed capturing S. invicta workers. In the same area, 16 other species of spiders were observed to be predators of S. invicta.

An estimated 400–800 million tons of prey are annually killed by the global spider community

Spiders have been suspected to be one of the most important groups of natural enemies of insects worldwide. To document the impact of the global spider community as insect predators, we present estimates of the biomass of annually killed insect prey. Our estimates assessed with two different methods suggest that the annual prey kill of the global spider community is in the range of 400–800 million metric tons (fresh weight), with insects and collembolans composing >90% of the captured prey. This equals approximately 1‰ of the global terrestrial net primary production. Spiders associated with forests and grasslands account for >95% of the annual prey kill of the global spider community, whereas spiders in other habitats are rather insignificant contributors over a full year. The spider communities associated with annual crops contribute less than 2% to the global annual prey kill. This, however, can be partly explained by the fact that annual crop fields are “disturbed habitats” with a low buildup of spider biomass and that agrobiont spiders often only kill prey over short time periods in a year. Our estimates are supported by the published results of exclusion experiments, showing that the number of herbivorous/detritivorous insects and collembolans increased significantly after spider removal from experimental plots. The presented estimates of the global annual prey kill and the relative contribution of spider predation in different biomes improve the general understanding of spider ecology and provide a first assessment of the global impact of this very important predator group.

Spiders associated with selected European hay meadows, and the effects of habitat disturbance, with the predation ecology of the crab spiders, Xysticus spp. (Araneae, Thomisidae).

In hay meadows near Zurich, central Europe, the faunistic composition of the foliage‐dwelling spiders was evaluated using sweep‐net technique. Eleven families were represented from the 1502 spider individuals captured, the majority of which (> 85%) belonged to the families Theridüdae, Linyphiidae, Araneidae, Tetragnathidae, Lycosidae, Thomisidae and Salticidae. Of the adults collected (39 species, N = 203) 38% were orb‐weavers, 33% were irregular or sheet‐web weavers, and 29% were cursorial hunters. The crab spiders (Thomisidae), Xysticus cristatus (Clerck) and X. kochi Thorell, were the species most frequently collected.