The colour pattern of Linyphia triangularis closely resembles that of North American Pityohyphantes costatus and P. phrygianus. Both species have a bifurcated “tuning-fork” marking on the dorsal surface of the carapace, which is less obvious in L. triangularis males. The ventral surface of the femora are devoid of dark spots, while black or brown spots are usually present in P. costatus and P. phrygianus. Unlike Pityohyphantes, the tuning-fork markings do not extend to the posterior eye rows in L. triangularis (Jennings et al., 2003).

The web of L. triangularis consists of a flattened sheet, slightly arched in the centre and held in place by scaffolding thread above and below the sheet. The web lacks a retreat, with the resident spider hanging upside-down near the center of the sheet. The webs of L. triangularis in Maine resemble the webs of Pityohyphantes species (Jennings et al., 2003).

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“Linyphia triangularis meets 7 of the 8 criteria that characterize a successful invader (Ehrlich 1986), i.e., 1) abundant in original range; 2) polyphagous instead of monophagous or oligophagous; 3) short generation time; 4) fertilized female able to colonize alone; 5) larger than most relatives; 6) associated with Homo sapiens; and 7) able to function in a wide range of physical conditions. Only its genetic variability, compared to that of non-invaders, remains to be ascertained” (Bednarski et al., 2010).

Linyphia triangularis is univoltine in Europe. It overwinters as eggs in leaf-litter beneath trees and shrubs. Juvenile spiderlings emerge from the egg sac in the spring (May), and reach maturity by late July or August. Development is protandrous, with males reaching adulthood about a week earlier than females. The sexually mature males enter the webs of subadult females, where they remain until the female reaches maturity. Mating takes place in the web, and oviposition occurs in October or November. The life history of this species in North America has not yet been investigated (Jennings et al., 2003 and references therein).

The most favourable habitat for L. triangularis in Maine is along coastal margins, roadsides and forest-edge areas. Webs are found in seedlings, saplings, shrubs, forbs, ferns and other low vegetation (Bednarski et al., 2010; Houser et al., 2005). Specimens have also been collected from the lower-crown foliage of red spruce (Picea rubens) and in loose bark and tree boles of paper birch (Betula papyrifera) (Jennings et al., 2003).

The large size, competitive ability, and aggressive nature of Linyphia triangularis may have contributed to its successful establishment in Maine (Bednarksi et al., 2010). This invasive spider can reach very high densities, e.g. in parts of Acadia National Park, Maine population densities can reach 12 individuals/m2 (Houser et al., 2005). Correlational evidence suggests that L. triangularis has a negative effect on native spiders; in areas with high density of L. triangularis native linyphiid spiders are scarce (Houser et al., 2005) or virtually absent (Jakob, in prep in Bednarksi et al., 2010).

Reductions in native spider density are likely to be due to competitive displacement by L. triangularis. It exhibits aggressive behaviour including web “takeovers” towards conspecifics and congenerics (Jennings et al., 2003). Bednarski et al. (2010) found that native Frontinella communis spiders abandoned their webs when L. triangularis were added to plots and were less likely to establish webs in plots containing L. triangularis. Where L. triangularis took over webs, they evicted (or possibly consumed) F. communis, and reshaped the web to their own typical shaped webs, thereby making use of energetically valuable silk. Loss of a web comes at great fitness cost to spiders, in terms of time and calories (Venner et al., 2003 in Bednarksi et al., 2010).

The high density that L. triangularis populations reach in some habitats may mean that it reduces the amount of insect prey available for native spiders. However, sticky trap censuses found no evidence that L. triangularis reduces flying insect abundances (Houser et al., 2007 in Bednarski et al., 2010). Further work is needed to establish definitively whether competition for prey is important in Maine spider communities (Bednarski et al., 2010).

Competition and predation by L. triangularis on native sheet-line weavers and other spiders could lead to displacement of these species, and thus lead to a decline in biodiversity in invaded areas (Jennings et al., 2003).

Members of the Argyrodes genus are known for their diverse foraging strategies, including kleptoparasitism, web-stealing and predation on other spiders species. In Maine, the native spider A. trigonum was noted inhabiting some Linyphia triangularis webs. Surveys conducted in 2003 found that overall 27.0% of L. triangularis webs contained A. trigonum. As A. trigonum uses both L. triangularis and native spider webs, it could reduce or increase the effects of the invader on native populations. Further studies investigating preferences for different host webs would be useful in assessment of the impact. However, because of its generalist behaviour, the regulatory effect of A. trigonum on populations of L. triangularis is suspected to be minimal (Houser et al., 2005).

Instead, assemblages of natural enemies (e.g., parasites, parasitoids, predators, and pathogens) may be needed for control or containment of this invasive spider (Houser et al., 2005).