Linepithema humile (Mayr 1868)
Synonyms (WWW5 )
Hypoclinea humilis Mayr, Iridomyrmex humilis (Mayr)
There are 28 species and subspecies of Linepithema. Only one of these occurs outside Central and South America, L. humile. Now established in many countries (including Australia) well beyond its original range, this species was first recorded as established here in January 1990, at Mt Smart stadium, Penrose, Auckland (Green 1990).
Distribution (see map)
Since its discovery, L. humile has not only spread in the Auckland area but is now to be found in Northland, Coromandel Peninsula, Bay of Plenty, Waikato, Hawkes Bay, Wellington City, Nelson City and Christchurch.
The mandibles have 5–8 teeth and 5–13 denticles, with the first (apical) tooth elongate and much longer than the second (subapical) tooth. The front margin of the clypeus has a broad, shallow concavity.
Diagnostic features of the worker
Length 2.2 to 2.6 mm; antennae 12-segmented; upper and rear faces of the propodeum convex; colour uniformly light brown; similar to some Iridomyrmex in overall body shape and colour, it differs in the number of teeth on the mandibles, a more tear-drop shaped head and the relatively low placement of the eyes (Shattuck 1999).
Several biological factors ensure the success of this species as a notorious pest. These include: multi-queened (polygynous) colonies — hence a high fecundity; large numbers of offspring and rapid recruitment that lead to dominance over larger ants at food sources; an ability to exploit a diversity of habitats and food sources; a propensity for forming supercolonies through successful mixing of individuals from separate nests linked by foraging trails.
Harris (2001) writes of L. humile in this country: "Foragers move steadily in defined continuous trails and tend to walk over objects placed on trails. They have only a slight greasy odour when crushed, as opposed to the strong formic acid smell of some ant species when crushed. Trails can often be found along smooth surfaces, and in an urban environment they are commonly found by pulling back the grass at the edge of the footpath. Foragers collect honeydew from scales [scale insects] and aphids and collect nectar from flowers. Trails can often be found ascending flowering trees and shrubs. Nests can be found under wood, stones, piles of leaves in potted plants, etc."
L. humile rank highly as a domestic nuisance species. They invade houses and are capable of penetrating food containers (Davis & Van Schagen 1993). They infest gardens, making outdoor dining difficult. When nests are disturbed, foragers will run up legs and arms, and some people are sensitive to their bite. L. humile has the potential to carry and hence spread disease (e.g., Staphylococcus , Candida , and Enterococcus ) around buildings, including hospitals (Fowler et al. 1993).
L. humile frequently displace most ant species (e.g., Ward 1987), which can have flow-on effects on the ecosystem as ants not only constitute a large component of the total animal biomass, but can also act as engineers affecting soil processes (Folgarait 1998). The strong competitive ability of L. humile , together with its broad diet, mean that through direct predation (Human & Gordon 1997), competition, interference, and egg predation (e.g., cerambycids, Way et al. 1992), it will interact with many invertebrate species. When the total ant biomass is increased following the invasion of L. humile, the invertebrate community is negatively impacted (Human & Gordon 1997; Cole et al. 1992).
L. humile feed extensively on the honeydew produced by homopterans (Lester et al. 2003) and actively disperse the homopterans and protect them from predation. This may increase adventive homopterans in native habitats and domestic and commercial orchards, interfere with native predators of homopterans, and help transmit diseases between plants.
Argentine ants have been predominantly sampled in urban areas and on the margins of native habitats so their potential impact on native systems remains unknown, although forest habitat appears unlikely to be utilised (Harris et al. 2002).
Compiled by Warwick Don & Richard Harris