Gorse, Ulex europaeus L. (Fabaceae), is a perennial, spiny shrub which can form dense, impenetrable thickets that compete strongly with pasture and other desirable plant species (Richardson and Hill, 1998). Gorse is a Weed of National Significance (Thorp, 1999) and has a serious impact on agricultural land and environmentally significant regions in South Eastern Australia. As part of an integrated management strategy, a guild of host-specific biological control agents are currently being introduced to gorse in Australia (Ireson et al., 2004).
One of these agents, the gorse spider mite, Tetranychus lintearius Dufour (Acari: Tetranychidae), has been introduced to several countries where gorse is a serious weed including New Zealand (Richardson and Hill, 1998), Chile (Norambuena et al., 2000), Australia (Ireson et al., 2003), Hawaii and Pacific coast states of the USA (Markin et al., 1996). T. lintearius is of European origin but was introduced to Australia in 1998 using populations previously established in New Zealand. It is now distributed throughout Tasmania, is widespread in Victoria and is established in southern New South Wales (Ireson et al., 2006). T. lintearius is considered to be a promising biological control agent for gorse as high levels of damage have been observed in the native range of both species in Europe (Zwolfer, 1963), it is capable of producing several generations a year (Stone, 1986), is host specific to members of the genus Ulex (Hill and O'Donnell, 1991 a, Ireson et al.,2003) and is reproductively isolated from pest Tetranychus species (Hill and O'Donnell, 1991 b).
Mites in the family Tetranychidae use piercing and sucking mouthparts to puncture cells and remove cellular contents. This process changes physiological processes such as decreasing the photosynthetic rate and altering the transpiration rate (Tomczyk and
Kropczynska, 1985), which can reduce plant growth and even alter flowering and seed production patterns. Feeding by T. lintearius in high numbers can result in a distinctive widespread ‘bleaching’ damage to gorse.
During surveys for potential biological control agents of gorse in Western Europe, Zwolfer (1963) observed large populations of T. lintearius on gorse and stated that heavy infestations of T. lintearius could result in the death of single branches or of the whole plant. However, this statement was based on visual assessments without
supporting experimental data. In the only published experimental study of the impact of T. lintearius on gorse, Fowler and Griffin (1995) found that a treatment of T. lintearius significantly reduced shoot growth of gorse plants by approximately 50% over 34 days in the United Kingdom. In a New Zealand study (by P. McGregor, Landcare Research New Zealand Ltd., unpublished data), a T. lintearius treatment caused growth reductions of approximately 18% of whole plant weight after two successive years of attack. Richardson and Hill (1998) also cite the unpublished data from another New Zealand study (by T.R. Partridge) which showed that “heavy feeding by mites over consecutive seasons can kill individual shoots and cause severe reduction in the growth of plants in the third year”. In another unpublished study conducted in Oregon, USA, Rice (2004) concludes that under certain conditions T. lintearius can reduce the growth of gorse over one season.
day preoviposition period. Adult females had an mean longevity of 17.8 days (at 25oC) and at 23oC laid on average 2.3 eggs per day for 8 days. The mean sex ratio for
T. lintearius was 1:2.5 (males:females) but can be highly variable in natural
conditions due to aggregating behaviour displayed by the females. T. lintearius does not appear to display diapausing behaviour and is thought to spend the winter in the form of slowly breeding colonies (Stone, 1986). Colonies of T. lintearius move
slowly as a group, feeding and web spinning as they travel. Both the eggs and inactive stages are left behind amongst large colonies protected by fine webbing as the active stages in the colony move on (Stone, 1986).
Mites in the genus Tetranychus have numerous natural enemies, these include the Chilean predatory mite Phytoseiulus persimilis Athias-Henriot (Acari: Phytoseiidae) and species of mite eating ladybirds, Stethorus spp. (Coleoptera: Coccinelidae). These predators are known to occur in T. lintearius colonies in Tasmania (Ireson et al., 2003).
Another biological control agent for gorse, the gorse seed weevil, Exapion ulicis
Forster, (Coleoptera: Brentidae), has been present in Australia since 1939 (Evans, 1943). Adult E. ulicis feed on gorse foliage and flowers, and oviposit into young, green pods. The larvae of this species then feed and develop within gorse pods destroying a proportion of the seed (Hill et al., 1991). As T. lintearius is a foliage feeding agent that produces large colonies protected by fine webbing, it is possible that T. lintearius may interfere with E. ulicis oviposition on young green pods.
The major aim of this study was to quantify the damage caused by T. lintearius on the growth and reproductive capability of gorse. As T. lintearius may interfere with E. ulicis oviposition and as predators of T. lintearius are present in Australia, additional
objectives were to (a) test whether the presence of T. lintearius affects numbers of and the subsequent damage caused by the seed feeding biocontrol agent E. ulicis and (b) monitor T. lintearius colonies for the presence of predators. The use of T. lintearius as a biological agent for gorse in Australia, the interaction between agents and the