Gorse Biological Control

Overview

Gorse has three approved biological control agents in California: the pod weevil, Exapion ulicis (Coleoptera: Brentidae), the gorse spider mite, Tetranychus lintearius (Acari: Tetranychidae), and the gorse thrips, Sericothrips staphylinus (Thysanoptera: Thripidae).

The pod weevil is widespread in California and occurs wherever gorse is found (Fig. 1). The adult female emerges from overwintering in early spring when gorse is in flower and feeds on the yellow flower petals and pollen. Later, when the pods form and the seeds begin to swell, the female chews a hole in the pod and lays a clutch of eggs. The larvae burrow into the developing seeds and consume them from inside (Fig. 2). The larvae develop to adults inside the pod. Adults leave when the mature pod dries and splits open, and they hide in sheltered places such as soil litter during the summer and winter. About 60% of pods are infested with weevils, resulting in destruction of 30-40% of viable seed.

The gorse spider mite is the first spider mite approved for use as a biological control organism in the United States (Fig. 3). Host specificity of the mite was thoroughly examined, and it was found to be safe. Both immatures and adults feed on the leaf tissue, and the stress from heavy damage can reduce flowering and seed production. Soon after release, the spider mite built up high populations, and whole blocks of plants were covered with their webbing (hence their name “spider” mite; Fig. 4). Later, predaceous mites and small ladybugs that specialize on mite predation moved into these areas, and their feeding caused severe declines in the abundance of the gorse spider mites. Currently, the mite occurs at low levels primarily around its original release sites in Marin, Sonoma, Mendocino, and Humboldt counties. Usually spider mite populations are too low to provide much damage.

The gorse thrips was approved in 2019 as the third biological control agent for gorse in the United States (Fig. 5). Host specificity testing showed this insect to be highly specific to gorse and safe for introduction. Its native range is western Europe and overlaps the native range of gorse in Europe. It has been introduced as a biological control agent in New Zealand, Australia, and Hawaii. Adult females deposit their eggs in slits in young gorse stems, and nymphs and adults feed on leaf and stem tissues (Fig. 6). In California, we expect 2-3 generations per year. This is a new agent and its first release occurred on June 5, 2020 in Marin County.

The leaf moth, Agonopterix nervosa (Lepidoptera: Depressariidae), is an accidental introduction that commonly occurs on gorse plants (Fig. 7). Its larvae tie leaves together with silk, and in some localities, high populations can cause severe damage to the growing tips. The leaf moth is not permitted for use in California.

Biological Control Agents

Species	Common name	Distribution	Impact	Notes
Agonopterix nervosa	leaf moth	wide	low	Accidental introduction, first recovered in the 1920s. Not a permitted agent.
Exapion ulicis	gorse pod weevil	wide	moderate	First released in CA in 1964
Sericothrips staphylinus	gorse thrips	new agent	unknown	First released in CA in 2020
Tetranychus lintearius	gorse spider mite	limited	low	First released in CA in 1994

How the Technique Is Employed

The gorse pod weevil is widespread and probably occurs wherever gorse grows in California.

The first release of the gorse thrips occurred in 2020 at a field research site in Marin County. A culture of the thrips is being maintained at the USDA-ARS Invasive Species and Pollinator Health Research Facility, and additional releases in Marin, Sonoma, and Mendocino counties will occur for the next few years.

The spider mite is not recommended because it is usually subdued by predatory mites. See additional information in Andreas et al. (2017).

Special Tips

The pod weevil does best in open, sunny sites, and poorly at sites exposed to saltwater spray.

Caveats

The pod weevil is present on plants from March through July. Adult weevils emerge from split pods in July and are off the plant until flowering in the spring.

The gorse thrips is present on plants all year round. Herbicides that kill plants will likely to prevent establishment or persistence of this biological control agent.

Where Can I Get These?

The pod weevil is widespread and likely occurs wherever gorse grows in California. There are no commercial sources for this insect. Adults can be knocked off plants in spring by beating branches with a stick or racket so that they fall into an open sheet or sweep net. This is most effective at cold temperatures (e.g. early morning), when insects are less able to fly. Adults can also be reared from mature pods that are collected in late spring (June) before they split open. Place the pods in a container with a fine screen top and wait for the adults to emerge.

The gorse thrips is a part of a new release program by the USDA-ARS and is not yet available. Releases in 2020 are planned for research sites. Once populations establish and build up to high levels, collections for redistribution will become available. There are no known commercial vendors of this beneficial insect.

References

Andreas, J.E., R.L. Winston, E.M. Coombs, T.W. Miller, M.J. Pitcairn, C.B. Randall, S. Turner, and W. Williams. 2017. Biology and Biological Control of Scotch Broom and Gorse. USDA Forest Service, Forest Health Technology Enterprise Team, Morgantown, West Virginia. FHTET-2017-01.

Coombs, E.M., J.K. Clark, G.L. Piper and A.F. Cofrancesco, Jr. 2004. Biological Control of Invasive Plants in the United States. Western Society of Weed Science, Oregon State Univ. Press, Corvallis.

Pratt, P.D., E.M. Coombs, and B.A. Croft. 2003. Predation of phytoseiid mites on Tetranychus lintearius (Acari: Tetranychidae), an established weed biological control agent of gorse (Ulex europaeus). Biological Control 26: 40-47.

USDA APHIS. 2019. Field release of the thrips Sericothrips staphylinus (Thysanoptera:Thripidae) for biological control of gorse, Ulex europaeus (Fabaceae),in the contiguous United States, Environmental Assessment July 2019.
https://www.aphis.usda.gov/plant_health/ea/downloads/2019/sericothrips-staphylinus-gorse-biocontrol-ea-fonsi.pdf

Contributing Authors

Dr. Michael J. Pitcairn, Program Manager, California Department of Food and Agriculture

Dr. Lincoln Smith, Research Entomologist, USDA