Scotch Broom Biological Control

Overview

Scotch broom has two approved biological control agents in California: the stem moth, Leucoptera spartifoliella (Lepidoptera: Lyonetiidae), and the pod weevil, Exapion[=Apion]fuscirostre (Coleoptera: Brentidae). The pod weevil is widespread in California and occurs wherever Scotch broom is found. The stem moth is also widespread but its abundance is generally low and patchy. Recently, two other natural enemies have moved into California, apparently on their own: the gall mite, Aceria genistae (Eriophyidae: Acari) and the seed beetle, Bruchidius villosus (Coleoptera: Bruchidae).

The stem moth lays eggs on the green stems after bloom in the spring (Fig. 1). The larvae burrow into the stem and tunnel up and down the length of the stem to feed (Fig. 2). Larvae overwinter in the stem and leave their tunnels in spring to pupate, spinning a white cocoon attached to the stem ridges. The stem moth usually occurs in low numbers which appears to cause little damage in California. On occasion, when large numbers do occur and stem die back is observed, plants usually regrow from below the damage.

The pod weevil has one generation per year. Adults emerge from overwintering when broom is in flower and feed on Scotch broom stems, flower petals and pollen (Figs. 3 & 4). Later, when the pods form and the seeds begin to swell, the female chews a hole in the pod and deposits an egg on on individual seeds. Upon hatching, the larva burrows into the developing seed and consumes it from inside (Fig. 5). Eggs may be deposited on several seeds within a pod. The resulting larvae develop to adults in the pod and leave when the mature pod splits open in summer. Adults are inactive during the summer, fall and winter, hiding in sheltered sites off the plant. Weevils infest about 60% of pods, on average, which results in destruction of 30-40% of viable Scotch broom seed in California.

The gall mite is an accidental introduction that was first discovered in Washington State and has spread south into California on its own (Pratt et al. 2019). It is most common on Scotch broom growing in the foothills of the Sierra Nevada Mountains. The occurrence of the gall mite is variable, with some plants being heavily infested with galls while other plants nearby have only a handful of galls (Fig. 6). In some patches, almost all plants are galled, but in other patches, only one or two plants have just a couple of galls. As this is a new organism in California, the population levels are continuing to increase in abundance. Galls form in young flower and leaf buds, and high densities cause extensive stem die-back and prevent flower production. The mites are microscopic and are visible at 20x magnification (Fig. 7). The galls are 0.2-1.2 inches in diameter and are hairy in appearance. The number of generations is unknown but preliminary observations suggest that mites leave galls to infest new plants in June and hide under bud scales until the following spring. Mite dispersal is typically by blowing in the wind (like pollen). Host specificity of the mite is currently being evaluated by Dr. Paul Pratt, USDA Agricultural Research Service in Albany, CA. Field surveys to date have not found it on any native species.

The seed beetle was accidentally introduced in the eastern USA before 1919. The State of Oregon obtained a permit to move it from North Carolina into Oregon and introduced it starting in 1998. The beetle has been spreading on its own from Oregon south into California. It was first discovered in Siskiyou County in 2014 and has increased and spread steadily southward. It is now found in Marin County in the Coast Range, in El Dorado County in the Sierra Nevada Mountains, and everywhere in between. The seed beetle also has been observed in pods of French broom, Genista monspessulana, but at a much lower level than in Scotch broom. However, given its rapid spread, it is likely that the seed beetle uses French broom as a bridge between isolated Scotch broom populations. The seed beetle is still expanding southward into California, and how far south it will spread is unknown. French broom populations extend further south into California than Scotch broom, and it is not known if the seed beetle will continue to move south following the French broom. Given that the pod weevil and the seed beetle exploit the same resource, it is not known how the two will interact: will they combine to increase the overall destruction of seed or will the seed beetle simply replace the pod weevil so that the combined impact is unchanged? It is still too early to determine the outcome.

The seed beetle has one generation per year (Fig. 8). Adult females deposit their eggs on the outside of the young green seed pod in May. The egg hatches, and the larva burrows into the pod and into a seed where it feeds. Each larva completes its development within one seed (Fig. 9). Adults emerge from the seeds and wait inside the pods until they open in summer. The seed beetle now occurs wherever Scotch broom occurs in California. Surveys in California have found the seed beetle to destroy over 60% of seed at some locations. The seed beetle has been observed to develop on some nontarget species in the field in France and New Zealand, including a native California lupine (Lupinus arboreus) and a forage species (tree lucerne, Cytisus proliferus[= Chamaecytisus palmensis]) (Sheppard et al. 2006,Haines et al. 2007), and it is not likely to be permitted in the future.

Both the gall mite and the seed beetle are not permitted for use as biological control agents in California because their risk to nontarget species has not been full evaluated.

Biological Control Agents

Species	Common name	Distribution	Impact	Notes
Leucoptera spartifoliella	stem moth	wide	low	First released in CA in 1960
Exapion fuscirostre	pod weevil	wide	moderate	First released in CA in 1964
Aceria genistae	Scotch broom gall mite	wide	unknown	Accidental introduction, found in CA in 2014. Not a permitted agent.
Bruchidius villosus	seed beetle	wide	high	Accidental introduction, found in CA in 2014. Not a permitted agent.

How the Technique Is Employed

Of the permitted insects, the pod weevil provides the highest impact through direct destruction of seed. The pod weevil can be found by breaking open mature (black) pods and looking for adult weevils among the seeds. Adult females can be seen crawling on the green pods in May, and the empty eggshells remain on the outside of the pods for several weeks and can be seen with careful observation. It occurs wherever Scotch broom occurs in California.

The stem moth has too little impact in California to be recommended for use as a control organism.

See additional details in Andreas et al. (2017).

Special Tips

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

Pod weevil larvae damage the inside and outside of seeds, which helps to distinguish them from seed beetle larvae, which feed completely inside a seed. Adult pod weevils have longer snouts and wing covers (elytra) than the seed beetles.

Recent surveys in California (2014-2019) have found that the pod weevil and seed beetle combined destroyed over 80% of seeds in some locations.

Herbicides that kill Scotch broom plants during the fall, winter or early spring (before flowering), may reduce plant populations and not affect weevil abundance.

The gall mite and the seed beetle are increasing and will likely add to the level of control now provided by the pod weevil and the stem moth.

The seed beetle also attacks French broom, which currently has no insects attacking seeds, so it may provide some benefit in reducing French broom seed production.

Caveats

The pod weevil and the stem moth attack only Scotch broom and have not been found on any native species.

The gall mite and the seed beetle are not permitted, so it is not legal to move them in CA.

Where Can I Get These?

The pod weevil and stem moth are widespread and likely occur wherever Scotch broom grows in California.

To date, there are no commercial sources for these insects.

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., G.P. Markin and T.G. Forrest. 2004. Scotch broom. In: E.M. Coombs, 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, pp. 160-168.

Haines, M.L., J.F. Martin, R.M. Emberson, P. Syrett, T.M. Withers, and S.P. Worner. 2007. Can sibling species explain the broadening of the host range of the broom seed beetle, Bruchidius villosus (F.)(Coleoptera: Chrysomelidae) in New Zealand? New Zealand Entomologist 30: 5-11.

Pitcairn, M.J. 2018. Weed biological control in California, USA: review of the past and prospects for the future. BioControl 63: 349-359. doi.org/10.1007/s10526-018-9884-6

Pratt, P.D., M.J. Pitcairn, S. Oneto, M. Brent Kelley, C. J. Sodergren, F. Beaulieu, W. Knee, and J. Andreas. 2019. Invasion of the gall mite Aceria genistae (Acari: Eriophyidae), a natural enemy of the invasive weed Cytisus scoparius, into California, USA and predictions for climate suitability in other regions using ecological niche modelling. Biocontrol Science and Technology 29: 494-513.

Sheppard A., M. Haines, T. Thomann. 2006. Native-range research assists risk analysis for non-targets in weed biological control: the cautionary tale of the broom seed beetle. Australian Journal of Entomology 45: 292-297.

Contributing Authors

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

Dr. Lincoln Smith, Research Entomologist, USDA.