20 Pauline Syrett and Rowan Emberson

Non-target host use by the broom seed villosus (F.) (Coleoptera: Chrysomelidae: Bruchinae) in Canterbury

Pauline Syrett1 and Rowan M Emberson2 1Landcare Research, PO Box 69040, Lincoln 7640 2Department of Ecology, PO Box 85084, Lincoln University, Lincoln 7647 Email: [email protected] Abstract The unexpected record from tree lucerne (Cytisus proliferus) of the seed-feeding beetle , introduced for biocontrol of broom (C. scoparius), indicated that tree lucerne and possibly other close relatives of broom might be significant alternative hosts. This simple, qualitative study aimed to compare the levels of infestation by B. villosus of seeds of tree lucerne with those of broom at five sites around Christchurch. In addition, white broom (C. multiflorus) and Spanish broom (Spartium junceum) were each checked at a single location. The level of infestation was highly variable in both broom and tree lucerne. However, at all but one site, where there was only a relatively small amount of broom, broom was strongly preferred over the alternative host plants. In spite of its acceptance in original host tests, white broom showed only low levels of attack and no were found in seeds of Spanish broom. Keywords: , Cytisus proliferus, Cytisus multiflorus, Spartium junceum, biological control Introduction Broom seed beetle, Bruchidius villosus (F.) (Coleoptera: Chrysomelidae: Bruchinae), was introduced into New Zealand from the United Kingdom in 1986 for biological control of broom, Cytisus scoparius (L.) Link (Syrett et al. 1999). Subsequently beetles were recorded from a non-target host, tree lucerne (or tagasaste) (Cytisus proliferus L.f., also known as Chamaecytisus palmensis (H.Christ) F.A.Bisby & K.W.Nicholls), an exotic shrub established in milder The Weta 46:120-27 and drier areas of New Zealand (Syrett 1999). Tree lucerne is native to the Canary Islands (Webb et al. 1988), where neither broom nor broom seed beetle occur.

In the original tests, conducted prior to the introduction of broom seed beetle into New Zealand, some development occurred on the closely related white broom (C. multiflorus (L’Her.) Sweet), but these tests failed to identify tree lucerne, Spanish broom (Spartium junceum L.) or any native New Zealand legumes as suitable hosts (Syrett & O’Donnell 1987).

Later studies revealed that broom seed beetle had a wider host range than originally thought, and was able to develop on a range of species in at least three genera (Cytisus, Genista and Spartium) within the subtribe Genistinae in the family , in the South of France (Sheppard et al. 2006). However, Haines et al. (2004) showed that populations of beetles from the United Kingdom, where the beetles imported to New Zealand came from, strongly preferred broom, as did the New Zealand populations that had been exposed to the alternative host for some 15 generations, though there was some increase in acceptance of the alternative host (Haines et al. 2013). Paynter et al. (2004) surveyed some possible non-target plants for a number of weed biocontrol agents that have been introduced into New Zealand, but did not include B. villosus in their field studies. The primary aim of this small study was to assess the relative field use of tree lucerne compared with broom in areas where the beetle has now been established for up to 25 years. Single populations of beetles were also sampled from two other plants suspected of being possible alternative hosts in New Zealand: Spanish broom (S. junceum) and white broom (C. multiflorus), both European species now well established in New Zealand. Methods Some preliminary sampling was undertaken in 2012 at four sites (Sign of the Takahe, Mt Pleasant, Corsair Bay and McCormack’s Bay) around the Port Hills, Canterbury. Sites were selected where

22 Pauline Syrett and Rowan Emberson tree lucerne was growing in close proximity to broom. Two hundred mature pods were collected from each of broom and tree lucerne on 4–5 January 2012 at each site. Because infestation rates in broom were found to be much higher than in tree lucerne, subsamples of 50 broom pods were used for three of the sites (Mt Pleasant, Corsair Bay and McCormack’s Bay). In 2013, the same sites were re-visited, as well as an additional Port Hills site for Spanish broom (Victoria Park) where both broom and tree lucerne were also present, and two adjacent sites at Chertsey, on the Canterbury Plains, for white broom where broom grew alongside (Table 1). Fifty pods were collected (10 from each of 5 plants) from each of the two hosts at each site on 7 January 2013. As Spanish broom pods were not mature at this stage, the Victoria Park site was re-visited and Spanish broom was sampled on 4 April 2013. Each sample was placed in a nylon mesh bag and hung in a cool garage until beetles emerged from seeds. The seeds were then checked under a microscope for the presence of beetles. Many seeds had emergence holes, some had beetles partly emerged, and some had beetles still inside. The total number of seeds infested in each sample was recorded, together with the total number of seeds in the sample, and the percentage infestation was then calculated.

Results and Discussion The number and percentage of seeds infested by broom seed beetles that were recorded in the seasons 2011/12 and 2012/13 respectively are presented in Tables 2 and 3. Of the four sites that were sampled in two successive seasons, broom seeds from Corsair Bay had the highest infestation rate in both years. Broom seeds at the Sign of the Takahe had the lowest infestation rate. However, infestation levels at this site may have been abnormally low because there were only a few, scattered plants there. Broom at Mt Pleasant and McCormack’s Bay had similar intermediate levels of infestation. Except for the Sign of the Takahe, where broom plants were even harder to find in the second season, infestation rates were higher in 2011/12 than in 2012/13. Infestation levels of tree lucerne seeds by broom seed beetle were substantially lower than those of broom in both years at all sites except for the The Weta 46:120-27

Sign of the Takahe in 2012/13. It is likely that the higher levels there were associated with the relative abundance of tree lucerne compared with broom. Although initial host screening tests showed that broom seed beetle was able to develop on white broom (Syrett & O’Donnell 1987), infestation levels at the two adjacent Chertsey sites sampled were low. No seeds of Spanish broom were infested at the single site sampled (Victoria Park) even though a quarter of the seeds from adjacent broom plants were found to have hosted broom seed beetle. Because seeds of Spanish broom mature much later than those of broom it is possible that phenological asynchrony contributes to the lack of acceptability of this species. However, Harman (1999) demonstrated that broom seed beetle in New Zealand has the ability to adapt to its host’s phenology, so a possibly more likely explanation is that United Kingdom populations of broom seed beetles (the source of beetles established in New Zealand) are more conservative in their host range than populations from continental Europe where there is a much greater diversity of plants within the Genistinae (Sheppard et al. 2006). It is interesting, however, that broom seed beetle has adapted relatively well to the novel host, tree lucerne, the phenology of which is not completely synchronous with broom. Tree lucerne starts flowering much earlier than broom although its seeds mature at a similar time. Only two seeds (0.9%) of the tree lucerne sample from the Victoria Park site were infested, though, so a similarly low infestation of Spanish broom at this site could have passed undetected. We did not sample Genista, another potential host , because we were unable to find a suitable local site where Genista plants were growing abundantly adjacent to broom. We have not examined populations of alternative host plants growing in the absence of broom. Such sites are not easy to find because in Canterbury broom is a very common shrub whose populations move regularly over time as a result of land disturbance and successional processes. The shift in the beetle population from broom to tree lucerne that occurred at the Sign of the Takahe site is possibly indicative of what we could expect to happen more generally during the decline of a

24 Pauline Syrett and Rowan Emberson broom population. However, we do not know how easily broom seed beetles might colonise an alternative host plant in the absence of broom. Conclusions After 25 generations broom seed beetle in Canterbury shows little evidence of exhibiting the broad host range within the Genistinae indicated by European host records (Sheppard et al. 2006). Our results support those of Haines (2004) that broom is strongly preferred as a host plant. The only other plant we looked at that supports significant development is tree lucerne, a close relative that does not occur in the natural range of broom. Acknowledgements We are very grateful for Christine Bezar’s expert editorial assistance.

References Haines ML. 2004. The unpredicted host use of Chamaecytisus prolifer by the introduced weed biocontrol agent, Bruchidius villosus; a retrospective analysis and explanation. (Unpublished PhD thesis). Lincoln University, Canterbury, New Zealand. Haines ML, Syrett P, Emberson RM, Withers TM, Fowler SV, Warner SP. 2004. Ruling out a host-range expansion as the cause of the unpredicted non-target attack on tasasaste (Chamaaecytisus proliferus) by Bruchidius villosus. In: Proceedings of the XI International Symposium on Biological Control of Weeds (eds JM Cullen, DT Briese, DJ Kriticos, WM Lonsdale, L Morin, & JK Scott) pp. 271–276. CSIRO Entomology, Canberra, Australia. Haines ML, Emberson RM, Sheppard A, Syrett P, Withers TM, Worner SP. 2013. Implications of individual variation in behaviour for host specificity testing in weed biocontrol. Biocontrol: 58(5): 703-713. Harman HM. 1999. The effect of variability in the phenology of the reproductive stages of Scotch broom (Cytisus scoparius) on the synchronization of the life stages of broom seed beetle (Bruchidius The Weta 46:120-27 villosus) in New Zealand. Biological Control 14: 228–234. Paynter QE, Fowler SV, Gourlay AH, Haines ML, Harman HM, Hona SR, Peterson PG, Smith LA, Wilson-Davey JRA, Winks CJ, Withers TM. 2004. Safety in New Zealand weed biocontrol: a nationwide survey for impacts on non-target plants. New Zealand Plant Protection 57: 102–107. Sheppard A, Haines M, Thomann T. 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. Syrett P. 1999. Broom beetle gets taste for tagasaste. Biocontrol News and Information 20: 50. Syrett P, O’Donnell DJ. 1987. A seed-feeding beetle for biological control of broom. Proceedings of the NZ Weed and Pest Control Conference 40: 19–22. Syrett P, Fowler SV, Coombs EM, Hosking JR, Markin GP, Paynter QE, Sheppard AW. 1999. The potential for biological control of Scotch broom (Cytisus scoparius) (Fabaceae) and related weedy species. Biocontrol News and Information 20: 17N–34N. Webb CJ, Sykes WR, Garnock-Jones PJ. 1988. Flora of New Zealand, Vol. IV: Naturalised Pteridophytes, Gymnosperms, Dicotyledons. Botany Division, DSIR, Christchurch, New Zealand. 1365 p.

26 Pauline Syrett and Rowan Emberson

Table 1. Collection site information

S,

Sign of the Sign Takahe 200 43°34′47″ 172°38′03″ E Gently slopingsite, sparse broom amongtree and lucerne other shrubs

S,

facing

-

Mt Pleasant Mt 210 43°34′23″ 172°43′18″ E Along onroadside north slope, andbroom lucerne tree stands separated 50m c. by

S,

Corsair Bay Corsair 40 43°36′25″ 172°41′59″ E Slopingsite, southerly aspect, andbroom lucerne tree stands separated 50m c. by

S,

McCormack Bay s 45 43°33′52″ 172°43′39″ E Slopingsite, north facing, mixed andbroom lucerne tree

S,

Victoria Park 275 43°35′43″ 172°38′30″ E Along roadside, slope, steep facingNW, andbroom lucerne tree separated 50m, c. by broom intermixed with Spanish broom

S,

Chertsey1 120 43°47′45″ 171°56′53″ E Flatsite along roadside, white broom intermixed broom with

S,

Chertsey2 120 43°47′56″ 171°56′32″ E Flatsite along roadside, white broom intermixed broom with

SiteName Elevation (ma.s.l.) Grid Reference Site description

The Weta 46:120-27

Table 2. Percentage of seeds infested by broom seed beetle, Bruchidius villosus, 2011–2012.

Site Cytisus scoparius (broom) Cytisus proliferus (tree lucerne) No. of seeds Infested No. of seeds Infested (%) (%) Sign of the Takahe 2092 18.1 1199 6.1 Mt Pleasant 423 52.5 1261 1.1 Corsair Bay 595 70.9 1181 7.5 McCormacks Bay 526 51.9 1382 9.0

Table 3. Percentage of seeds infested by broom seed beetle, Bruchidius villosus, 2012–2013.

Site Cytisus Cytisus Cytisus Spartium scoparius proliferus multiflorus junceum (broom) (tree lucerne) (white (Spanish broom) broom) No.of seeds No of seeds No. of seeds No. of seeds Infested (%) Infested (%) Infested Infested (%) (%) Sign of the 448 21.2 294 19.4 - - - - Takahe

Mt Pleasant 555 28.6 408 2.0 - - - -

Corsair Bay 289 51.2 334 15.3 - - - -

McCormack s 566 29.0 357 14.3 - - - - Bay

Victoria Park 530 26.2 227 0.9 - - 512 0

Chertsey 1 471 67.5 - - 450 0.7 - -

Chertsey 2 449 53.7 - - 431 3.1 - -