Observations on the beetle fauna (Insecta - Coleoptera) of Scotch Broom

1.   Introduction

Fig. 1: Scotch Broom Cytisus scoparius (a) habitus; (b) unripe fruit pods with feeding marks of recently hatched larvae of Bruchidius

Since several years, the lead author of this paper has observed a red-flowered shrub of Scotch Broom (Cytisus scoparius, Fig. 1) in a gravel pit located close to the highway A20 near Neukloster (Meck­len­burg-Vor­pom­mern, Nord­west­mecklen­burg district). In fall 2008, a few ripe yet closed fruit pods of the shrub were collected for the first time to harvest the seeds.

Upon opening the pods, several live specimens of two beetle species were discovered. They were later determined by the second author as broom seed beetle Bruchidius villosus (F., 1792) (Col., Bruchinae, Fig. 2b) and Scotch broom seed weevil Exapion fuscirostre (F., 1775) (Col., Apionidae, Fig. 2a). Both species are of similar body length. According to literature, Bruchidius villosus is 1.7-3.5 mm large, Exapion fuscirostre 2.4-3.0 mm.

Fig. 2: (a) Scotch broom weevil Exapion fuscirostre, (b) Broom seed beetle Bruchidius villosus

In the following years (2009–2011) the occurrence of both beetle species in various populations of Scotch Broom near Bad Doberan (Satow, Kritzmow, Hohenfelde) in the district of Rostock was further investigated.

During all years since 2008, both species have been present in the sampling areas. However, their relative abundance varied depending on year, location and sampling date. A sample of 101 closed fruit pods, collected from the sampling site Kritzmow in early August 2010 gave the result shown in table 1. Of those fruit pods infested by Exapion, 17 contained parasitic wasps at the same time. In the case of fruit pods infested by Bruchidius, four also contained parasitic wasps. In seven fruit pods only parasitic wasps were found, but no living adults of either Exapion or Bruchidius. In the latter case the hosts of the wasps were not identified, although determination is in principle possible by examination of the feeding marks of the beetle larvae on the seeds. It should be noted of course, that the sample and the results in table 1 are just a snapshot. It cannot be excluded that the seeds contained further unhatched beetles.

2.   Development of Bruchidius villosus

The Broom seed beetle Bruchidius villosus places its oblong eggs on the surface of the fruit pod, where they are firmly affixed (Fig. 3a). The newly hatched larva either penetrates the husk immediately towards the inside of the pod, or makes a small burrow in the superficial tissue of the husk before finally entering the pod.

The young larvae penetrate the unripe broom seeds near the hilum (Fig. 3b). Initially the development of the seed is not impaired by the feeding activity of the larva. From the outside the seed exhibits no visible signs of damage (Fig. 3c). At the time of pupation inside the seed, the endosperm is almost entirely consumed. The beetle hatches inside of the seed and gnaws its way through the pericarp by making a ring-shaped incision (movie 1). Through the thus produced opening it leaves the seed and gets into the closed fruit pod, where the beetle stays until the pod ripens and bursts open.

In 2011, the first ovipositions in the sampling areas near Bad Doberan were observed in early June. The number of eggs deposited on normally developed pods sampled on June 25th, 2011 varied between three and 17. An enumeration of the development stages on June 30th, 2011 gave the following result: recently deposited eggs 13%, eggs with unhatched larvae 39%, abandoned integuments 48%. Two short months after the observation of the first egg deposition, the first adult beetles were observed end of July. The period of hatching continued throughout the whole August.

3.   Development of Exapion fuscirostre

So far the authors did not succeed to observe the egg deposition of the Scotch broom weevil Exapion fuscirostre. According to literature references the eggs are deposited in the pods.

Following our own observations, the larva penetrates the seed similar to the larvae of Bruchidius and feed on the endosperm. Contrary to the larvae of the broom seed beetle, the larvae of Exapion feed on the pericarp and occasionally on nearby seeds at a later stage of their development. (Fig. 4a).

Remarkably, the larva uses a yellow cementing substance to seal the seeds partially and to glue the seed to the inside wall of the pod. In the puparium thus produced, pupation (Fig. 4b) and hatching (Fig. 4c, movie 2) takes place. The adult beetles are released from the pod upon ripening and bursting open. It could be assumed, that the development duration is similar to that of Bruchidius, as the adult beetles were observed in the pods at the same time.

4.   Parasites

Fig. 5: Braconid Triaspis cf. simulator: (a) female (left) and male (right) pupa; (b) female adult.

In 2010, the abundance of parasitoid wasps (Hy­me­no­pte­ra, Braconidae) in the fruit pods was exceptionally high. A specimen has been determined by Prof. Kees van Achterberg (Depart­ment of Terrestrial Zoo­logy, Netherlands Centre for Biodiversity Naturalis, Leiden) as a member of the genus Triaspis, the species is most probably Triaspis simulator (SZEPLIGETI, 1901) (Fig. 5).

In 2011, parasitic infestation was present as well, however to a much lower extent than in 2010. Initially, the parasitic larvae were observed on the surface of the beetle larvae, however, at later stage of their development, the braconid larvae also partially penetrated the beetle larvae (movies 3 and 4). The infested beetle larvae seemed to be paralyzed and did not show any sign of defending moves. Depending on their host, the braconid larvae pupated within or at the empty seeds. Interestingly, the adult braconid wasps were also able to gnaw through the pericarp of the seed after hatching.

In early June 2011, besides the beetle larvae also larvae of gall midges (Diptera, Cecidomyiidae) were noted by their conspicuous lifestyle and agility. Under their feeding activity the young seeds shriveled, in most cases all seeds within the pod. Whether their presence disturbs or even prevents a successful development of Bruchidius and Exapion remains to be clarified. According to WALOFF (1968), the gall midge Contarinia pulchripes (KIEFFER, 1890) develops in Scotch Broom in England. The larvae of the gall midges were parasitized by wasps as well, most probably by chalcid wasps (Hymenoptera, Chalcidoidea, Fig. 6c).

Closing remarks

This topic page shall present snapshots from the life and development of beetles to the interested reader. It shall encourage the reader to explore and investigate seemingly inconspicuous creatures. Meanwhile, the two described beetle species have received widespread attention in a number of countries as a potential agent against excessive spreading of Scotch Broom on pasture land.

Acknowledgements

The authors thank Prof. Kees van ACHTERBERG for the determination of the braconid species. For the review of the draft and helpful discussions the authors want to thank Irina WÜRTELE and Klaas REISSMANN.

Literature

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