Monday, 13 October 2014

Lots of beetles = less Hypericum

I have mentioned from time to time the disastrous impact of St John's Wort (Hypericum perforatum) on the grassy areas of the ACT.  This morning a colleague mentioned in an email that he had seen some Chrysomelid beetles munching on the wort and wondered if something could be done to enhance its spread.  Another colleague then advised that they come and go and probably nothing needs to be done.

By chance I was in a paddock off Pialligo Avenue in the morning and noticed a good squad of leaf-beetles giving some serious mandible to the Wort there.
As there are (according to Hangay and Zborowski some 2250 species within the family Chrysomelidae in Australia I am not game to say which these were.  However I have included some close ups in case any ento-types are feeling bold.


 Judging by the amount of faecal matter behind a couple of these specimens they were having a really good time.

Here are some words about the beetles from a NSW DPI page on the wort.  I have emphasised one sentence in red:
Chrysolina beetlesTwo species have established in Australia: Chrysolina hyperici and C. quadrigemina. They are black with bronze, dark-blue or purple reflections, and are oval in shape. Chrysolina quadrigemina is slightly larger (6.0 to 7.1 mm) than C. hyperici (5.3 to 6.1 mm). Some individuals of C. quadrigeminaare distinctly bluish.

The Chrysolina larvae and beetles feed on the leaves of St John’s wort. The larvae attack the winter growth and the adult beetles attack the spring growth. At favourable sites, beetles may reach densities high enough to cause complete defoliation, and this suppresses flowering and seed production. The best control is achieved when the beetles and larvae attack the weed in the same or consecutive years. The damage produced by the beetles can appear spectacular, but the impact tends to be sporadic and inconsistent. They can provide effective control in open, unshaded situations, but without follow-up pasture improvement the weed frequently re-establishes. The beetles are not effective in timbered country, as they mate only in sunlight.

Chrysolina beetles have low mobility, and therefore it may be worthwhile to assist their movement by collecting batches in spring and transferring them to new infestations.

Do not use herbicides as an additional method of control when high numbers of Chrysolina beetles are present, because partially defoliated plants are unlikely to absorb enough herbicide to kill them.



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