|Gastrophysa viridula on a dock leaf|
Belonging to the Order Coleoptera [Beetles], they reside in the Family Chrysomelidae which is a taxon with an enormous profusion of highly specialised herbivorous species. There are nearly 40,000 species in this taxon and most are phytophagous (plant-feeders). Some species although having specialised on a narrow range of plant species, that is choosing only to eat a few different types or even just one, have not yet entered any type of evolutionary dead-end. In fact they are able to switch to new host species if required; for example if the predation pressure on an ancestral host plant becomes too much. For a herbivore to do this it needs to contend with many different factors including locating the new plant, utilising its nutrients and dealing with its defensive mechanisms. All these abilities are conferred by the pheno- and genotypic plasticity of the herbivore and indeed the plant. Other important herbivorous taxon such as the Lepidoptera (moths and butterflies) have different ecological niches for the larvae and adult stages but not so with the Chrysomelidae and so they need to choose a host plant suitable for egg deposition, larvae and adults. It is therefore beneficial to the insect to take note of the change of host plant quality throughout time and season and also to account for previous oviposition and feeding that has taken place. As one can see the demands on a herbivorous insect for successful choice and subsequent utilisation of a host plant are immense. The perceptive abilities for location of the host plant and the insects digestive abilities to gain adequate nutrients - when plant is encountered the beetles will be required to efficiently digest the plant material and cope with the plant toxins - as well as contending with herbivores and carnivores already present, all come into play during host choice. One must not forget too that phytopathogens are known to alter the attractiveness of the plants. (1)
Our leaf beetle, Gastrophysa virudula, fed and oviposited less on dock plants (Rumex obtusifolius) which were infected by the rust fungus Uromyces rumicis than it did in healthy plants in laboratory environments. The alteration in nitrogen contents were observed in rust-infected dock plants and were not fed on by G. viridula. The diseased plants also contained higher levels of calcium oxalate but whether these cues caused the reduced feeding is yet unclear. (1 & 2)
Beetles reared on infected dock leaves had greater larval mortality and slower development. Fecundity was also reduced and regarding oviposition few larvae survived from eggs laid on rusted leaves in the field. (2)
Having said all this I am glad to report that no such rust fungus was evident on the leaves shown in the pictures. Our fellas were happy out!!!
Note the distended black abdomen which is evident on the female species when ready for oviposition (3) and in fact is a type of sexual dimorphism. The following picture [different location] shows this dimorphism with the smaller beetle being the male:
|Sexual Dimorphism in Green Dock Beetle|
And so we went our separate ways and the beetles had their dock leaves and I my coffee and pizza and custard and what not.It's funny what you'll find in a garden and exciting to know that multitudes of life cycles are turning around every single second on this earth. For them it is a matter of survival and for us merely a more than trifling curiosity. Still it was great to see them.
1. Fernandez and Hilker, Host plant location by Chrysomelidae, Basic and Applied Ecology, Volume 8, Issue 2, 1 March 2007, Pages 97-116
2. Hatcher, Paul, Ayres and Whittaker, The effect of foliar disease (rust) on the development of Gastrophysa viridula
(Coleoptera: Chrysomelidae). Ecological Entomology, 19, November 1994, Pages 349–360
3. Insectoid.Info, Green Dock Beetle, http://www.insectoid.info/beetles/leaf-beetle/green-dock-beetle/