A beneficial insect that we are lucky enough to have on the farm is the green dock leaf beetle (Gastrophysa viridula). The male, as seen in the feature image above, grows to about 4mm in length – this particular specimen was closer to 3.75mm. The female is larger at 6 or 7mm long. Both genders have a beautiful metallic green lustre causing them to sparkle in the sunshine like tiny green jewels.
Mating is something of a competitive & feisty affair. Males are often to be seen attempting to mate with heavily egg laden females as in the picture above. Note how distended the female’s abdomen becomes due to the large number of eggs she is readying to lay. If you look long enough you will also find the occasional fracas breaking out between males. In the picture below, one can see a male attempting to mate with a female whilst another beetle grabs his leg in his mandibles & attempts to pull the mating male away.
When the female’s eggs are ready she will lay her them on the underside of dock (Rumex) leaves in small groups of about thirty eggs. She will lay many such egg clusters. The eggs themselves are yellow and barely 1 mm long.
After a few days the grey-black larvae hatch from the eggs and begin to consume the dock plant. Whilst they may occasionally ( & mistakenly?) feed upon other broadleaved plants they are almost exclusively consumers of docks, thus being very useful as a natural controller of docks in grassland. We have found that large dock infestations are a little too much for our local population to consume but they are excellent for general ongoing control especially if supported by a little late season cutting of the dock plants. The larvae are pictured below:
The larvae will go through several moults (perhaps 3 instars) before falling to the ground to pupate in to adult beetles. Thus these beautiful, beneficial beetles complete their life-cycle. Below is a diagram to illustrate the cycle (click image to view / download large version) :
Bonus image: in common with other insects, these dock beetles have amazing compound eyes, as shown in the micrograph below: