March of the green vomit-grubs

As you may know, I am a stakeholder in a nearby community farm, here in the sometimes-sunny south of England. We grow many different crops and one of these is a small patch of asparagus. So, I was interested to see some beetle larvae eating the leaves and stems. Fortunately they don't seem to be affecting the asparagus growth noticeably so they've been left where they are (we don't use pesticides but could remove them by hand if they become a problem). Handily, I specialise in chrysomelids (leaf beetles) and recognised them as larvae from this family: the small shiny head capsule, and the body widening towards the rear then coming to a blunt point is typical. I've been looking at writing a guide to juvenile chrysomelids so in this case, knew they were larvae of the asparagus beetle Crioceris asparagi, and the presence of the distinctively colourful adults confirmed it.

Larvae of Crioceris asparagi along an asparagus stem. Adults in the background.

Late instar larva of C. asparagi eating an asparagus stem. This mature larva is close to pupating and shows a typical chrysomelid larval shape (though some differ greatly especially the tortoise beetles in the subfamily Cassidinae).

Larvae such as this are clearly potential prey for insectivorous predators that like juicy grubs. However, while some chrysomelid larvae hide and protect themselves with shields of their own faeces and shed skins, this species has a different approach. When threatened, it arches backwards, raises its head and regurgitates a droplet of partly digested food onto its attacker. This might not do much to a human but could be noxious to a smaller organism, causing the larva to be avoided or giving it time to drop off the plant to safety.

C. asparagi larva with a regugitated droplet on its head.

C. asparagi larva having used its droplet against my threatening finger.

In contrast, a chrysomelid larva (unidentified) with a shield of faeces and shed skins.

Assuming the larvae survive to pupate and emerge from their soil cocoons as adults, their defence changes greatly as they are colourful, warning potential predators that they are (or in the case of this species, are pretending to be) toxic - something known as 'aposematic mimicry'.

C. asparagi adult.

C. asparagi adults creating the next generation.

So, for now I'll enjoy these interesting beetles - hopefully they won't become too numerous...