Thorny Devils in the Garden

I was recently out and about in the garden and found some fascinating insects, Keelbacked Treehoppers, Antianthe expansa. They were on some of our celery plants and are, according to Vanessa Vobis Master Gardener and Museum Gallery Interpreter, "a very annoying pest on our tomatoes."

Adult Keelbacked Treehopper on celery

(Approximately ¼ inch or 7 mm long)

When I found the Keelbacked Treehoppers, all but one were in the nymphal (immature) stage. As nymphs, these insects do not have wings (this is true for all insects—just look at caterpillars, grubs, and maggots), and are bound to the area in which they were deposited as eggs by their mother. The nymphs are often attended by ants, which feed on their sugary excreta and provide a level of defense against the treehopper's predators. Though it should be noted that this is not always the case, there were no ants in attendance around the treehoppers I found, so this isn't a reliable mode of identification.   

When I found the treehoppers in our Edible Garden, they were feeding on one of the celery plants. They feed on the juices (phloem sap to be exact) inside the plant by inserting their tiny straw-like mouthparts and sucking up the liquid. Although many sources say that these insects cause little, if any, damage to the plants they are on, this is not always the case! When populations of these insects are high enough they can cause serious stunting and sometimes lead to the loss of plants. Many gardeners in our area complain of these pests on their tomatoes. It is not clear if the decline in plant health is due to excessive feeding by these insects or by secondary infections spread to the plant during feeding. 

Immature Keelbacked Treehopper

photo courtesy of Vanessa Vobis

From a naturalist's perspective these insects are some of the more weird and wonderful. Adult treehoppers (family Membracidae) can be recognized by the prominent enlargement of the pronotum (segment directly behind the head). This enlargement gives many treehoppers a humpbacked or thorny look, hence the other common name for these insects: thorn bugs. However, some tropical species go beyond the thorny devil look and opt instead for something a bit more insectuous! Case in point, the Cyphonia treehopper has a pronotum that mimics an aggressive ant species, which not only looks awesome, but is a great defensive mechanism against predators.      

Cyphonia treehopper with ant-like pronotum

image from Nicolas Gompel/Nature