Sap-sucking bugs

Sap-sucking bugs (Insecta: Hemiptera: Sternorrhyncha) are small and cryptic plant parasites. Phloem sap is not the easiest thing to eat. It’s sweet and refreshing, but it’s hard to get to, low in protein, and protected by plant defenses. Sap-sucking bugs tap into phloem tissue with needle-like mouthparts. They have guts that function as filter-chambers, and cellular mechanisms to transform excess sugars into long-chain polysaccharides that are excreted as honeydew. They get the protein they need by teaming up with endosymbiotic microorganisms. And by spitting plant-jacking effector molecules into their hosts, they hack their defenses.

Crypticerya tabernicola

Crypticerya tabernicola — What a beauty!

As the primary consumers of phloem, sap-sucking bugs are important in nutrient cycles. Honeydew is much easier to eat than phloem sap. It is candy for some birds, and is even more important in the diets of other insects, especially ants. In other words, sap-suckers are ecosystem energy pumps. They are also agricultural pests and are disproportionately common in invasive faunas. Most obviously, they damage plants by taking their sap. This is often compounded when honeydew fertilizes sooty molds which inhibit photosynthesis. Many species, for example the Asian citrus psyllid (Diaphorina citri), also act as vectors of horrible plant pathogens. 

Some of us think that sap-suckers are lovely to look at. Most don’t. But what they may lack in physical beauty, they more than make up for in fun biology. Here’s an example. Most aphid species do cyclical parthenogenesis, that is, they alternate between sexual and asexual reproduction. In hundreds of species, these changes in reproductive mode are tied to obligate alternations in host-use. In most host-alternating species, a colony foundress induces a gall on the primary (where sex happens) host. And in some host-alternating-and-gall-inducing species, some of the colony foundress’s clones grow up to become gall-protecting venomous soldiers. And in some soldier making species, the soldiers cheat and sneak off to another gall, where they shirk their protective duties, loaf around, and make babies.

In the Hardy Lab, one thing about sap-sucking bugs that we are especially interested in is the evolution of host-use. Sap-suckers can have extremely broad diets, and we’d like to figure out how and why they do it. Just as interesting is that most sap-sucking bug species are host-plant specialists. This is typical for plant-feeding insects. The reason that it is interesting in sap-suckers is that it applies to groups like scale insects, that are wind dispersed as delicate, and short-lived first-instar nymphs. This should drastically increase the cost of host specificity.  And from some of our comparative phylogenetic work, it appears as having a broad diet is especially cheap in sap-sucking bugs. In short, the evolution of host-use in sap-sucking bugs doesn’t make sense. By making sense of it, we could learn a lot about how the evolution of biodiversity works.