Sap-sucking bugs

Sap-sucking bugs are small and cryptic plant parasites. Plant sap is not the easiest thing to eat; it’s sugary and abundant (and must be thirst-quenching), but it’s hard to get to, low in protein, and protected by plant defenses. Sap-sucking bugs have evolved tricks to solve these problems. They tap into plant vascular tissue with needle-like mouth parts. They concentrate ingested sap with filter-chamber guts, and they excrete excess sugar as honeydew. To get the protein they need, they teaming up with endosymbiotic microorganisms. And to thwart host-plant defenses, they use complex metabolic systems.

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 for other animals to eat than phloem sap. Birds eat it, and so do other insects, especially ants. The upshot is that sap-suckers are ecosystem energy pumps. That’s good! One the other hand, they are also agricultural pests and are disproportionately common in invasive faunas. Most obviously, they damage plants by siphoning off 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 changes in diet. 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 selfishly make clones of themselves. Fun!

In the Hardy Lab, one thing about sap-sucking bugs that we are especially interested in is the evolution of their diets. 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 diet 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. (The pickier they are about what they eat, the lower the odds of actually encountering it.)  And from some of our comparative phylogenetic work, it appears that having a broad diet is especially cheap in sap-sucking bugs. (So why wouldn’t you have a broad diet?) 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.