Cannibalism in Mosquito Larvae Confounds Egg Counts
By Meredith Swett Walker
The mosquito Anopheles gambiae is one of the most important vectors for malaria, and, unfortunately for us, humans are its preferred blood meal. Along with humankind’s efforts to control this vector, many predators help keep An. gambiae’s populations (at least somewhat) in check. Fish, amphibians, birds, bats, spiders, and other insects all feed on the mosquito. A new study published in April in the Journal of Medical Entomology adds another predator to that list: Anopheles gambiae itself.
At first glance, cannibalism in a major malaria vector might seem like good news for Team Human. But the discovery of this behavior may force mosquito researchers to reconsider some of their data on oviposition (egg laying) in this species, as Juan Huang, Ph.D., and her colleagues at Michigan State University argue in their study of egg and neonate larvae cannibalism in An. gambiae.
Researchers working to control An. gambiae want to understand oviposition in this species because, if they can find a method to deter or prevent oviposition near human habitation, they may be able to reduce transmission of malaria. Some studies have suggested that female An. gambiae avoid laying their eggs in pools of water where An. gambiae larvae are already present. Presumably, this is so that their offspring won’t compete for food with too many other larvae. Researchers hoped to discover what cues female mosquitoes used to detect the presence of other larvae and develop these into a water treatment that would reduce egg laying.
But Huang and her co-authors experiments suggest that An. gambiae females might not be avoiding An. gambiae larvae at all. This species lays its eggs at night, so many oviposition experiments present females with oviposition sites in the evening and count the eggs present in each site the following day. But what happens during that time interval? Apparently a lot. Huang and colleagues report direct observations that older (third and fourth instar) An. gambiae larvae readily eat An. gambiae eggs and first instar larvae.
In addition, further experiments demonstrated that An. gambiae do not avoid ovipositing in water where fourth instar larvae are present. To show this, Huang and collaborators used super glue to seal the mouth parts on fourth instar larvae. (How exactly does one glue shut the mouth of a 1 centimeter-long mosquito larva? “That takes lots of practice. I used a tip of smallest insect pin to pick up a tiny drop of glue, then applied it to the mosquito under a dissecting microscope,” says Huang, who must have very steady hands.)
Gravid female An. gambiae were presented with three choices for oviposition: water without larvae, water with larvae whose mouthparts were glued shut, and water with control larvae who had glue applied to their thoraxes but had unhindered mouthparts. When eggs were counted, similar numbers were found in the water-only treatment and water with mouth-glued larvae. But treatments with larvae with unhindered mouthparts had 86- to 148-fold fewer eggs (and presumably some very satisfied larvae.)
Huang and co-authors argue that previous conclusions that female An. gambiae avoid laying eggs in sites where older An. gambiae larvae are present were incorrect because they didn’t consider cannibalism and assumed that fewer eggs present in the morning meant that fewer eggs were laid.
So, in other words, don’t count your eggs before taking cannibalism into account!
Journal of Medical Entomology
Meredith Swett Walker is a former avian endocrinologist and wannabe entomologist. She now studies the development and behavior of two juvenile humans in the high desert of western Colorado. When she is not handling her research subjects, she writes about science and nature. Find a sampling of her work at www.magpiescicomm.com.