Avoiding cadavers is good for you

There is a caveat to the title of this post. Avoiding cadavers is good for you if you’re very close to the cadaver. But that’s getting ahead of the game, so let’s back up:

There is growing evidence that uninfected animals can often sense and avoid infected individuals or infectious agents in the environment.  Or, conversely, uninfected animals may be attracted to infected individuals (see my old post about how disease is sexy).  Gypsy moths fall under the first category. Specifically, gypsy moths can become infected by a lethal (to them) bacilovirus when they ingest leaves contaminated by infectious gypsy moth cadavers, and previous work with these gypsy moths showed that they will preferentially eat uncontaminated leaves over contaminated leaves.

Obviously, these studies on the avoidance behavior of individuals were conducted because the authors thought that avoidance should affect an individual’s probability of becoming infected. These behaviors may affect population-level infection dynamics, too. So, does avoidance behavior affect pathogen transmission?

Eakin et al. (2015) used a really cool combination of laboratory experiments, field data, and modeling to answer this question. First, they parameterized two submodels using data from a laboratory experiment where they allowed individual caterpillars to feed on leaves that had one infectious cadaver on the surface. In the first submodel, they used model selection to figure out which mechanistic model best predicted a caterpillar’s probability of becoming infected. They found that the best model incorporated both how close to the infectious cadaver the uninfected caterpillar fed and how much the uninfected caterpillar ate. Just one bite of an infectious cadaver (which are ~78 bites total) increased the risk of infection by 0.4-4.7%. In the second submodel, they used model selection to figure out which stochastic simulation model best explained a caterpillar’s feeding decisions. They found that the best model included avoiding infectious cadavers. But here’s an interesting thing: the caterpillars don’t really detect and avoid cadavers until they are 0.7 mm away. Because eating a bite or two of infectious material doesn’t increase infection risk dramatically (just 0.4-4.7%), Eakin et al. (2015) suggest that caterpillars shouldn’t really go out of their ways to avoid cadavers; they should keep munching away until there is a cadaver right in front of them. Neat!

Finally, using the two parameterized submodels, Eakin et al. (2014) showed that the model predictions fit field data quite well, and the model that includes cadaver avoidance slightly outperformed the model without cadaver avoidance. At the highest densities of infectious cadavers, avoidance can reduce infection rates by 7% in a single transmission bout in the field. Cool!

I glossed over some cool math – like stochastic simulation models – so you should go check out the paper. Also, here is a relevant cartoon that, if nothing else, demonstrates my peculiar unique brand of humor:

(I don’t think Eakin et al. (2014) are selling caterpillar art on the Interwebs. But if they suddenly start to sell caterpillar art, I want 10% of the profit. Just saying. PS – this is a thing.)

Reference:

Eakin, L., M. Wang, and G. Dwyer. 2015. The Effects of the Avoidance of Infectious Hosts on Infection. The American Naturalist 185:1.

Caterpillar Campfire

(Prepare yourselves. This post is about the sexiest paper that I have read in a long time.)

Many plants are covered in tiny hairs, called trichomes, which defend the plant by trapping insects. These can be hooked trichomes that snag insects, or glandular trichomes that produce secretions that the insects stick to. These trichomes may be beneficial because they trap and kill insects that might harm the plant. But they also seem somewhat detrimental, because predatory insects (e.g., lady beetles) that might benefit the plant by killing herbivorous insects (e.g., aphids) might also get stuck in the trichomes and die (Eisner et al. 1998).

However, some predatory arthropods can avoid getting stuck in the trichomes. For instance, the common tarweed has glandular trichomes, but it is still visited by five types of predatory arthropods (Krimmel and Pearse 2013). Interestingly, those predatory arthropods will eat both dead and living insect prey. AND GUESS WHAT?! Just like plants use extrafloral nectaries and food bodies to attract ant defenders, the dead insects trapped in the sticky trichomes of the tarweed attract those five predatory arthropods. Those predators reduce herbivory from a caterpillar by 60%, and that reduced herbivory leads to greater plant fitness.  A. Maze. Ing. Go read the paper. It’s beautiful.

Fun facts from the paper:

20-30% of vascular plants have glandular trichomes. So, this specialized predator attraction may be a widespread phenomenon.

Individual plants had up to 40 insect corpses at a time!!

References:

Eisner T, Eisner M, Hoebeke ER. 1998. When defense backfires: Detrimental effect of a plant’s protective trichomes on an insect beneficial to the plant. PNAS 95(8): 4410-4414.

Krimmel BA, Pearse IS. 2013. Sticky plant traps insects to enhance indirect defense. Ecology Letters 16: 219–224.