Monday, August 4, 2014

Chasing butterflies at Fort Worth Nature Center

Eastern swallowtail (male?) resting on oak.
I took advantage of the unseasonably cool weather Saturday morning to chase butterflies around Fort Worth Nature Center. I only tried to catch them with my camera, but I think next time I'd do better to bring a net-- they are fast and a little too wary for me to sneak up. I'm still learning butterfly ID, and taking pictures and flipping through my guidebook has been a pretty effective way to make species stick.
Along the causeway. River on the right, marsh on the left.

I wasn't the only person out enjoying the brief reprieve from the heat of Texas summer. One couple had undertaken the marital trial-by-fire of paddling a two-person canoe (much to the amusement of the fisherman and anyone else within earshot). For my own part, I was hot-footing to dodge fire ants and ant lion pits.  Trumpet creeper, button bush, and liatris were all still blooming along the river despite being deep in the late summer dormant season.

Gulf fritillary (Agraulis vanillae) on liatris
Specifically, I was out looking for the gulf fritillary. I've seen them in parks and around town, but they don't sit still much. When they do it's mainly up high in a tree out of reach. The picture at left is as close as I've been able to get. Unlike the northern fritillaries whose larva feed on violets, gulf fritillary caterpillars only eat passion vines (Passiflora spp). How tropical. Gulf fritillaries, like squash bees, have probably expanded their range with human settlement. In both cases, the domestication and propagation of their host plants in our gardens has allowed these pollinators to move into new landscapes [1].

Grasshopper on button bush. What is he doing?
Gulf fritillaries are not closely related to 'true' fritillaries, and they are the only extant representative of their genus. Generally lumped with long-wings, their nearest relatives are tropical in distribution. Gulf fritillaries themselves are migratory, and they have been known to make exploratory forays as far north as the central plains [2].  However, their US population centers are in the southern third of the lower 48, in areas where their larval host plant is either native or naturalized.

It's amazing to me how most butterflies are quite specific in their larval host plants. As first glance, it seems like a high-risk strategy. Why specialize at all when specialization means fewer food options? There must be some benefit, or host plant specialization wouldn't be the dominant strategy.

After doing a little digging in the literature, it appears that host plant chemistry and avoidance of parasitoids are proposed drivers for female butterflies to lay their eggs on one type of plant [3, 4]. Most of us are familiar with Monarch butterflies and how they use toxic alkaloids from the milkweed they eat as caterpillars to discourage predation. The gulf fritillary uses Passiflora spp. alkaloids towards similar ends. Unsurprisingly, tropical lepidoptera are more likely to be specialized than their temperate kin [4]. Dyer et al. hypothesized increased competition as the driver of increased host plant specialization at lower latitudes [4], so perhaps it is the opposing pulls of predation and resource availability that drive this selectivity. Who knows, but it's certainly fun to ponder!

Sulphur (little yellow?) butterfly next to leaf

  1. Graves, Sherri D., and Arthur M. Shapiro. "Exotics as host plants of the California butterfly fauna." Biological Conservation 110.3 (2003): 413-433. 
  3. Thompson, John N., and Olle Pellmyr. "Evolution of oviposition behavior and host preference in Lepidoptera." Annual review of entomology 36.1 (1991): 65-89.
  4. Dyer, Lee A., et al. "Host specificity of Lepidoptera in tropical and temperate forests." Nature 448.7154 (2007): 696-699.

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