Drama among sand wasps
Naturalist(s): W.J. Davis
Date: May 22, 2012
Location(s): Leffis Key, Manatee Co., FL (map)
Leffis Key, Manatee Co., FL
Field notes
This past May, I encountered numerous sand wasps (Stictia carolina) on Leffis Key. In more rural areas, females will hover around livestock to catch horse flies (Tabaninae) hence the common name "Horse Guard." The flies are subsequently paralyzed and buried alive in an underground nursery. Although livestock is absent on the island, the wasps make do by preying on other insects. Over time I discovered many intriguing behaviors of these large wasps.

    How does a female keep track of the nursery burrows that she builds in her short lifetime? Decades ago, Niko Tinbergen discovered the answer by conducting simple experiments by positioning pinecones around the entrance of a wasp's nest. Typically, when a wasp leaves to hunt it will fly in circles above its current burrow before zooming out of view. During such recon flights, she will memorize the position of nearby landmarks such as rocks, pinecones, prominent plants, etc. When Tinbergen rearranged such landmarks, returning wasps searched in the wrong locations for their burrows. As I discovered, you don't have to repeat Tinbergen's experiments to observe wasps circling above their nests.

   Being solitary, each female digs her own nests in the sand. At the end of the tunnel (typically 30 -78 cm long) a chamber is dug where a female lays one egg. Before sealing off a nursery, she will stock each chamber with live, but paralyzed prey which serves as food for her offspring. Over three days, she stuffs as many as twelve flies and other insect larvae into the nursery chamber.

    I videotaped several females excavating burrows. When digging, a wasp enters the tunnel head first all the while rapidly pushing sand beneath her body with specially modified front legs. She also backs out of the burrow during early stages of construction. Occasionally, because sand that has accumulated at the entrance needs to be removed, a wasp may start a bout of sweeping some distance from the burrow's entrance. As seen in the video, you can get a good view of how the wasps manage to shovel so much sand efficiently. While digging, a wasp folds its front legs inward to form a crude spoon. Presumably males do not excavate tunnels, but their front legs are shaped similarly to those females (see photos). Perhaps the size and shape of the forelimbs also helps in capturing and transporting prey.

    To videotape a wasp carrying prey, I attached a macro lens to my camcorder and waited at the entrance of a completed nest. I was rewarded with an unexpected surprise when I discovered I was not the only one staking out the nest. Soon after the wasp arrived, a small fly zoomed in and landed on the wasp's abdomen. As evident from the contortions of the fly, its intent was to lay eggs on the wasp's victim before the wasp disappeared underground (see video). Evidently, female wasps returning with prey shouldn't linger at the entrance of the nest as they risk being ambushed, a fate that sacrifices their own larva to fly maggots. (Presumably, maggots of a fly develop faster than larvae of a wasp. Furthermore, given that flies are present in other videos posted on the internet, parasitic flies are, ironically, a common problem for sand wasps.)

   Also on Leffis Key, males were seen gathering around a park bench (see video). Why are so many males attracted to this particular spot? Consider the following facts:

  • Although most of the wasps seen near the bench were males a few females have been observed digging burrows under the bench;
  • Next to the bench is a garbage can that potentially attracts flies; and
  • Groups of female sand wasps often dig nursery burrows near sources of food (e.g., garbage cans or pastures filled with horses.
male horse guard
   As pointed out by my friend and colleague Jessie Williams, the presence of flies does not directly impact males since they lack a stinger to subdue prey (adult sand wasps, male and female, feed on nectar). However, males are attracted to breeding sites of females. As males wait for virgin females to emerge from their nursery burrows, they vigorously defend plots of land where females are constructing burrows -- in this case, under the park bench located next to a garbage can. As reported in the literature, an emerging female can be swamped by amorous males to such an extent that a ball of wasps forms around her, a behavior that I would like to document for our archive.