The period of rain here in Texas a few weeks ago was intense but welcome. The moisture brought a revival of plant life, and the return in Austin for some insects that depend on these plants. This was largely due to heavy rains that broke the drought 150-200 miles south of us. One such insect that has been seen in greater numbers here is the snout butterfly, Libytheana bachmanii.
This butterfly has been the object of long term observations by Dr. Larry Gilbert, Director of the Brackenridge Field Lab (BFL), where the presence of the snout butterfly has increased dramatically since the rains. In the past, he has written about some of his observations on how rainfall and drought affect this butterfly.
To best understand the effects, here are some general understandings of the snout butterfly. Migratory populations, sometimes called “swarms,” of this butterfly can get up to 248 miles (400km) wide. These populations have been reported several times in south Texas over the last century. In 1921, there was a recorded event estimated to be a 25 million-per-minute swarm of a flight that lasted 18 days. It involved more than 6 billion butterflies.
Population size is positively correlated to different hackberry trees’ growth cycles. Some of these trees are the Spiny hackberry (Celtis pallida) and the Sugar hackberry (Celtis laevigata). The tender new leaf growth of these trees support this butterfly’s larvae. The migration of the snout butterfly is highly dependent on when this tree is creating new growth
Photo: Larry Gilbert.
How do rains or lack of them affect swarms like these and cause them to migrate? Periods of drought arrest growth in many plants, including some of the trees like the hackberries mentioned above. Without tender new leaves, there is limited food source for larvae. Additionally, parasitoid activity is low at the end of a drought, and Gilbert provides a deeper explanation of the impact of parasitoids: “Parasitoids of different species can attack eggs, larvae or pupae. They impact the population growth of the butterfly. When they are present, they kill most of the butterfly’s early stages thus allowing developing [plant] leaves to survive and the trees are not defoliated. The impact of the parasitoids on leaf production is indirect via impacts on the butterfly population.”
Texas also has patchy rainfall, with some areas receiving high amounts while areas right next door get nothing. However, this situation does not always impact migration as the snout butterflies can cross dry regions to a certain extent to find hospitable areas.
Migrations like these are most noticeable in less-urbanized places where researchers work. This is where the value of field stations steps in. Rob Plowes is the Operations, Research and Teaching Coordinator at BFL and elaborates on what this means. “Here in Austin, BFL serves as a local knowledge base against which researchers can observe long term changes and episodic events, like migrations.”
Photo: Larry Gilbert.
When we think about migrations of insects, the first thing that comes to mind is one of the most well known of butterflies, the monarch. But even among butterfly populations, migration is not the same process. Plowes states: “The distinctive difference between snouts and monarchs is the episodic one way migration of snouts, like lemmings, versus the annual return migrations of monarchs.”
Either way, field stations are essential areas where things like differences in insect migrations can be seen in real time and studied. As Plowes says: “Much can be learned about both at BFL.”
