Overview
The collective behavior of ant colonies, and locomotion of individuals within a colony, both respond to changing conditions. The invasive Argentine ant (Linepithema humile) thrives in Mediterranean climates with hot, dry summers and colder, wet winters. However, its foraging behavior and locomotion has rarely been studied in the winter. We examined how the foraging behavior of three distinct L. humile colonies was related to environmental conditions and the locomotion of workers during winter in northern California. We found that colonies foraged most between 10 and 15˚C, regardless of the maximum daily temperature. Worker walking speed was positively associated with temperature (range 6–24˚C) and negatively associated with humidity (range 25–93%RH). All colonies foraged during all day and night hours in a predictable daily cycle, with a correlation between the rate of incoming and outgoing foragers. Foraging activity was unrelated to the activity of a competing native ant species, Prenolepis imparis, which was present in low abundance, and ceased only during heavy rain when ants left foraging trails and aggregated in small sheltered areas on trees.
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Methods and contributions
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Findings and contributions
- Colonies foraged most between 10 and 15˚C regardless of maximum daily temperature.
- Worker walking speed was positively associated with temperature and negatively associated with humidity.
- Colonies foraged during all day and night hours in a predictable daily cycle, with incoming and outgoing forager rates correlated.
- Foraging was unrelated to the low-abundance native ant P. imparis and stopped only in heavy rain, when ants aggregated in sheltered areas on trees.
- Winter walking speed rose about 0.10 cm/s per ˚C, half the rate reported for L. humile at higher summer temperatures.
Methods
- Field observation of three L. humile winter aggregations on Stanford campus — Three winter aggregations nesting at the base of Coast live oak trees were monitored simultaneously in three 17–27 h observation sets in winter 2016.
- Hourly trail counts of foraging rate and timed walking speed — Ants crossing a rate line 1 m up the trunk were counted in sixteen 30 s intervals, and 1–10 foragers were timed over a 10 cm speed line.
- Arduino-based logger for temperature, humidity, and light — A DHT22 sensor, CdS photoresistor, and Arduino UNO logged ambient conditions every 5 s; P. imparis abundance was also counted on trunks.
- Linear vs quadratic mixed effects models of foraging and speed — Log-transformed foraging rate and walking speed were modelled against environmental predictors with colony and day random effects, comparing linear and quadratic fits.
- Autoregressive partial correlations of up- and down-tree traffic — Partial correlation coefficients between normalized hourly foraging rates in both directions were computed at time shifts of -12 to 12 hours.
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