The black carpenter ant (Camponotus pennsylvanicus) is one of the more common pests encountered by pest management professionals. Colonies of C. pennsylvanicus normally occur in dead wood or in decayed sections of living trees and buildings, as well as naturally occurring voids in buildings. The egg-laying queen and the brood, as well as most of the workers, remain in this nest, which is known as the parent colony. A small portion of the worker population periodically leave the nest to forage for food in the form of honeydew from aphids and scale insects; arthropod prey are scavenged for use as a protein food source. When a colony resides within a living tree, foragers often climb into the canopy to collect honeydew from sap-feeding honeydew producers, while in urbanized areas, they will also look for food in and around buildings.
Depending on food availability, C. pennsylvanicus may form permanent ground trails from the parent colony to additional, relatively stable food supplies. This is normally in the form of another tree or bush that contains populations of honeydew producers, but it often is a resource site within a structure. These trails are well managed by the ants, and they will remove any grass or other obstructing debris. When the trail passes through thick grass, it is often fairly constricted to a width of two or three centimeters. In urban areas, worker ants will also follow structural guidelines (edges of flower beds and sidewalks, telephone lines leading into a building, etc.). Most ant traffic occurs along the trail at night, although some daytime foraging may occur. Our research has investigated environmental variables that affect the foraging behavior of C. pennsylvanicus.
HOW WE DID THE WORK. To investigate the behavior of C. pennsylvanicus, capacitive proximity sensors were used to detect ant movement along their ground foraging trails. These sensors are normally used in industry, but were adapted to detecting ants by suspending the sensors in special holders above the ants’ foraging trails (see photo above right). As ants walked beneath the sensors, they were counted and this information was stored in a datalogging computer. The datalogging computer was programmed to provide five-minute totals of ants counted by the sensor.
Concurrent with collection of ant activity data, temperature, humidity, light intensity, rainfall, wind speed and air pressure data were also collected. These data were analyzed with the ant count data to determine which variables were most important in regulating ant activity. The data were used to develop a predictive model of carpenter ant foraging activity in response to factors within the environment.
The combination of sensor and datalogging technology allowed the amount of ant traffic to be measured every minute of every day. An advantage to using this technology is that it requires minimal human intervention to monitor ant behavior. Thus, we did not have to visually count ants passing a particular point during a specified time period, and we avoided this extremely time-consuming and laborious process that provides only a "snapshot" of foraging activity.
The sensors and datalogger eliminate these hardships. They can be left in the field for weeks, or even months, at a time with little maintenance while providing researchers with a more complete picture of ant activity.
IMPORTANT FINDINGS. In a preliminary study, sensors were placed above the trail of a large colony and data was recorded for several days. It is well known that C. pennsylvanicus is a nocturnal species, and our foraging data confirm this. As light intensity decreased throughout sunset and dusk (6 p.m. to 8 p.m.), ant activity dramatically increased. Shortly after sunset, large numbers of ants aggregate at the nest entrance and, when conditions are dark enough, begin to stream out of the nest. At this time of high activity (about 8:10 p.m.), we recorded as many as 150 ants in a five-minute period, or equivalently, two ants per second (see chart on page 118). During the hours of 7 to 8 p.m. and 8 to 9 p.m. there were about 10 to 400 ants counted by the sensor, respectively. This is a 40-fold increase in activity in just one hour. This response to nightfall is closely tied with light level and ants were observed to forage earlier in the evening as the season progressed and days became shorter.
The burst of ants leaving the nest typically occurs over a period of about an hour, with the majority of the ants leaving the nest within the first 20 minutes. Following this initial ant "rush hour’" we recorded an hour of low activity. Thereafter, activity steadily increased through the night. This same pattern occurred nearly every night that data were collected C. pennsylvanicus colonies.
We believe that this is an indication of nightly carpenter ant foraging strategy. Initially, a large number of ants leave the nest at nightfall to search for food, resulting in the large burst of activity. These ants then search for a time away from the nest. The time it takes for the initial foragers to travel to another location and find food may explain the relatively low level of activity seen after 9:30 p.m. Some foragers will encounter food sooner than others. The ants that do find food return with it to the nest. As more ants return to the nest and some ants leave the nest to forage again, activity slowly increases and plateaus at a somewhat constant level until morning, when activity drops off again.
A larger scale study using the ant sensors was ultimately conducted. Sensors were placed on the foraging trails of four colonies and data were collected from mid-June through late October. The number of ants counted were totaled for each night and related to air temperature, humidity, rainfall, wind speed and air pressure data.
Our results indicate that temperature strongly influences the number of ants foraging from a colony on any given night. On most nights, as temperature increased, more ants were counted by the sensors. The greater ant activity may be either from ants making more foraging trips or because more ants leave the nest on warmer nights than during cooler nights.
Although temperature was found to be an important environmental factor regulating foraging activity, other, colony-specific factors may also play a role. For example, ant foraging may increase or decrease relative to food availability. During another study, one of the colonies being monitored was fed a large amount of crushed crickets. On the nights following this feeding, ant activity from this colony was considerably less than would be predicted by temperature alone. We speculate this decrease in activity may be caused by satiation from the cricket feeding. In effect, ants from colonies that are physiologically "full" are under less pressure to leave the nest and forage than ants that are from colonies that are starving from lack of food.
This information is of value in that it allows pest management professionals to make informed decisions on both inspections and control tactics. For example, it might be used to predict the intensity of carpenter ant activity depending upon past and/or present environmental conditions (i.e., for a particular month or season), or perhaps it could suggest potential bait preferences given a set of environmental conditions. If it can be shown that ants forage more intensely during a period of drought, indicating that they are experiencing water stress, this could make a liquid bait more effective. From an applied research standpoint, this technology might be used to measure recruitment rates to baits or to measure treatment efficacy via changes in ant activity.
The use of sensor and datalogging technology has much to offer from both a basic and applied research standpoint. With our current research, we hope to unravel further mysteries of C. penns-ylvanicus that will allow us to better understand and control this insect.
Gary Bennett is professor of entomology and director of Purdue University’s Center for Urban and Industrial Pest Management. Andrew Nuss is currently working towards a Ph.D. degree at the University of Georgia.