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METHODS


Photo is Courtesy of Susan Mahr from the University of Wisconsin-Madison. Here the Aphidius matricariae wasp is pictured [photo by Max Badgley], a 1/8 inch long, black wasp from Europe. It is one of the most common and effective parasites of the green peach aphid

STUDY SPECIES

The name of the species of aphid observed in this experiment is Umbonia crassicornis. Its native habitat ranges from Mexico through Northern South America, with introduced populations present in the Caribbean and the US. Female aphids usually lay about 100 eggs on the end of stem or branch, and remain to protect offspring until they are mature, 6-8 weeks later. Immature aphids are called nymphs.

The experiment took place in Costa Rica near the town of San Luis on the Pacific side of the Cordillera de Tilaran Mountains at an elevation of 700 meters (approximately 2,2906 feet). Enterolobiam cyclocarpum and Acacia centralis tree saplings were those on which the U. crassicornis were observed. Aphids are most common in the region during the rainy season from May through December. The study was conducted between July 18th and August 13th of 1996.

Pseudopolybia compressa wasps are the most common predator of the U. crassicornis, which approach from the air in their attack on the aphid nymphs. If a nymph is captured, it is taken back to the wasp nest where it is cut into small pieces and fed to the wasp larvae. P. compressa do not hunt socially.

BEHAVIORAL OBSEVATIONS

In order to observe the predatory activities of the wasp on many groups of aphids, Dr. Cocroft recorded the insects on film. In his own words he states, "I positioned a camcorder (Canon ES 2000 Hi-8 with a 200x optical zoom lens) on a tripod 1.5-2m from the group, with the long axis of the stem in the focal plane of the camara. Observations on a focal aggregation consisted of continuous monitoring for 2-6 hours." This filming began whenever a wasp was observed to be 1-2m from the aphid grouping, and ended about 1 min. after the departure of the wasp. Field notes were also used in addition to the recorded footage.

PRESENCE OF THE MOTHER AND THE PREDATOR’S SUCCESS RATE

To determine how effective the mother aphid was at defending her young from P. compressa, it was necessary to define the parameters of the experiment. The success rate of the wasps capturing of a nymph was related to whether or not the mother Umbonia crassicornis was present during a wasp encounter. An "encounter" was defined as a wasp approaching within 20 cm of the aphid aggregation, hovering around the nymphs and/or landing near their location, and finally leaving and no longer being within 20 cm of the nymphs for more than 30 sec.

CENTER VS. EDGE POSITIONS

In his research, Dr. Cocroft decided to address the question: are individuals on the edges of groups more vulnerable to predation than those in the center? And if so, how more susceptible are these border nymphs to predation? To answer these questions, Dr. Cocroft scored whether the individual contacted by the wasp, was on the inside or outside of the group. He hypothesized, or predicted, that predation risk is independent of position. However, upon tallying the results, he discovered that depending on the number of nymphs present 30-50% of those that were attacked were those on the outside. These results will be further discussed later.

DISTANCE FROM THE FEMALE AT THE TIME OF CONTACT

A second factor in Dr Cocroft’s experiment was the question: is the nymph’s likelihood of capture related to its distance from the female? To identify location in relation to the position of the mother aphid, each nymph was classified as proximal, medial, or distal third of the group, where the individuals closest to the base of the branch were considered to be "proximal." To gauge the distance between the female and nymph being attacked, a ruler was placed on the TV screen to determine how far the aphid mother needed to travel in order to defend her offspring. The results of this portion of the experiment will be discussed shortly.

FEMALE AND WASP BEHAIVOR

The behavior of both the female aphid and attacking wasp was the last important element in the study methodology used by Dr. Cocroft. He measured two aspects of the timing of female-predator interactions: first the speed at which the mother moved across the group to protect a nymph, and second, the time required for a wasp to remove a nymph once an encounter began. Turnaround time for the female to move into a different position to defend offspring was another important issue. Dr. Cocroft measured these times by observing these events on the TV screen and using a stopwatch and counting the number of video frames necessary for the behavioral event to occur.

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