Passage III

Southern pine beetles (SPBs) construct nursery galleries for their larvae within the bark of pine trees. SPBs introduce Fungus M into the nursery gallery to serve as a food source for developing SPB larvae. If a competitor fungus——Fungus A——outcompetes Fungus M in the gallery, SPB larval development will be disrupted.

SPBs also introduce into the nursery gallery a bacterial species that is thought to help maintain the relationship between SPBs and Fungus M. Two strains of this bacterial species, Strain W and Strain R, have been isolated from SPBs. Scientists did 2 experiments to study whether the introduction of Strain W or Strain R would be more likely to protect the larval food supply.

Experiment 1

An agar growth medium contained in each of 160 petri dishes was inoculated at its center with Strain W. The medium in each dish was also inoculated, at a point near its edge, with a fungal species: in 80 dishes with Fungus M and in the other 80 dishes with Fungus A. At the end of weeks, the shortest distance across the zone of inhibition (ZOI: the space between the edges of fungal growth and of bacterial growth) was measured for each dish. The shortest distance for a dish, defined here as the width of the ZOI, determined the degree of inhibition exhibited by the dish (see Table 1).

Figure 1 shows, for each fungus, what percent of the 80 dishes exhibited a given degree of inhibition.

Experiment 2

Experiment 1 was repeated, except with Strain R instead of Strain W (see Figure 2).