For Instructor and Students
Biological control is a costly means to control invasive species and also a risky endeavor, as not all biological control agents introduced manage to effectively control target species. Further, introduced biocontrol agents have been found to have unanticipated and negative effects on the community to which they are introduced. Typically, such reported negative effects have included instances of host-switching or non-target effects. Less frequently reported are the indirect effects that introduced agents can have on food web interactions. The figures used in this jigsaw activity come from peer-reviewed articles which report information on the indirect effects of two Urophora species (gall flies) which were introduced to control populations of the invasive spotted knapweed (Centaurea maculosa).
Two species of gall flies (U. affinis and U. quadrifasciata) were introduced in the early 1970s to control populations of spotted knapweed. The gall flies lay their eggs in the undeveloped flower heads of knapweed. The developing larvae of the gall flies effectively decrease seed production of the knapweed. However, this decrease in seed production does not translate into reductions in the populations of the invasive knapweed. (Another instance of individual level effects not translating into population level effects - see previous figure sets in this issue). Therefore, the gall flies continue to persist in populations of knapweed and (as we will see) have the potential to indirectly affect human health.
The purpose of this jigsaw activity is to piece together the relationships between the invasive knapweed, the biocontrol agents (gall flies), deer mice, hantavirus, and human health.
For Instructor Only
Although the gall flies did not successfully reduce populations of knapweed, they do provide an additional food source for native deer mice (Peromyscus maniculatus), which allows deer mice populations to increase in areas with knapweed populations. Deer mice are known reservoirs for hantavirus, a virus that can be deadly to humans. Therefore it is postulated that increases in deer mice populations could lead to higher risks of hantavirus in humans.
The following figures come from three papers, each of which investigates separate aspects of the indirect effects of an introduced biological control agent.
The first figure (3.1), from Pearson, McKelvey, and Ruggiero (2000), illustrates that the monthly variation in % stomach content (of deer mice) that consists of gall fly larvae corresponds with the yearly cycle of gall fly and deer mouse life cycles. Students should realize after seeing this figure that deer mice are consuming gall flies as a large portion of their diet, and are only consuming gall flies when they exist as larvae within the seed heads of the knapweed. Instructors may wish to pose a question to students to address this portion of the figure. As an example, ask students: why does the dependent variable decline in June-Aug?
The second figure (3.2) is from Pearson and Callaway (2006), which illustrates that gall fly larvae density is higher in areas with higher knapweed density.
The third figure (3.3), also from Pearson and Callaway (2006), illustrates that the abundance of deer mice, the abundance of seropositive (carrier of virus) mice, and the proportion of seropositive mice are all higher in populations containing higher densities of knapweed.