Interpreting host-test results for classical biological control candidates: can the study of native congeners improve the process?
deJonge, R.B., Jones, I.M., Bourchier, R.S., Smith, S.M. Interpreting host-test results for classical biological control candidates: can the study of native congeners improve the process?. Biological Control, [online] 145 http://dx.doi.org/10.1016/j.biocontrol.2020.104237
Plain language summary
When an insect is chosen as a potential candidate for the biological control of an invasive plant, a key requirement for screening is that it eats only the target plant and not related species. In general most of the host-range testing of potential biological control insects must be done under contained laboratory conditions or at best under semi-field conditions in the countries where the insects originate. Typically insects will eat a lot more plant species in laboratory testing, when they have no alternative choice, than they actually do in the field. Using a case study of a North America beetle compared to a European species in the same genus, this paper reviews the testing of plant species eaten by biological control agents and suggests a new tool to improve the interpretation of laboratory testing. Specifically the paper demonstrates that by experimentally assessing the feeding response of existing North American insect species that are closely related to the foreign insect, we can better understand what the foreign insect will actually feed on, if it were to be released in North America.
When an insect candidate is identified for classical biological control, the first step is to identify its
fundamental host-range. An insect’s fundamental host range, however, is typically broader than its
ecological host range. This discrepancy can lead to ‘false positives’ in host testing and hamper the
development of potential agents. Here, we propose a novel tool for interpreting host-range tests and
identifying ‘false positives’ by studying native insects closely related to the biological control candidate.
We conduct a series of laboratory and field studies comparing the fundamental and ecological host ranges
of Chrysochus auratus Fabricius (Coleoptera: Chrysomelidae), a beetle native to North America, and
present throughout the range invaded by pale swallow-wort, Vincetoxicum rossicum (Kleopow) Barbar.
(Apocynaceae). We use the results to re-evaluate the risk associated with releasing the closely-related
European beetle, Chrysochus asclepiadeus (Pallas) (Coleoptera: Chrysomelidae), a biological control
candidate for V. rossicum that has raised some concerns because of no-choice feeding on North American
milkweed species (Asclepias spp.) during laboratory host-testing. Laboratory and greenhouse trials here
show that native North American C. auratus adults can feed and complete larval development on several
native Asclepias species (fundamental host range), however in the field where both closely-related plant
genera are present, this species specialized only on plants in the genus Apocynum. It appears then that
Asclepias species generate ‘false positives’ for Chrysochus beetles when only the fundamental host range
is assessed in the laboratory, and there is a need to re-evaluate C. asclepiadeus for potential biocontrol in
North America taking into account its ecological host-plant range. We advocate for the inclusion of
closely-related native congeners, where appropriate species exist, to aid in interpreting host-plant testing
for potential classical biological control agents.