Host Suicide Induced by Parasites ─ Hemlock Woolly Adelgid
Donald A. Windsor
Suicide induced in animal hosts by animal parasites has been known for decades (1).
But suicide induced in plant hosts by animal parasites is new to me. The Hemlock Woolly Adelgid (HWA) (Adelges tsugae, Hemiptera) is currently invading Chenango County in central upstate New York, where I live (2). At a training session by the NY State Department of Conservation in Sherburne on 19 January 2019, I learned that the crawler stage of the adelgid inserts its feeding tubes into the stems of Eastern Hemlock (Tsuga canadensis) trees, just proximal to a needle and then feeds on tree sap for the rest of its life. However, this parasite does not kill its host tree. The tree kills itself by shutting off the flow of sap. This defensive reaction eventually kills all of its needles and twigs, resulting in the death of the entire tree, in about 4 years. This suicidal outcome seems to be an overreaction by the host's defense mechanism (3, 4).
The Western Hemlock (Tsuga heterophylla) is also parasitized by a lineage of HWA, but its similar defense reaction is muted enough so that infestations are not lethal (5).
A comprehensive recent review of HWA is provided by Limbu et al. (6).
All of which makes me wonder. When a person has a fatal allergic reaction to a bee sting or a peanut, is that really a suicide? Not in the sense that humans have a free will and killing oneself has to be intended to qualify as suicide. However, the body, sans mind, does indeed commit suicide.
Host suicide may confer a selective advantage if it gets rid of, or retards, its parasite. Consider the interactions of the fungus causing anther-smut disease in several species of alpine carnations (7).
1. Trail, Deborah R. Smith. Behavioral interactions between parasites and hosts: Host suicide and the evolution of complex life cycles. The American Naturalist 1980 July; 116(1): 77-91.
2. Anon. Early detection of the Hemlock Woolly Adelgid (Adelges tsugae) in small northeastern hemlock (Tsuga canadensis) woodlots. Forest Connect Fact Sheet, Cornell University Cooperative Extension. 4 pages.
3. Radville, Laura, et al. Variation in plant defense against invasive herbivores: Evidence for a hypersensitive response in Eastern Hemlocks (Tsuga canadensis). Journal of Chemical Ecology 2011 June; 37(6): 592-597.
4. Gonda-King, Liahna ; et al. Tree responses to an invasive sap-feeding insect. Plant Ecology 2014 March; 215(3): 297-304.
5. Foley, Jeremiah R. ; Salom, Scott ; Minteer, Carey.
6. Limbu, S. ; Keena, M.A. ; Whitmore, M.C. Hemlock Wooly Adelgid (Hemiptera:Adelgidae): a non-native pest of hemlocks in Eastern North America. Journal of Integrated Pest Management 2018; 9(1): 27:1-16.
7. Bruns, Emily L. ; Antonovics, Janis ; Hood, Michael. Is there a disease-free halo at species range-limits? The codistribution of anther-smut disease and its host species. Journal of Ecology 2019; 107: 1-11.