Low host-plant nitrogen contributes to poor performance of heather beetle, an introduced weed biocontrol agent in New Zealand

Heather, Calluna vulgaris, is a non-native plant that displaces indigenous vegetation in Tongariro National Park (TNP), New Zealand (NZ), and was targeted for biocontrol with introductions of European heather beetle, Lochmaea suturalis. Heather beetle was chosen as a biocontrol agent for NZ because in its native Europe it is a host-specific, outbreak-pest of low-fertility, heather-dominated ecosystems. However, when released in NZ in the 1990s heather beetle did not perform as expected: establishing field populations proved difficult, and population growth rates were lower than in European outbreaks.

European research links heather beetle outbreaks to air pollution that causes increased host-plant nitrogen (N). We hypothesized that low N in NZ heather, probably caused by low air pollution combined with low-fertility soils, contributes to poor heather beetle performance. We confirmed that heather in TNP has low mean N (1.14% dry weight) compared to UK samples (1.48%). In TNP, fertilizing heather increased mean N to 1.35%, which, in caged bioassays increased heather beetle oviposition (by 35%) and adult body mass (by 28%). From 2001 to 2015 in TNP a field release trial showed improved heather beetle performance on fertilized heather plots versus controls.

We conclude that low host-plant N probably contributed to poor heather beetle performance in TNP. Further research is needed to make a causal link between low N in heather in TNP and low air pollution, and to determine the underlying mechanisms for reduced performance of heather beetle, such as reduced fecundity, increased overwintering mortality, and/or Allee effects. Host-plant quality has rarely been implicated in poor performance of weed biocontrol agents and deserves further investigation.