Lace Bug Does Best in Shade
Woolly nightshade devastated by lace bugs. Image - Andrew Blayney
Since the first release in November 2010, the woolly nightshade lace bug (Gargaphia decoris) has been released widely throughout New Zealand. The lace bug has established readily at many sites, but it became clear early on that predation was possibly going to prevent it from achieving its full potential.
However, in June we received reports of some spectacular damage in the Bay of Plenty. Heavy damage through to total defoliation of woolly nightshade plants was observed over an estimated 15-ha area in the Ngapeke Forest Block. This estimate is based on only what can be easily seen from tracks so the damage is expected to be present over a much larger area. By contrast nearby plants in open areas remain healthy. The photos sent to us looked uncannily like those of a major lace bug outbreak in South Africa, which also occurred where woolly nightshade plants were growing under pine trees. Unfortunately that site was destroyed by fire soon after the outbreak and could not be studied further. Lace bugs have been established in South Africa for 15 years but populations have mostly remained at low levels due to predation of the immature life stages. We shared the Ngapeke Block photos with our collaborator in South Africa, Dr Terry Olckers (University of KwaZulu-Natal). Terry told us that one of his students had recently done some work confirming that the lace bugs do best in partial shade.
In the South African study potted plants were grown in a shade house to ensure uniformity of size and quality before being put out in either full sun, partial shade or full shade. The plants were inoculated with 20 adult lace bugs, and monitored daily for 2 weeks. Other plants were inoculated with a batch of newly-hatched nymphs, which were left undisturbed for a week and then monitored daily for 4 weeks. Because the early instars are most susceptible to predation, all generalist arthropod predators were collected after the first week to gain some insight into their abundance but were then left undisturbed. During the 2 weeks that the adults were monitored, numbers declined steadily in all three treatments due to dispersal. The nymphs also steadily declined, with few reaching adulthood. “Partially shaded locations consistently proved to be the most suitable for both life stages, with fully shaded locations the least suitable,” confirmed Terry. Partially shaded sites had on average the least predators. These results, combined with the fi eld observations in both countries, suggest that any further releases of the lace bug should target partially shaded woolly nightshade infestations to increase the chances of establishment and impact.
Meanwhile efforts are continuing to find other woolly nightshade biocontrol agents for New Zealand. After the flowerbud-feeding weevil (Anthonomus santacruzi) failed host testing by unexpectedly completing development on poroporo (Solanum aviculare) in a field test in South Africa, it was back to the drawing board. As Terry had no immediate plans to develop new agents, we contacted the collaborator in Brazil who had originally assisted him (and helped us more recently with our tradescantia project). Professor Henrique Pedrosa Macedo (University of Parana) agreed to help and we drew up a plan.
Based on previous experience with other woolly nightshade insects, host-testing is likely to be complex and require field trials. “We therefore decided to move directly to field testing in Brazil rather than attempting to undertake testing in containment in New Zealand, which might provide ambiguous results, and ultimately see us still needing to undertake field trials,” explained Simon Fowler. This meant setting up test gardens in Curitiba. Most test plants could be sourced in Brazil but we needed to ship over seeds of poroporo (S. aviculare, S. laciniatum). The poroporo plants were initially grown in a containment facility in Brazil to check for disease, but have now been planted out ready for trials to begin as soon as insects are available. Unfortunately, due to unusual weather conditions in Brazil last spring/summer, the insects we wish to test were unusually rare and could not be collected in sufficient numbers. The species of interest include a yet to be identified gall-former, another flowerbud-feeding weevil (Anthonomus morticinus) that may be more tightly host specific than the failed A. santacruzi, and a stem-boring weevil (Conotrachelus squalidus). Simon will visit Brazil in November to assist with field surveys and establishment of the field trials.
This project is funded by the National Biocontrol Collective.