Which Insects Pose the Greatest Risks?
Our expertise in determining which insects attack which plants has recently been put to a slightly different use, i.e. helping to predict risks to native plants. Quentin Paynter assisted Nick Martin (Plant & Food Research) in looking at the risk that exotic herbivores pose to our indigenous plants. In particular, they were interested in teasing out whether the taxonomic group the insects belonged to was important.
Despite the best efforts of border security, there is a constant influx of herbivorous insects arriving here either accidentally on imported goods or arriving under their own steam and one of the tasks of the Ministry for Primary Industries (MPI) is to assess the risk of any new incursions to our indigenous flora. This assessment is based on an estimate of the likelihood that a new herbivore will feed on an indigenous plant.
Although some taxa may be more likely to attack indigenous plants than others, until now nobody has looked into what has actually happened in New Zealand. Nick and Quent searched the literature and talked to experts in the field to document known associations between adventive species and indigenous plants. They produced a summary showing the number of adventive mites and insects known to have arrived in New Zealand and which plants they are hosted by. Seven major arthropod orders were used in the analysis with the largest group being Hemiptera (true bugs). The other six comprised Thysanoptera (thrips), Lepidoptera (moths and butterflies), Hymenoptera (wasps, bees and ants), Diptera (flies), Coleoptera (beetles and weevils) and Acari (mites), with a total of 624 species included in the assessment (other orders, such as Dermaptera (earwigs), were omitted from the analysis because there were insufficient data on their host associations in New Zealand).
“Hemiptera were the main group found attacking our indigenous plants, with 32% of the adventive species caught in the act,” commented Quent (see graph). One order, Hymenoptera, and some families within orders, e.g. gall mites (Acari: Eriophyoidea) and gall-flies (Diptera: Cecidomyiidae), are a low risk since they are not known to attack indigenous plants.
“This may reflect the degree of host-specificity exhibited by different insect groups,” said Quent. “The New Zealand flora has evolved in isolation and contains a high proportion of endemic plants found nowhere else in the world, so arthropod groups with a high proportion of specialist herbivores are less likely to attack endemic native plants. Plants that are most at risk of attack by specialist herbivores tend to be those that are not confined to New Zealand, such as mānuka (Leptospermum scoparium), which is also native to Australia.”
In general the more polyphagous species (able to feed on many plant species) there are in an arthropod family, the more likelihood there is that any particular adventive species will feed on our native plants. Risk assessments should therefore factor in the taxonomic group insects belong to as this will give a clue as to the likely risks they pose and which plants are likely to be on their menu.
Further information
Martin NA, Paynter Q 2013. Predicting risk from adventive herbivores to New Zealand indigenous plants. New Zealand Entomologist 37: 21–28. Available online: http://doi.org/10.1080/00779962.2012.759308
