Digging Deep: What determines pest occurrence in an area? Here’s what this study showed

Pests damage crops to the tune of billions of dollars each year, and yet few studies have attempted to understand their proliferation outside their native ranges. In particular, what are the biotic, abiotic and socioeconomic factors that determine pest occurrence in a particular area?
In a recent study in PNAS, a group of ecologs from Canada have assembled a dataset consing of 310k pest populations from 206 countries in order to statically model their dribution in host native ranges. This was supplemented another dataset of more than 210k pest occurrences to determine pest populations in the non-native ranges of their host species. ‘Host’ here refers to the tree species the pest devours.
Host tree species, -and-large, are more likely to have pests in their native ranges than in their non-native ones, suggests a Gougherty et al. (2022) report. Host trees, even when playing the role of invasives in their non-native ranges or when being grown for agricultural purposes, had 15.76 per cent fewer pests than those in their native range. Trees grown commonly grown for our day-to-day utilities are good examples.

Pine trees, for instance, that are native to the Northern Hemisphere, but are grown extensively in the Southern for timber/forestry were observed to have hundreds of pests more natively than in their non-native ranges. Another good example would be the apple, cottonwood and plum trees that are grown extensively outside their original native ranges for agriculture. Exceptions, however, do ex. The eucalyptus, for example, showed more pests in their non-native habitat than in its native habitat (i.e. the Australian archipelago).
Furthermore, results show a notably high tendency for pests to occur in warm and wet countries within the hosts’ native ranges, particularly in regions where there are a large number of host trees, and when hosts are similar to native flora. Overall, pests are more abundant in countries with a large area or high population (e.g. India, China, United States etc.) than in smaller ones. Most of the ‘absences’ inferred pertained to south Asian countries like Bhutan, Nepal, Myanmar etc. However, a country’s GDP was observed to have little effect on pest occurrence.
Pests are not always ‘bad’, however. Some of them, particularly in their native ranges, play a key role in carbon cycling, forest regeneration and maintain tree diversity keeping the populations of their host species in check. Moreover, contrary to popular belief, neither are all pest species insects – in fact, according to this study, arthropods had ‘narrower geographic ranges’ while molluscs ‘tended to have the widest.’
So, what explains this gradient in pest occurrence? Why are some pest populations absent in areas that have suitable hosts and climate?
Gougherty et al. (2022) propose two explanations for the observed phenomenon. One, over evolutionary timescales, pest species had the opportunity and time to co-evolve with the defenses of the host species. Two, pests can ‘accumulate’ hosts. In other words, as pests spread to other areas within the native range of the host, they came in contact with other potential host species. This also means that, in the future, as pest and hosts respond to global climate change, pests can further expand their ‘host breadths.’ Many moth species, such as the spongy moth and winter moth, have acquired new hosts in their invaded habitats in North America.
Another factor, authors note, is trade – for the exchange of agricultural produce from two climatically similar regions can lead to a build-up of non-native pests. However, the effect of trade on the dribution of pests within the native host ranges.

The absence of pests outside the native host ranges is explained what is known as the ‘enemy release hypothesis.’ The hypothesis states that invasive species are less likely to be impacted parasites in non-native habitats, than in their original geographical locations where parasites kept their populations in check. The study, however, does add a caveat: the effects of the ‘enemy release’ might just be temporary. That is, the host might escape the parasite/pest pressure, but the pest might just soon catch up – just like eucalyptus. Eucalyptus has accumulated more pests, and even introduced it to more pests that were not there in its native range.
Authors suggest that the problem can be augmented identifying pests that can ‘catch up’ with their potential hosts, and also end up harming native tree species in the new locales. This is not a straightforward process, unfortunately for many pest species are more destructive outside the hosts’ native habitats than within.
The paper asserts that ‘while identifying mechanisms that allow both geographic and host jumps could further improve… understanding of the regions and hosts most at risk from non-native pests,’ their modelling approach indicates the principal drivers of spread – biological, abiotic and anthropogenic – as well as identifies the pest species most likely to spread and geographical areas under risk.