Insecticide resistance is well documented in non-social insects, but could it be impacting treatments for ants and termites too? A US review paper examines the evidence.
Insecticide resistance is often one of the first things mentioned when a pest control treatment doesn’t work. It is certainly a possibility with bed bug treatments and maybe with German cockroach treatments, but is insecticide resistance really a reason for treatment failure when dealing with ants and termites? A recent review paper from leading researchers in the US had a closer look at the possibility.1
‘Insecticide resistance’ – a definition
Firstly, it’s necessary to define ‘insecticide resistance’. The authors of the paper use the definition proposed by the Insecticide Resistance Action Committee (IRAC), which states resistance is “a heritable change in the sensitivity of a pest population that is reflected in the repeated failure of a product to achieve the expected level of control when used according to the label recommendation for that pest species”. There are two key phrases here – “heritable change in sensitivity” and “repeated failure”.
From an operational point of view, when carrying out treatments in the field, repeated failure of a treatment (once all other reasons for treatment failure have been excluded) could possibly be attributed to insecticide resistance. However, it can only really be confirmed by laboratory testing. When considering insecticide resistance in ants and termites, “heritable change in sensitivity” is at the core of the discussion.
Differences between non-social and eusocial insects
Crucial differences exist in the population structure and reproduction strategies of eusocial insects (bees, ants, termites) compared to the non-social insects (bed bugs, cockroaches, flies, mosquitoes).
All the adults in non-social insects can reproduce and their reproductive rate can be rapid. As a result, the population can quickly respond to selection pressures. For example, if a few resistant adults remain after a treatment, they can quickly breed, passing on their resistant genes to the next generation. Repeated exposure to the same insecticide will favour the more resistant individuals and thus a strongly resistant population can quickly develop.
For eusocial insects the situation is quite different, and their population dynamics provide a barrier to the natural selection pressures that drive insecticide resistance. In ant and termite colonies, the vast majority of individuals are non-reproductives, which means they cannot pass on their genes to the next generation. When a treatment takes place, it is often targeted at a single colony, often with a single reproductive (if present, any secondary reproductives will likely have the same genetics). As a result, there aren’t the number of reproductives nor the variation in genetics to drive the development in insecticide resistance in the same way that would occur with an insecticide treatment on a cockroach population. Lastly, although ants and termites produce a lot of reproductives, in many cases they only do so once a year – not the fast reproduction rate that allows for the development of insecticide resistance.
That’s not to say insecticide resistance couldn’t develop in eusocial insects. Firstly, natural variation in susceptibility to insecticides has been recorded in bees, ants and termites. Secondly, ants and termites contain the same number of detox-encoding genes as the non-social insects, which do exhibit resistance. It’s just that the insecticide pressure needs to be applied over a larger area over a longer period of time for the selection pressure to drive the development of resistance. Indeed, the few cases of resistance in eusocial insects have occurred in honeybees. The widespread use of DDT in cropping areas in the 1950s and 1960s led to the development of DDT-resistant populations of wild honeybees in California.
Behavioural resistance
With their unique social behaviours, the authors of the review pointed out that resistance to insecticide can be achieved through the development of behaviours that provide increased protection from insecticides. Recent research on red imported fire ants found that the ants used soil particles to cover insecticide-treated surfaces to reduce the effect of certain insecticides.2
Challenges in investigating resistance in social insects
Given the need to work with whole colonies and carry out research over extended timeframes, the authors acknowledged some of the key challenges in carrying out experiments to confirm insecticide resistance in eusocial insects. However, they suggested that studies comparing insecticide susceptibilities of colonies from different areas may provide evidence for resistance. For example, by comparing colonies from areas where insecticide use has been consistent over time versus an area of no insecticide use. One study did indicate that urban odorous house ant colonies were more tolerant to the neonicotinoid dinotefuran than colonies from natural non-urban areas. It was suggested that this may be correlated with the fact the urban colonies had a polygynous/polydomous colony structure whereas the natural colonies tended to be of a single queen/single nest structure.
This leads to another suggestion from the authors, namely that any ant studies on insecticide resistance should be focused on invasive ants; with their super-colony structure (multiple queens and rapid reproduction) they are more likely to develop resistance.
Termite gut symbionts
When it comes to studies on termites, the authors also pointed out that the gut symbionts should be part of any investigations. Not only are they metabolically active, but they also have rapid reproductive cycles, allowing for the quick selection of beneficial traits.
Bait avoidance in ants
For pest managers using baits in ant management, it is important to understand that bait avoidance is not insecticide resistance. Bait avoidance could simply be the result of changing food preferences, although it could also be a learnt behaviour whereby the colony learns to avoid a particular bait as a consequence of experiencing a sub-lethal effect. To minimise the chances of sub-lethal colony effects, it’s important to apply sufficient bait for the size of the ant population.
Summary
In summary, our current understanding indicates that although metabolic insecticide resistance in ant and termite populations is a theoretical possibility, there is little evidence to confirm insecticide resistance in field populations. So, if a product isn’t working it may be down to a quality issue with the product or an issue with application.
1 Scharf, M.E and Lee, C-Y (2024): Insecticide Resistance in Social Insects: Assumptions, Realities, Possibilities. Current Opinion in Science. https://doi.org/10.1016/j.cois.2024.101161
2 Wen, C et al. (2022). Red imported fire ants cover the insecticide-treated surfaces with particles to reduce contact toxicity. Journal of Pest Science. 95: 1135-1150. https://doi.org/10.1007/s10340-021-01474-0
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