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brown rat

The pest control industry has evolved beyond the wanton destruction of pests at all costs.  Sandy MacKay, Technical Lead and Alex Wade, Technical Manager at PelGar International, seek to explain what makes an animal a pest, and consider whether there is still a place for pesticides, in this constantly evolving industry.

We are embracing the concept that what makes an animal a pest is not its species, but its location and behaviour.  We understand that animals, such as rats and mice, are an important part of a delicate equilibrium within the ecosystem of our planet. We also understand that human behaviour, especially our ability to grow, store and waste food in huge quantities, encourages these animals and supports their populations artificially. This breaks that delicate equilibrium and turns them into pests.  The responsibility then falls to us to manage these unnatural populations of pests through exclusion, prevention and control.

Yet it is all too common for the media to paint pesticide usage as the ‘enemy’. With many (if not all) of the products, used in pest control, intended to kill, control or disrupt populations of pest animals. It does feel like a paradox to claim that their use can be of benefit to the environment.  So, is the media wrong then to be concerned? Is there still a place for pesticides within this constantly evolving industry?

The very simple answer is – yes. The purpose of pest control is to safeguard the well-being and interests of humans, the animals directly under our care and those animals who are indirectly affected by our actions.  In short, pest control has become less about ‘control’ and more about management.

The problem does not lie with the products this industry employs, but how we choose to apply them.  As with anything you do, from turning on a cooker in your home, using a plug socket, or baiting an area for rodents, the level of risk assigned to a situation comes not just from the products, but more importantly how and where they are used.  It is all too easy to overlook the positive impacts that a well-structured, diligent and precise pest management programme can have not just on society and public health, but also the environment.

Pesticides for preservation of wild landscapes

In addition to the estimated 40% of crops that are lost globally to pest animals, plants and fungi before harvest [1] each year in developed countries, almost 10% of the food harvested and stored will be lost to pest animals.  This number rises dramatically in developing countries to almost 30% [2].  Food that remains after this onslaught may still be spoilt, have a severely reduced economic value, or simply no longer be fit for its intended use.  This is clear evidence that pests have a direct impact on the economy of our society, yet the ramifications of these losses can spread much further.

The loss of crops, either stored or from the field, will ultimately lead to a deficit in food for the society that it is intended for. Immediately this can lead to starvation and famine of humans and associated livestock unless suitable replacements can be found.  The impact this can have on developed countries could be significant, but the impact that a 30% loss in stored foods will have on a community within a developing country, especially one of subsistence farmers, will be devastating. In Asia, a loss of 5% of rice production amounts to approximately 30 million t; enough rice to feed 180 million people for 12 months[3]. Post-harvest losses are probably of a similar magnitude to pre-harvest losses.

Therefore, to compensate for this loss, farmers will annex more land to establish greater crop yields to avoid a potential deficit. Whilst fields of crops will support an ecosystem of vertebrate and invertebrate animals alike, neither conventional nor organic farmland will ever be able to support the level of biomass or biodiversity that an unfettered countryside will.

By protecting crops both in the field and in storage, through using competent integrated pest management strategies, it is possible to reduce waste, therefore reducing the reliance on larger and larger areas of land to support the required crop yields.  The proficient use of chemical control methods as part of this integrated strategy, to quickly and effectively control pest infestations within stored crops, is an effective and essential tool. If done correctly this can be achieved without unnecessarily effecting the surrounding ecosystem, providing a net benefit to the populations of humans and livestock and, vicariously, the surrounding environments.

The benefits of these strategies can occur in several ways: firstly, as already described, by allowing more land to remain ‘wild’ a healthier and more diverse ecosystem is established. Secondly an unchecked and artificially sustained infestation of rodents within a crop or food store will put a greater pressure on the surrounding ecosystem.  As the pest population increases, and begins to fragment, individual animals move back into the countryside.  This constant migration of pests from safe harbourage ultimately increases food competition within the ecosystem between pest rodents and other wild rodents and birds.  This can potentially lead to other species becoming outcompeted or displaced.

Studies observing bird nests in rainforest ecosystems in the South Pacific noted that, ‘of a total of 178 artificial nests containing two eggs of three different sizes, placed either on the ground or 1.5 m high… 12.4% of the nests were depredated [by rodents] during the first 7 days’[4].  The pressure that an abnormal number of rodents can put onto a species can often result in the loss of that species from the environment.  In order to keep the population density of ‘pest’ rodents within these ecosystems at a natural equilibrium we must take care to reduce the migration of rodent species from areas of human interest back into the environment. It is vital to control these pest populations not only efficiently, but quickly through the precise and controlled use of rodenticide products within the framework of a greater pest management strategy.

Rodenticides for conservation

This may seem counter intuitive, but the presence of invasive animals to an ecosystem can be disastrous.  On island-based ecosystems invasive rodents can be the single biggest threat to a population of animals and, in some cases, an entire species.  Some papers state that ‘invasive mammals are the greatest threat to island biodiversity and invasive rodents are likely responsible for the greatest number of extinctions and ecosystem changes.’[5]  This statement is more alarming when you consider that ‘non-native rat species and mice have been introduced to more than 80% of the world’s island groups.’[6]

Huge programmes of work are undertaken to control invasive species of rodents which threaten these niche habitats and destroy entire populations of animals, especially ground nesting birds.  These programmes all employ a wide range of strategies to control rodents in these environments, but fundamental to their success is the careful and structured use of rodenticides.  The use of rodenticides to these conservation efforts is so vital, that of the 332 successful island rodent eradication programmes undertaken globally to date, only 2 did not employ the use of a rodenticidal compound.

These conservation initiatives have resulted in locations such as South Georgia being free from invasive rodent species for the first time in over a hundred years at the conclusion of the programme.  PelGar was involved in a similar strategy, under its Conservation in Partnership initiative, in the Isles of Scilly Seabird Recovery Project. Through the use of strategic baiting programmes, pest pressure of invasive rodents was sufficiently removed to allow for the return of the Manx Shearwater, the first chicks of which were observed on the island for the first time in living memory in 2017.

The application of pesticides will always be a paradox of creating safety through the use of ‘harmful’ chemicals.  The risks to environment, society and the health of the general public will always be of concern, but without the use of Integrated Pest Management strategies, that include the use of pesticides to manage pest animals, the impacts of not using these tools will create a risk to all these things that is untenable.

 

[1] OECD-FAO Agricultural Outlook 2012. Organisation of Economic Co-operation and Development and Food and Agriculture Organization of the United Nations

[2] Stored-Grain Ecosystems – Edited by Digvir S. Jayas, Noel D. G. White, William E. Muir

[3] Impacts of Rodents on Rice Production in Asia – Grant Singleton

[4] Invasive rats strengthen predation pressure on bird eggs in a South Pacific island rainforest – Curr Zool. 2017 Dec; 63(6): 583–590

[5] Invasive rodent eradication on Islands. Gregg Howald et al. 2007

[6] Proceedings of the 12th Wildlife Damage Management Conference (D.L. Nolte, W.M. Arjo, D.H. Stalman, Eds). 2007

This article appeared in International Pest Control, July/August 2019 volume 61:4