by Cassandra Olds, extension entomologist
As temperatures rise and we move into the throes of summer, many are evaluating their insect control options. Controlling insects and related arthropod pests has been a challenge since the dawn of human history. Descriptions of these early attempts exist as preserved archeological artifacts, some of which date as far as 4000 years ago! These records describe the use of elemental compounds such as sulfur and arsenic but also more complex derivatives made from toxic plants. The development of effective commercially available control methods began to accelerate after the end of World War II with the discovery of four main compounds, namely organochlorides, organophosphates, carbamates and synthetic pyrethroids. We still use many of these compounds today.
It is predicted that by 2050, the world will need to sustainably feed more than 9 billion people1. Safe and effective insecticide use will likely continue to be a key component of integrated pest management strategies. Although addressed less frequently, the development of insecticide resistance is as much of a concern for the future of livestock production as it is for crop production systems. The development of insecticide resistance is a natural process of adaptation. During this process, genetic changes occur in individuals within the pest population allowing them to survive the toxic effects of the insecticide. When the insecticide is used, individuals with the adaptation go on to reproduce spreading this genetic advantage to their offspring while the others without the change die. With each cycle of reproduction, the number of resistant individuals increases until entire populations become resistant. Genetic changes which confer a selective advantage form the basis of life, as such the development of insecticide resistance unfortunately is inevitable. The rapid generation interval of many insect species means that any genetic advantage has the potential to spread very rapidly. Thankfully, there are steps which can be taken to slow the rate of resistance.
Environmental management practices which reduce suitable breeding habitats can significantly reduce insect populations and thus the need for insecticide use. Larval stages of horn, face and house flies develop in manure, by removing or manipulating sources of manure making them inaccessible, fewer flies develop to the adult stage. Stable flies develop in decaying vegetation and the areas around standing hay bales are prime developmental sites for larvae. Patch burning grazing land and use of walk through traps, also known as Bruce’s traps have been shown to reduce adult horn fly populations by 40-50% without the use of any insecticide interventions.
When insecticide application is required, use judiciously following the manufacturer recommended dosages and formulation. Incorrect formulation dilutes the concentration of the active ingredient potentially below the level required to kill pests. This allows pests to be exposed to the chemical without the killing effect, promoting the development of adaptive mutations. Pesticides, including insecticides and acaricides are grouped by their method of killing and labeled with numbers from 1 to 32. Within these groups, there are lettered subgroups which indicate a different chemical structure although the method of action remains the same. Avoid using products repeatedly which fall into the same group even if the application method differs. Unfortunately, not all insecticide labels show the group number, however, you can use the search function from this website to easily get group information for the listed active ingredients (https://irac-online.org/modes-of-action/). Permethrin, a pyrethroid derivative and the organophosphate coumaphos are both effective for tick and fly control. Permethrin targets sodium channels in cell membranes, affecting the arthropod nervous system and is classified as a group 3A compound. Coumaphos also affects the arthropod nervous system however, it affects an entirely different target, the protein acetylcholinesterase and as such, is a group 1B compound. Alternating between compounds from different groups will result in resistance developing at a slower rate than if either were used alone to control ticks and flies. For tick and fly control, use organophosphate for two years, followed by pyrethroid for one year establishing a 2-1-2-1 rotation basis. Abermectin based products (avermectins B1a and B1b) can be used as a replacement if resistance to either pyrethroids or organophosphates is encountered. Reaching out to your local veterinarian or extension specialist can help you develop a sustainable control program.
The fallout from increased insecticide resistance extends further than controlling existing livestock pests, it has far reaching implications for controlling invasive pest species. Cattle fever ticks (Rhipicephalus (Boophilus) microplus) were once widespread throughout the southern United States but were successfully exterminated through an extensive eradication campaign which spanned decades. Strenuous efforts to maintain this status through a permanent tick eradication quarantine area keep the ticks from reestablishing and spreading northwards, for now… Alarmingly, ticks collected from this area show increased acaricide resistance rates from 3-4% in 2008 to over 50% in 20172. Such increases in resistance significantly reduce control options with potentially catastrophic effects. Reintroduction of cattle fever ticks could also result in the reintroduction of bovine babesiosis, a deadly tick-borne disease of cattle caused by Babesia parasites. With the scientific understanding of the mechanisms underlying insecticide resistance increasing, models and molecular techniques are better able identify resistant populations and predict the spread of this resistance. Classifying insecticides by method of action rather than mammalian toxicity allows trends in resistance spread to be more accurately monitored allowing us to better predict the future of insecticide resistance. On-going research focused on insect physiology will provide further information on new safe and effective targets. However, even with these advances, slowing the spread of insecticide resistance will require coordinated concerted efforts by us all.
1http://www.fao.org/fileadmin/templates/wsfs/docs/expert_paper/How_to_Feed_the_World_in_2050.pdf
2 https://doi.org/10.1093/aesa/saz053
