Insecticide resistance Flashcards
What is insecticide resistance?
Development of an ability in a strain of an organism to tolerate doses of toxicant which would prove lethal to the majority of individuals in a normal population
When and what was the first documented case?
1914 (San Jose scale insect to hydrogen cyanide)
How many species have shown resistance as of 2000?
> 700 species - growing exponentially
What major pest species have shown resistance?
Bollworms Mosquitoes Peach-potato aphids Diamondback moth Houseflies Whiteflies Planthoppers and leafhoppers Rice stem borer
What factors affect resistance development?
Dominance of alleles involved Generations per year Offspring per generation Mobility of pest population Single or multiple applications Persistence of the insecticide
Where is resistance impact the greatest? Why?
Tropical countries
- stable climate: ensures numerous generations per year, rapid pest reproduction
- greatest problems associated with vector borne disease transmission
- cotton primarily grown in tropical countries (many cotton pests are resistant to insecticides)
What types of resistance are there?
Single compound resistance
Multiple resistance - resistance to at least 2 actives caused by separate exposure to those actives
Cross resistance - insecticide resistance to a compound a pest was not exposed to
What are the trade offs in resistance development with regards to aphids?
Species exposed to multiple selective forces (insecticides being only one)
- Peach-potato aphid (Myzus persicae) there is evidence that resistance to insecticides has an impact on behaviour
- Aphids with enhanced levels of detoxifying enzymes (carboxylesterases) show reduced tendencies to move from senescing (aging) leaves
- results in greater mortality under adverse climactic conditions (food source is poorer)
What other trade off, as well as reduced movement from senescing leaves, do aphids “trade off” as a result to resistance?
Aphids with both enhance levels of detoxifying enzymes (carboxylesterases) and kdr resistance show reduced tendency to respond to alarm pheromone
How can trade offs be used in resistance development?
By changing the dominant selective force i.e. enhancing exposure to parasitoids/predators rather than insecticides may slow resistance development
What are the major resistance mechanisms?
Elevated levels of detoxifying enzymes - esterases, monoxygenases, transferases (gene amplification)
Decreased target site sensitivity - Sodium channels, AChE target site, GABA receptors
Increased cuticular thickening (lower absorption)
Behavioural changes in pest species - e.g. diamon-back moth larvae
What are the four target sites most insecticides act on and what has this resulted in?
Acetylcholine receptors (neonicotinoids) Sodium channels (DDT, Pyrethroids) GABA receptors (cyclodienes, BHC) AChE (organophosphates, carbamates) Has ensured the development of cross and multiple resistance to numerous active ingredients - 50% of the current world market targets AChE
How do neonicitionoids work?
Act as agonists of the post-synaptic acetylcholine receptors (ligand-gated ion channels)
Ligand gated ion channels recieve chemical signals e.g. from acetylcholine and then convert the signals to electrical impulses by opening ion channels
Unique mode of action means no cross-resistance with other insecticides and partly explains rapid uptake
Facts and figures on neonicitionoids
First use 1991 - imidacloprid Nitenpyram and acetamiprid - 1995 Thiamethoxam - 1998 Now 7 "neonics" in total ~25% of all insecticides used worldwide are neonics
Why was pre-existing resistance a worry?
Which species is especially worrying?
Some species may have developed resistance to neonics as a result of feeding on tobacco
Peach-potato aphid (Myzus persicae nicotianae)
What species was the first to exhibit resistance to neonics and to which?
How many species are reported as of 2014 to be resistant?
First reported resistance 1996 - Cotton whitefly (Bemisia tabaci) to imidacloprid
20spp. reported resistant as of 2014
Which majorly concerning species have shown neonic resistance to date?
Peach-potato aphid (Myzus persicae) Cotton whitefly (Bemisia tabaci) Cotton aphid (Aphis gossypii) Housefly (Musca domestica)
What does evidence from the whitefly indicate the resistance mechanism to neonics is?
Indicates increased ability to detoxify the active ingredient
As result of work with Piperonyl Butoxide it has been shown that monooxygenases are involved in breakdown
What is a synergist and what synergists are used with neonics?
A compound that does not have insecticidal activites but makes the insecticide more effective
Piperonyl Butoxide
- inhibits cytochrome P-450 enzymes
- these enzymes detoxify many insecticides
- commercially available since 1947
What gene is responsible for the excess amounts of detoxifying enzymes in Bermisia sp.?
CYP6CM1
Why has a lot of cross resistance to neonics been observed?
All neonics have similar mode of action
What gene does Myzus persicae have that leads to excess P-450 enzymes? What is an additional problem in this species?
CYP6CY3
Gene amplification leads some resistance individuals to have more than 100 copies of the gene
Also evidence of over-expression of the genes involved in cuticular thickening
Also evidence of amino-acid substitutions leading to binding site changes (arginine to threonine)
Additional Reading - Neonic points from Jeschke and Nauen., 2008
- Potent agonists selectively acting on nAChr, leading to build up of ACh and death
- Can be taken up by plants at root to kill sucking pests (Hemiptera) - may also be aiding in the decline of bees
- Soil treatments can be done by incorporation of granules, injection, drip irrigation, spraying and the use of tablets
- Plants and plant parts can be treated by seed dressing, pelleting, implantation, dipping, injection and painting
- All above methods have led to a more economic and environmentally friendly use of these products
- Biodegradable and hence no bioaccumulation
- Safety and effectivness attributed to the high selectivity for insect nAChr, fast penetration into insect CNS and most importantly the difference in target site interactions
Additional Reading - Imidacloprid success against which pests?
Jeschke and Nauen., 2008
Hemiptera - Aphis fabae, Aphis gossypii, Myzus persicae
Coleoptera - Colorado potato beetle (Leptinotarsa decemlineata)
- Rice leaf beetle (Lema oryzae)
Lepidoptera - Tobacco budworm (Heliothis virescens)
