W9L2 Insecticide resistance Flashcards
1
Q
Why do we need insecticides?
A
- Insects are vectors of some major diseases and important agricultural pests
- Insecticides provide (often broad-spectrum) control of insect numbers and spread
- More tailored approaches such as gene drive and Wolbachia will likely have important roles in future insect control, but they are relatively labour-intensive and species-specific
2
Q
Early use of insecticides
A
- Insecticidal compounds are widespread in nature, e.g. nicotine, fungal aflatoxins
- Some of these compounds have been extracted and used deliberately by
humans, but tend to be expensive to produce - The era of synthetic insecticides began with the identification of the insecticidal properties of DDT in 1939
3
Q
The DDT era: how did it work and how is it used now
A
- Dichlorodiphenyltrichloroethane interacts with sodium channels in neurons in such a way as to hold them open
- Co-ordinated changes in sodium-potassium balance in neurons are crucial to transmitting discrete signals - DDT therefore results in
spasming and ultimately death - Despite prominent successes of DDT in control of insect disease vectors in the 1940s, concern about off-target effects arose early
- The Stockholm Convention on Persistent Organic Pollutants (2004)
allows DDT use for vector control only
4
Q
How to kill insects
A
- Producing chemicals toxic to insects but not humans could be achieved in several ways:
- Targeting pathways unique to insects (e.g. tebufenozide, which activates the moulting hormone receptor)
- Targeting shared pathways with different exposure routes - insect and vertebrate blood-brain barriers are functionally distinct
- Targeting molecular differences in shared pathways
5
Q
Resistance in insect
A
- The broad mechanisms of insecticide resistance will mostly be familiar:
- Detoxification
- Target site alteration
- Reducing net uptake
- These mechanisms are not identical to those seen in pathogens, e.g. reduced net uptake relies on reduced movement through the cuticle rather than efflux
- Behaviour can affect exposure, but evidence for behavioural evolution in response to insecticides is limited
6
Q
Drosophila melanogaster and insecticides
A
- Much investigation of pesticide resistance has involved the genetic model insect Drosophila melanogaster
- As this species feeds on rotting fruit, it is not the direct target of pesticide use
- Nonetheless, particular strains show clear resistance to various insecticides
- Off-target effects of insecticides are a major environmental concern, contributing for example to colony collapse disorder in honeybees
7
Q
Detoxification enzyme in insect
A
- A relatively small number of enzyme families play an outsize role in detoxification of natural and synthetic insecticides
- Drastic variation is seen in the number of such enzymes found across insects
- These enzymes tend to have properties (expression site, reaction catalysed…) useful to acting against a broad range of molecules
8
Q
Cyp6g1 in Drosophila melanogaster
A
- Cyp6g1 was identified as a detoxification enzyme due to its overexpression in the DDT-resistant Hikone-R line
- Cyp6g1 expression level is affected by a duplication and insertion of transposable elements into regulatory regions
9
Q
What does CYP6G1 do
A
- Most cytochrome P450s function by adding hydroxyl groups to their substrates; these can often be excreted from the body more `efficiently than the parent molecule
- While the reactions catalysed by CYP6G1 acting on DDT have not been characterised, its activity on imidacloprid have been studied in more detail
10
Q
The curious case of neonicotinoids
A
- Neonicotinoids are synthetic chemicals with a structural resemblance to nicotine, developed in the 1980s
- Like nicotine, the neonicotinoids bind acetylcholine receptors, causing excitotoxicity and death
- The molecular composition of acetylcholine receptors is complex - all receptors are pentamers, but can contain various combinations of subunits (ten exist in D. melanogaster)
11
Q
Target site mutations in neonicotinoids
A
- Studies in D. melanogaster indicated that loss of the α6 subunit gene conferred resistance to imidacloprid
- However, analysis of target species resistant to neonicotinoids shows that target site mutations are rare
- As of 2015, 220 cases of Bemisia tabaci neonicotinoid resistance had been reported, none involving target site mutations
- Exactly parallel mutations (R81T in subunit β1) have been observed in two aphid species
12
Q
More parallel target site resistance
A
- Like DDT, pyrethroid insecticides (e.g. deltamethrin) bind the voltage-gated sodium channel required in neurotransmission
- Target site mutations in the para gene (which encodes the α subunit of the channel) are known from several species; in particular, M918T (super kdr) and L1014F (kdr) mutations have arisen independently at least six times
13
Q
Reducing insecticide uptake
A
- CYP4G enzymes have been observed to be overexpressed in pyrethroid resistant Anopheles mosquitoes, despite not being expressed in the usual detoxification tissues
- D. melanogaster CYP4G1 is responsible for the synthesis of cuticular hydrocarbons needed for desiccation tolerance
- Anopheles gambiae resistant to deltamethrin have higher levels of cuticular hydrocarbons than susceptible mosquitoes
14
Q
Population genetics of insecticide resistance
A
- Although insect generations are very rapid compared to those of humans, they are much slower than most pathogens
- Lateral gene transfer is far rarer in insects than in bacteria
- Resistance to insecticides may arise more slowly than antimicrobial resistance, but nonetheless is very widespread
- Does resistance persist in the absence of insecticides?
15
Q
Insecticide resistance fitness costs
A
How well fitness costs in the laboratory correlate with fitness costs in the field is not known
* Some evidence for higher mortality during field overwintering in resistant blowflies and mosquitoes
* Mosquitoes resistant due to esterase expression show reduced energy reserves
* Sex differences need to be considered - elevated CYP6G1 appears to increase female but decrease male fertility in D. melanogaster
16
Q
Alternatives to insecticides
A
- Overnight cessation of insecticide use without any alternative control strategies would devastate agriculture and disease control
- What alternatives exist?
- Use of endosymbionts in blocking disease transmission
- Gene drive to reduce population numbers
- Sterile insect technique
- Transgenic plants
- Refuge strategies
- Biological control
- Semiochemical manipulation
17
Q
Sterile insect technique
A
- Conceptually somewhat similar to gene drive - does not require transmissible gene modification, but far more resource-intensive
- Requires mass culture of target species, separation of sexes, and irradiation of males at high enough dose to reliably cause sterility
- Release of irradiated males into target population means most matings unsuccessful (assuming no female preference)
18
Q
Transgenic plants
A
- Bacillus thurigiensis bacteria carry plasmids encoding ‘Cry toxins’; these damage the gut epithelium of various insects, stopping feeding and promoting bacterial infection
- Insertion of cry genes into the genomes of plants can confer resistance to a range of insect pests
- Bt crops are widespread in Australia; cotton expressing Cry1Ac and Cry2Ab for protection against Helicoverpa moths is nearly ubiquitous
19
Q
Refuge strategies
A
- Providing ‘refuges’ (nearby host plants not expressing Cry toxins) is commonly used in the hope of reducing Bt resistance levels
- A supply of susceptible mates in refuges means:
- Mostly heterozygous progeny (susceptible to insecticides if resistance is recessive)
- Fewer eggs laid on crop plants
- Evidence for refuges slowing, but not entirely preventing, resistance development (especially where dominant resistant alleles arise)
20
Q
Biological control - failures and successes
A
- Cane toads were introduced to Australia in the 1930s in a largely unsuccessful attempt to control
the native beetle , a pest of sugar cane - However, many insects have been successfully controlled using natural predators
- Outbreaks of the Australian scale insect Icerya purchasi in other countries have been controlled using Australian ladybirds (Novius cardinalis) or parasitic flies (Cryptochetum iceryae)
21
Q
Semiochemical manipulation
A
- Insects rely on chemical cues for numerous purposes - these may be either attractive or repulsive
- The bird cherry-oat aphids shifts between host plants according to season
- bird cherry in winter, cereals in summer
- Methyl salicylate is key to the aphids’ recognition of bird cherry; applying it to barley in summer reduced colonisation by 50%
22
Q
Integrated pest management
A
- Insecticides are likely to remain important into the future, but there is increasing emphasis on them as one tool among many
- Possible interactions between different approaches should also be considered:
- Plant defensive chemicals can affect the attractiveness of sex pheromones
- Signals such as alarm pheromones can be used to attract predators
