LECTURE 2 Flashcards
factors affecting agrochemical insecticide use
- Insecticide resistance and use-cancellation or de-registration of some insecticides due to human health and environmental concerns.
- Environmental and health impacts of insecticides
Urgent need for NEW effective (bio)pesticides: transgenic plants and recombinant baculoviruses as delivery systems for a variety of insect-selective toxins.
Additional strategic approaches for the development of plant breeding for biocontrol attributes
Need to overcome perceived weaknesses of biopesticides and negative press of GM organisms in the global control of arthropod pests
sterile insect technique
1 The sterile insect technique is an area-wide pest control method that reduces agricultural pest populations by releasing mass-reared sterile insects, which then compete for mates with wild insects.
Contemporary genetics-based technologies use insects that are homozygous for a repressible dominant lethal genetic construct rather than being sterilized by irradiation.
Engineered strains of agricultural pest species, including moths such as the diamond- back moth Plutella xylostella and fruit flies such as the Mediterranean fruit fly Ceratitis capitata, have been developed with lethality that only operates on females.
Transgenic cropsecpressing insecticidal toxins
3 Transgenic crops expressing insecticidal toxins are widely used; the economic benefits of these crops would be lost if toxin resistance spread through the pest population. The primary resistance management method is a high-dose/refuge strategy, requiring toxin- free crops as refuges near the insecticidal crops, as well as toxin doses sufficiently high to kill wild-type insects and insects heterozygous for a resistance allele.
toxin-sensitive engineered males
4 Mass-release of toxin-sensitive engineered males (carrying female-lethal genes), as well as suppressing populations, could substantially delay or reverse the spread of resistance. These transgenic insect technologies could form an effective resistance management strategy.
Bacterial insect biopesticide : Cry toxins from Bacillus thuringiensis Bt
Bacillus thuringiensis Cry toxins have been widely used in the control of insect pests either as spray products or expressed in transgenic crops. These proteins are pore forming toxins with a complex mechanism of action that involves the sequential interaction with several toxin-receptors.
Cry toxins are specific against susceptible larvae and although they are often highly effective, some insect pests are not affected by them or show low susceptibility.
In addition, the development of resistance threatens their effectiveness, so strategies to cope with all these problems are necessary. The activity of Cry toxins can be enhanced by using additional proteins in the bioassay like serine protease inhibitors, chitinases, Cyt toxins, or a fragment of cadherin receptor containing a toxin-binding site. Different modifications performed in the toxin gene such as site directed mutagenesis, introduction of cleavage sites in specific regions of the protein and deletion of small fragments from the amino-terminal region lead to improved toxicity or
overcome resistance, representing interesting alternatives for insect pest control.
Mode of action of Cry toxins:
1, Crystal solubilization in midgut lumen.
2, Protoxin activation and
translocation through the peritrophic membrane.
3, Toxin binding to primary receptor and cleavage of helix α-1.
4, Oligomerization of the toxin and binding to secondary GPI-anchored receptors
5, Toxin insertion into the
membrane and pore formation.
i) Chitinase that improves access of the toxin to the epithelial membrane.
ii) Serine protease inhibitors that reduce degradation of Cry or of toxin-receptors.
iii) Introduction of intramolecular cleavage sites in the toxin that improves binding interaction.
iv) Introduction of more binding sites
CR12-MPE-peptide from cadherin receptor or the Cyt1A toxin.
v) Deletion of helix α-1 that induces toxin-oligomerization and skip
cadherin interaction.
3D structure of Cry1A showing three domains
Roles: Domain I Hydrophobic and amphipathic helices interact with membrane
Domain II Three antiparallel b sheets surface exposed loops for receptor binding
Domain III Tightly packed b sandwich protects C terminus active protein prevents
cleavage
Bt
Bacillis Thuringienisis
Mosquito control measures
- Mosquito development eggs->larvae->pupae happens on standing bodies of water so spray larvicide to the water.
- Adults acquire west nile virus by biting infected birds, so control this by spraying adulticides
Fungal insect biopesticide
a.. female mosquitoes contact fungal spores from treated surfaces as they rest to digest a bloodmeal.
b. Fungal infection progresses mosquitoes die and sporulate, producing a mat of fungal spores on the outside of the cadaver (top right).
c. Infection with the entomopathogenic fungus Beauveria bassiana dramatically reduces survival of Anopheles stephensi mosquitoes by day 14 (the time following an infectious blood feed at which an individual mosquito becomes able to transmit malaria).
d. In addition there appears an interaction with malaria (Plasmodium chabaudi) whereby daily mortality rates accelerate from day 11 in those mosquitoes carrying both fungus and malaria. Mosquitoes infected with the fungus exhibit a significant decline in propensity to blood feed as the disease progresses.
f . Finally, survivorship or development of the malaria parasite inside the mosquito is affected such that even if mosquitoes survive there is less chance of having the infectious stage of the malaria parasite (the sporozoites) in their mouthparts (shows mean (± SEM) proportion of the starting population of mosquitoes. The effect is an 80-fold reduction in potential of mosquitoes to transmit malaria.
The sustainability of chemical and biological interventions AUGMENTATIVE CONTROL
a In their normal life cycle female mosquitoes take a blood meal every 2-4 days and use this to mature sequential batches of eggs (x-axis). Natural mortality is generally high (survivors, y-axis) such that the majority of the reproductive output (vertical arrows) from a population accrues over the first 1-3 feeding/oogenic cycles. Relatively few mosquitoes actually survive long enough (12-14 days) in the field for the malaria parasite to complete its development, migrate to the mosquito mouthparts and get transmitted to a new human host (‘infectious’).
b Exposure to a fast acting insecticide following the first blood meal reduces survivorship and prevents malaria transmission. However, the rapid mortality carries a big fitness cost and creates a substantial selection pressure for development of resistance.
c Relative slow speed of fungal kill helps mitigate selection pressure as infected mosquitoes can still complete the important early oogenic cycles. An isolate that allows a high level of survival (and hence egg production) over the first 7-9 days, for instance, but then causes extensive mortality will still reduce malaria transmission but will impose little selection for resistance
Viral insect Biopesticide
s: Targets include: diamondback moth, codling moth, pine beauty moth, cotton bollworm, Soybean velvet bean caterpillar
BaculovirusesBaculoviruses
Ingested Slower acting Host specific UV sensitive Maintain beneficials Safe for vertebrates and non-target invertebrates No residues Expensive to produce
Chemical insecticides
Contact poisons Fast acting Broad spectrum UV stable Destroy beneficials Many have adverse effects to non-insect species Potential problems with residues Cheaper and easier to produce
Biofungicides and disease supression
Broad spectrum pesticides/herbicides target pest and microbiome often in addition
targeting predators of the pest
Disease suppression natural process and key function of microbiomes
Biological control via consortia offers long-term resilient pest control
Protection can be afforded to crops, livestock, human populations
