Ent 207                                                                                            April 2002

During many previous wars disease often killed or incapacitated large numbers of troops; many diseases carried and spread by insects and their relatives.

Efforts to reduce the losses led to research for chemicals that could be used against insect pests (vectors) during WW II.

- testing chemicals off the shelf.
- screening identifies 1 in 10,000 kills insects -> potential insecticide.

Geigy Chemical Co.

e.g. DDT -  killed numerous insect species.
- determined - low toxicity to humans.
- immediate use in war - mosquitoes, fleas, lice.

- success led to:
- change in attitude.
- applied chemistry to control insect pests.
- management of pest -> ERADICATION.

- deficiencies in this approach based on:

1.  ecological grounds: unrealistic
2.  chemical grounds: nerve poisons
3.  evolutionary grounds: selection

1927-1952:

growth of number of papers on insecticide testing           J. Econ. Ent.
by 1952: ca. 60% of all papers.

led to prediction to resistance
not a new observation                                             

1946: house fly resistance                           DDT
selected resistant population

currently >500 resistant to one insecticide.
= populations in discrete distributions, not entire species.

answer to resistance - spray more/develop new insecticides

->"pesticide treadmill"

Resistance Categories

1) Cross Resistance
2) Multiple Resistance
3) Duplicate Resistance

1) Cross Resistance

-development of resistance to one insecticide leads to resistance  to some other insecticide (different categories-similar mechansims).

e.g. DDT - tends to lead to pyrethroids

2) Multiple Resistance

-resistant after treatment by several types of insecticides.
-implies different mechanisms as well

e.g. Spodoptera in Africa; Colorado potato beetle in N.A.

3) Duplicate Resistance

-more than one mechanism giving resistance to an insecticide

e.g. housefly
SKA strain

INSECTICIDES

- most widely used technique to reduce insect pest populations.

Advantages:

- properly applied at the right time causes rapid death to most insects.
- broad spectrum effects.
- exert a density independent effect.
- well suited to mechanized agriculture.
- relatively inexpensive.
- return on investment on average $4 per $1 invested.

Classification

Inorganics

- lack carbon.
- most metallic compounds or salts.
e.g. sulphur, copper, arsenic or lead.
- some apparently used several thousand years ago.
- increased frequency of use in 19th and early 20th centuries.

mode of action

- stomach poisons - limited use to chewing insects.

e.g. lead arsenate

- formulated for use against gypsy moth.
- became widely used in orchards.
- early cases of resistance (codling moth and leafrollers).
- phytotoxic to trees; replacement trees grew poorly.
- remained in soils for long periods.
- most use dropped with the development of organic insecticides.
- sulphur-based compounds still in use.

Organics

      A. botanicals - derived from plants  (and B. synthetics).

- plants contain numerous "secondary" compounds that deter or destroy insects.
- although derived from plants ("natural")  - safer than synthetic compounds??

mode of action:

- variable - depends on the compound.

e.g. nicotine sulphate (alkaloid - caffeine, etc)

- tobacco extract used as early as 1690.
- very toxic to insects (also to humans).
- interferes with nerve transmission.

pyrethrum (mixture of two pyrethrins).

- extracted from chrysanthemums.
- had rapid "knockdown" of insects; some insects recovered.
- low mammalian toxicity (acid environment) and degrades rapidly.
- derivatives synthesized - 100X cost of equal amount of DDT.
- interferes with nerve transmission.

azadiractins

- extracted from neem tree (Azadirachta indica).
- low toxicity for non-target arthropods and vertebrates.
- degrades rapidly.
- deters insect feeding - phytophagous insects.
- also interferes with growth, development, reproduction and oviposition.

B. synthetics

- includes organochlorines, organophophates, carbamates and pyrethroids are major groups.
- different types of molecules, different modes of action and very different levels of toxicity.

insecticide

- implies selective toxicity for insects.

- not always selective, and can be highly toxic to mammals.

Acute oral LD₅₀  (rats)

Mode of Action

- stomach, muscle, nerve poisons**.
- insecticide outside animal.
- most are lipid soluble.
- many are contact poisons - insect comes in contact with insecticide uptake through body surface (multiple points of entry trachea? digestive system).

See your sketch

1. insecticide enters waxy layer - mobility - spreads over surface.
2. enters and moves through tracheal system.
3. may be ingested and enter through digestive system.

- once internal possible fates:

i. degraded/detoxified  -  mixed-function oxidases (MFOs)
ii. excreted via Malpighian tubules.
iii. absorbed/bound to macromolecules in blood.
iv. carried to and penetrate blood-brain (nervous system) barrier.

Insecticides have been chemically manipulated to suit a particular need:

 1. broad spectrum - parathion and relatives.
2. selective (kills flies and leaves natural enemies)- trichlorfon (Dylox).
3. systemic in plants - imidacloprid (Gaucho) dimethoate (Cygon).
4. systemic in animals - fenthion.
5. short life - TEPP (<24 hrs).
6. persistant - azinphosmethyl (Gusathion).
7. low mammalian toxicity - malathion (head and body lice).