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SELECTIVE USE OF PESTICIDES

(Contacts)

 

Selective Pesticides.

 

A. "Selectivity" defines the capacity of a pesticide to spare natural enemies while destroying their pest host.

 

B. Two types of selectivity:

 

     1. physical: arises from differential exposure of pests and natural enemies to a pesticide.

 

     2. physiological: arises from a differential inherent susceptibility on the part of the pest and its natural enemies to a

            pesticide.

 

I.  Factors that can determine physical selectivity.

 

                A. Preservation of natural enemy reservoirs during treatment, either within treated areas or within easy migrational distances from

                       them.

     1. maintain adjoining untreated crop areas or stands of untreated alternate host plants.

 

     2. recolonizing treated areas with mass-reared natural enemies.

 

     3. staggering chemical treatments of portions of large plantings.

 

     4. employing spot or strip treatments of chemicals.

 

B. Timing pesticide treatments to allow for the differential susceptibility and seasonal occurrence of the various developmental

       stages of natural enemies.

 

     1. the pupal and prepupal stages of parasitoids are relatively immune to pesticides.

 

     2. the eggs of many predators are laid in protected spots or are otherwise inherently unsusceptible.

 

     3. adult parasitoids and predators are generally the most susceptible stages.

 

C. Physical selectivity may also be conferred by the feeding habits of various natural enemies.

 

     1. internal parasitoid larvae are protected within their hosts from contact poisons.

 

     2. adult entomophagous insects vary in susceptibility to stomach poisons in relation to their propensity to ingest insecticide

           contaminated hosts, plant exudates or honeydew.

 

D. Physical selectivity also can be conferred by manipulating the dosage and persistence of pesticides.

 

II.  Physiological selectivity is conferred by a pesticide that is more toxic to a pest species than to its natural

             enemies. But, unfortunately, the reverse is usually true.  

                      

 

A. A few pesticides have been developed that are fairly specific against certain groups or species of arthropods.

 

B. Physiological selectivity is a costly achievement. The costs involved in the research and development of pesticides are

     tremendous, well in the range of 20-40 million dollars per compound. If more of the highly specific pesticides are to be

developed for integrated control, something probably will have to be done to offset those tremendous developmental costs to 

      industry, for obviously the marketing potentials of selective and specific pesticides are much less than those of broad-

      spectrum compounds.

 

C. To make matters worse for industry, successful integrated control programs have resulted in smaller demands for pesticides

       and a reduced demand for broad-spectrum compounds. The continuation of this trend could deter industry from trying to

       find additional specific compounds with limited market potentials.