Why did the population of pests in Area A increase in 1978 when, in 1977, it appeared that the pest had been almost completely eliminated

Introduction a pest from other regions or formation of resistance due to regular application of same pesticide.

There may be two reasons for their presence, first it may be introduced from surrounding regions of that area where no pesticides are used for their elimination. This introduction of pest through human or animals entering the area.

Secondly, there may be some of pests survive due to gain resistance against the pesticide which increases their population in the next year so we can conclude that introduction a pest from other regions or formation of resistance due to regular application of same pesticide.

Learn more about population of pest here: link

Learn more: link

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