062 IOWA DEPARTMENT OF AGRICULTURE. 



METHODS OF COMBATING INJURIOUS INSECTS OF FARM, GAR- 

 DEN AND ORCHARD. 



S. W. Snider, Center Pomt, Iowa, Before North Linn County Farmers' 



Institute. 



All organic nature is extremely complex. There is a definite relation- 

 ship between the various species of animals and plants, a relationship 

 as yet too little comprehended by man. In the constant struggle, in- 

 dividuals of one species destroy individuals of another and are in turn 

 destroyed by still others. Nature tries to keep peace by allowing some 

 of all to survive. She is always trying to maintain an equilibrium, and, 

 if undisturbed, succeeds fairly well. But the equilibrium is so delicate 

 that the introduction or destruction of a single species may destroy it. 

 If none of them were destroyed, the progeny of a single female aphid or 

 plant louse, such as is often found upon apple twigs, in a single season 

 would become numerous enough to completely cover a strip two hun- 

 dred miles long and eleven and one-half feet wide. But as long as an 

 equilibrium is maintained there can be no such rapid increase. A host 

 of natural enemies feed upon the aphids and are instrumental in reduc- 

 ing their numbers. The natural enemies of this insect are largely other 

 insects; these are preyed upon or parasitized by other natural enemies 

 and perhaps they in turn by still others. To a greater or less extent the 

 same is true of most injurious insects. The old dogma, "Big fleas have 

 little fleas upon their backs to bite 'em and little fleas have lesser fleas 

 and so on ad infinitum," becomes applicable to almost any insect. In- 

 sects affecting cultivated plants usually have a food supply which ia 

 practically unlimited so that their natural enemies are the chief factor 

 in determining their abundance. These natural enemies include a large 

 number of species of parasitic and predaceous insects, fungous diseases, 

 birds and other small animals. Many of these "will feed upon but one 

 species of insect, consequently food supply is the chief factor in controll- 

 ing their abundance. When something happens to reduce its natural 

 enemies, an insect becomes unusually abundant for a time. Then its 

 enemies have an unlimited food supply and increases rapidly, soon be- 

 coming so numerous that the host insect disappears suddenly and we 

 wonder what became of them. 



The parasites then die for want of food and the host insect again 

 increases. Theoretically, this explains the unusual abundance of certain 

 insects at times and their scarcity at other times; practically other 

 factors may also enter in. There is now less danger than formerly of 

 new injurious insects being imported, because of the greater care taken 

 to prevent it. Many of our insect pests are native of the United States. 

 Some notable exceptions are San Jose scale, the Gypsy moth, codling 



