453 



various species resorting to it. It may be said in general terms, how- 

 ever, that when the life history of a food plant or the common history of 

 a group of such plants exhibits sufficiently constant characters to serve 

 as an adaptive matrix, an adaptation to it of the life history of those in- 

 sects strictly or mainly dependent on it for food is more or less likely tu 

 follow. 



Mutual Biographical Adjustments of Competitors 



An example of the competitive relations into which corn insects of 

 widely different character, origin, habit and life history may be brought 

 by their dependence on the same food plant may be found in Diabrotica 

 loiigiconiis and Af'liis iiiaidiradicis. Both pass the winter as eggs in the 

 earth of the corn field, the aphis hatching sooner than the root-worm and 

 developing two or more of its short-lived generations before the Dia- 

 brotica larva is out of the egg. gaining thus the advantage of an earlier 

 attack in greater numbers. It is also able to take much more rapid pos- 

 session of a held of corn because of its command of the services of ants 

 in finding its way to the roots of the plants which ihe tiny and feeble 

 Diabrotica larva must search out for itself. 



Later the root-aphis gives origin to young, many of which acquire 

 wings and may thus disperse as their local attack upon the plant becomes 

 unduly heavy, while the root-worm must take its chances for the year 

 in the field where the eggs were left the previous fall. The aphis fet'ds 

 at first on the sap of young weeds common in spring in all cultivated 

 fields, and may thus save itself even though the ground is planted to 

 wheat, or oats, an event which causes the death by starvation of every 

 root-worm hatching from the egg. 



In respect to rate of multiplication, the root-aphis has of course a 

 truly enormous advantage as compared with the corn root-worm, and yet. 

 notwithstanding all these facts favorable to the aphis, its injuries to corn 

 in Illinois are seemingly no greater than those done by the corn root- 

 worm. This is due partly to the fact that, through the winged members 

 of the early generations, the percentage of which increases as conditions 

 become locally less favorable, the aphis largely leaves the field in which 

 it originally started and early breaks the force of its attack by a general 

 distribution of it. The depredations of the root-worm, on the other 

 hand, increase with the growth of the insect until about Se])tember first, 

 and increase also at a rapid rate from year to year in a field kept continu- 

 ously in corn. It follows as a consequence that the principal damage by 

 Aphis maidiradicis is done to the corn while it is young, and that by Dia- 

 l)rotica to the well-grown plant. 



This serial order of injuries to the corn plant, due to the relation of 

 the life histories and rates of multiplication of these two competing in- 

 sects, is an advantage to both of them and. indeed, to all three, corn in- 

 cluded, since the plant would be more seriously injured or more certainly 

 t'.estroyed if both its insect enemies attacked it together than it is where 



