October 19, 1917] 



SCIENCE 



391 



As the spring season advanced several 

 species of ants began visiting this bed. Later, 

 colonies of aphis were found upon the roots of 

 the corn, and finally heavy infestations upon 

 the upper parts of a number of the corn plants. 

 During four months of almost daily scrutiny 

 no aphis was ever discovered upon either the 

 teosinte or the hybrids. Ants were noticed 

 occasionally upon these plants but their visits 

 were apparently fruitless. A dozen or more 

 hills of teosinte had been grown in 1909 and 

 again in 1910 in cornfields heavily infested 

 with aphis, but none had been noticed on the 

 teosinte, although no particular attention was 

 given to this question at the time. 



The colonies of aphis in the greenhouse 

 were sprayed effectively at intervals, but new 

 colonies again appeared on the corn. It is 

 well known that corn plant aphis, when not in 

 the winged state of metamorphosis, frequently 

 depend upon ants for transportation over 

 short distances, e. g., from one plant to a 

 neighboring plant, as well as from one region 

 to another on the same plant. The ants act 

 as herders and protectors of the aphis, taking 

 their toll in the sweet sticky fluid secreted by 

 the aphis. The aphis are moved to a new 

 feeding spot or pasture when the supply of 

 fluid is not satisfactory to their herders. 



The occasional appearance of ants searching 

 over the teosinte and hybrid plants, indicated 

 that the ants imdoubtedly were willing and 

 perhaps did perform their share of the com- 

 pact, and that the aphis were unable to sub- 

 sist upon the tissue of these host plants. 

 Aphis in the winged stage, probably, lodged 

 many times upon the immune plants. 



It was learned that there were two recog- 

 nizd forms of aphis involved in the problem, 

 namely, the corn root-aphis. Aphis maidi- 

 radicis, and the corn plant-aphis. Aphis maidis; 

 one working only or almost entirely on the 

 roots and the other on the culm of the corn 

 plant. The former is more numerous and 

 destructive in corn fields. 



The attacks of the sap-sucking aphids do not 

 produce disease other than depleted plant 

 tissue and local lesion in the area upon which 



they are at work. A portion of the insect 

 secretions is waste — popularly known as 

 " honeydew " — and produces the characteristic 

 sticky, gummed and soiled surface where the 

 aphis and the ants have been operating. 



Just as there is no important grape-growing 

 region free from the devastating woolly aphis 

 or phylloxera of the vine; so, also, there is 

 probably no corn-growing region of impor- 

 tance, in North America at least, which is 

 free from the root-aphis of corn. There is no 

 way of estimating accurately the enormous 

 loss in reduction of yield caused by these in- 

 sects which are steadily increasing in num- 

 bers and extent of migration, but this loss is 

 known to be very great indeed. In Central 

 Illinois the damage by the corn root-aphis 

 sometimes causes total failure of the crop in 

 limited areas. 



Careful cultural methods may reduce con- 

 siderably, for the time being, the number of 

 aphis in a cornfield; yet the field may become 

 reinfested from surrounding, untreated fields 

 of corn and from other plants upon which the 

 aphis are known to subsist. 



Forbes reports finding from eleven to twenty- 

 two generations of corn root-lice in one season. 

 He estimates three hundred and nineteen bil- 

 lion lice and three trillion eggs left in the 

 groimd at the end of the season for each louse 

 hatched in the spring. These figures are based 

 upon the average rate of production with no 

 undue break in the cycle. 



It is this high rate of multiplication by a num- 

 ber of successive generations which makes the root 

 lice so destructive, even in fields first entered by a 

 few winged lice borne on the wind from some 

 neighboring field which has become more or less 

 overstocked. 



We may conclude that improved cultural 

 methods and other common treatment will, at 

 best, protect the cornfield only for a short in- 

 terval. A more effectual remedy and per- 

 manent solution of the problem is to be wel- 

 comed. The amount of work connected with 

 a thorough investigation of this discovery 

 would have been far greater than the writer 

 was able to undertake personally in connection 

 with his other duties. 



