Pa PC 1 6 



BETTER FRUIT 



March 



Peculiarities of the Destructive Plant Lice, or Aphids 



By Dr. A. L. Melander, Professor of Entomology, Washingrton State College, Pullman 



DESTRUCTIVI-: though they :iro, 

 the I^lant Lice, or Aphids, show 

 so many peculiarities in the 

 course of their lives that we cannot 

 help but marvel at their nature. 

 Throughout the sunmier generation 

 after generation is born alive from 

 virgin females. To the student of 

 heredity this phenomenon is more than 

 interesting, for the successive genera- 

 tions are often widely unlike. Some- 

 times winged individuals are born of 

 wingless mothers, and they in turn 

 produce wingless offspring. Sometimes 

 the color differs, sometimes the struc- 

 ture. From a practical standpoint the 

 aphids are further remarkable in that 

 some species change their diet during 

 this alternation of generations, the 

 summer broods ac(|uiring food habits 

 totally different from those for the re- 

 mainder of the \ear. 



Aphids breed rapidly. In a few 

 weeks they are mature and then, except 

 for the sexed final generation of the 

 year, they reproduce living young with- 

 out the necessity of mating and fer- 

 tilization. This rapidity of develop- 

 ment explains why aphids frecpiently 

 become abundant to excess despite the 

 usual sprayings. Huxley, the English 

 scientist, once calculated that were a 

 single plant louse to repro(Uice her full 

 number of olTspring and were they 

 and all their descendents to live to old 



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age the amount of aphids resulting in 

 one season would weigh ten billion 

 pounds. This has been translated as 

 about eiiuivalcnf to the weight of the 

 entire population of the L'nited Slates. 

 That such staggering amounts of jdant 

 lice fail to appear is due to unnatural 

 death, conung not only through the 

 agency of tiny wasp parasites, of lady- 

 bird beetles or of Syrjihus fly maggots, 

 all of which greedily i)rey on plant 

 lice, but also through such causes as 

 wind and rain. However, it is sufTi- 

 cient to say that aphids are prolific. 



The curious alternation of genera- 

 tions fliat occurs through the year can 

 be illustrated by the life cycle of the 

 (ireen Apple Aphis. The winter eggs, 

 which are commonly located on the ex- 

 posed bark of watersjjrouts and of the 

 new growth, hatch when the buds are 

 swelling. The tiny emerging lice are 

 rather dark green in color and soon 

 work into the opening buds. These in- 

 dividuals are all females, but arc 

 cajjable of reproducing the species by 

 themselves, a ])henomenon known as 

 parthenogenesis, which is a Greek word 

 meaning "virgin's birth." They are all 

 wingless when mature, never lay eggs, 

 but bring forth from three to twelve 

 living young a day. When about a 

 huntlred young have been born the 

 stem mother dies. 



The lice of the second generation are 

 paler in coloi- than their parent. They, 

 too, are parthenogenctic, vivii)arous 

 females, and except for a rare indi- 

 vidual now and then are likewise 

 devoid of wings. Their offspring, the 

 thiril generation, usualh develop wings. 



in which case they pass dining tlieir 

 growth through a sort of pupa stage 

 chai acterized by the possession of small 

 wing-pads. This generation, whether 

 winged oi' not, still comprises iiartbeno- 

 genetic, viviparous females. Winged 

 indivifluals appear in diminishing num- 

 bers after midsunnner. Their purpose 

 in life is ol)viously to spread the species 

 from tree to tree, and hence they are 

 called "ndgrants." 



The Green Apple Aphis spends its 

 entire existence on apple trees, or more 

 rarely on pear, hawthorn, quince or 

 flowering crab. In the late fall some 

 small lice are born, of a more yellow- 

 ish color than the sunnner generations. 

 'Ihese are the wingless males and fe- 

 males, the only sexed individuals of the 

 year, .\ftcr mating the males perish, 

 while the females crawl out on the 

 twigs to deposit fheii- single egg and to 

 die. Thus are produced the only eggs 

 of the year whose dormancy exhibits 

 Nature's splendid adaptation to tide a 

 delicate insect through the rigors of 

 winter. 



Closely related to the Green Apple 

 Aphis is the Rosy .\pple .\phis. Like 

 the other, this species also lives in the 

 leaves, causing them to curl by inject- 

 ing a poison into the growing leaf- 

 tissue, l3ut unlike the Green Aphis the 

 Rosy Aphis confines its attacks mainly 

 to the leaves around blossom clusters. 

 The poison injected is very subtile and 

 aifects the young fruit, inhibiting its 

 growth so that the tree develops a crop 

 of "gall-apples." This stunted fruit 

 varies in size according to the extent 

 Continued on page 41 



STEM MOTHER 



a GENERATION 



J 7 A 



THE GREEN APPLE APHIS 



APHIS 



POM) 



