8.24 



Aphides, or Plant Lice. 



[JAN, 



may take place to a number of quite unrelated species of 

 plants. 



Aphides damage plants in two ways : (i) by sucking away 

 the sap and so weakening the plant, and (2) by their excrement 

 falling on the leaves and clogging the stomata and so interfer- 

 ing with gaseous interchange. Further, the excrement, con- 

 sisting partly of a sweet gummy substance called " honey- 

 dew," is a favourable germinating medium for the spores of 

 some fungi, and spoiled sooty-coloured patches show on twig, 

 leaf, and fruit. 



Natural Checks. — Several insects prey upon Aphides, and 

 should be encouraged. The chief of these are Ladybirds and 

 their larvae (Coccinellidce) ; Hover-fly larvae (Syr phidce) ; and 

 the larvae of the Lace-wing Flies , (Chry sop idee). Various 

 minute Hymenopterous parasites (Chalcididce) lay their eggs 

 in the bodies of Aphides, those parasitised being destroyed. 

 Man cannot, however, rely solely on the services of these 

 beneficial insects, but should check the increase of the Aphides 

 by washes as soon as they appear upon his plants. 



The Bean Aphis (Aphis rumicis, Linn.). — This Aphis, 

 known variously as Black Fly, Collier, and Black Dolphin, is 

 in some years a very severe enemy of the Bean Crop, shoots 

 and pods being quite smothered with the insects. Harm 

 results not only from the weakening of the plant by the drain- 

 ing away of the sap, but also from the masses of excrement 

 which cover and clog the outside of the plant. Important 

 points in the biology of this Aphis are : (1) It does not confine 

 itself to the Bean Crop, but is found on many other plants in 

 widely separated Natural Orders, examples being docks, 

 thistles, furze; (2) There are regular migrations to these 

 plants and back to the bean again ; and (3) Various generations 

 with somewhat dissimilar individuals are found at different 

 stages in the life-history. The following are recognisable : — 

 (a) The early wingless female, on the bean, black in colour 

 with ochreous tints on the shanks and the middle joints of the 

 antennae. This female produces live young; the young 

 are slate-grey to black. At certain times a stage is found 

 when the insects show dusky wing-cases or wing rudiments, 

 and the abdomen is black, with white spots; this is the stage 



