Aphids on Your Plum Tree? E EDI ™ M - PAT S CH 



X hntomologist Maine r-xpenment btation 



[Editor's Note: This story identifying the plum aphis with the one on aquatic plants in summer is too interesting to be buried from the 

 general reader in a scientific bulletin where the author put it a year or so ago, after tracing the wonderful life history of the insect concerned. So 

 the marvel of it is rewritten in words that are not technical because of its practical interest to the gardener .] 



WELL, spray promptly, of course. 

 Use kerosene emulsion if you know 

 how to make it so it will not injure 

 the foliage. If not, try some good 

 tobacco extract and apply freely — don't miss 

 one of the pests! Kill 'em quickly — the 

 whole messy sticky colony that the ants are 

 ambling among! If you don't, your plums 

 are doomed (Unless, of course, the syrphid 

 maggots, lady beetles and aphid lions under- 

 take the job for you and save you the expense 

 of the spraying equipment). 



"What are aphids for any way?" "Why 

 to keep the horticulturist from becoming lazy, 



Where does the "black fly" of your plum tree come from 

 each year? Ever notice lice on Water-lilies? 



to furnish the entomologist with a salary, and 

 for the naturalist to tell you a story about." 



Yes, aphids have stories — and this is the 

 tale that belongs with one of the plum feeding 

 species. 



More than a century and a half ago Linne' 

 (or as he is more usually called Linnaeus) 

 found an aphid on a Pond-lily and called 

 it Aphis nymphaeae after the plant it was 

 living on, an old and very good way of nam- 

 ing these insects, for many of them stick 

 remarkably close to a single species of plant 

 or a few nearly related ones. This kind, 

 however, accepts on its bill-of-fare several 

 water plants and not infrequently thrives in 

 the aquarium. 



It was in such a situation that a man, about 

 ten years ago in Ohio, found a colony of these 

 aphids and called them Aphis aquaticus, never 

 suspecting that they had had a name ever 



since 1761. This is not so strange as it may 

 seem and is the same thing that has happened 

 to many of Linnaeus' species since it is not 

 always easy to tell from his descriptions 

 just what insect he had in mind. 



To be sure all insects are remarkable in 

 one way or another, but listen to what the 

 man from Ohio found out about this one. 



"One peculiarity which attracted my at- 

 tention was the ease with which the aphids 

 walked over the surface of the water, or were 

 found half submerged in an attempt to feed 

 on aquatic plants. An accurate examination 

 of the forms showed that on either side of the 

 thorax were located three pairs of wax glands. 

 It is quite clear that this waxy secre- 

 tion would be of the greatest value in keeping 

 the insect from getting wet, as it not only 

 projects out from the body, but is also pow- 

 dered over the entire ventral portion of the 

 thorax. While walking on the water this se- 

 cretion is always in contact with the surface 

 and serves as a float while the insect pushes 

 itself along, moving quite as rapidly as on a dry 

 surface." 



A few years previously an account of the 

 same insect appeared in Science under the 

 title, "A Lacustrine Aphid." 



In 1910 another entomologist found out 

 that Linne' and the man from Ohio had been 

 dealing with the same species and that its 

 name, if written in full, should be Rhopalosi- 

 phum {Aphis) nymphaeae (Linne') {aquaticus 

 Jackson) Davis. 



"DUTjwhat has all this to do with the aphid 

 -*-* on your plum tree? Nothing so far as 

 any one knew until 1915, when an aphid de- 

 tective in the backwoods of Maine fell to 

 puzzling over the species attacking plum in 

 that state. All except one answered to names 

 associated with Prunus, but this one had 

 never been named apparently. That little 

 oversight was easily corrected — down went 

 a name in manuscript. What it was does 

 not matter now for it never got into print. 

 Just before the paper went to press a publica- 

 tion arrived from Russia containing the 

 description of a "new species" on Prunus — 

 Aphis prunorum it was called. It looked, 

 judging from its picture, remarkably like 

 the unnamed plum aphid in Maine. 



But what business had either the Maine 

 species or prunorum from Russia in the genus 

 Aphis ? The swollen cornicles brought it 

 nearer Rhopolosiphum didn't they? 



Perhaps that clue didn't make the aphid 

 detective jump! Why? Well, because there 

 was a common species knocking around on 

 water plants o' summers ever since the time 

 of Linnaeus. It was a migratory species, too, 

 in all probability; for how was it to winter on a 

 succulent water plant? And didn't that 

 species have swollen cornicles and an antenna 

 like the "new species" on plum? 



Out came the microscope, and the aphis from 

 the plum and the aphis from the water plant 

 could not be told apart on their structural char- 

 acters! 



Supposing, O, just suppoing it should turn 

 out that Rhopalosiphum nymphaeae (Linne') 

 migrates to plum in the fall and deposits 

 her overwintering eggs on its twigs and that 

 the species lives on the plum in the spring 

 until it is for it to go to the waterside for the 

 summer months! 



Now it is a difficult matter to follow an 



164 



aphid in its flight and find out where it goes 

 when it leaves something else. But there is one 

 time in the life of a migratory aphid when it 

 can often be made to confess its own secret. 



Potted water plants were brought to the 

 laboratory early in the season, before the 

 aphids were winged on the plum; and was it 

 coincidence that at this time there were no 

 aphids on the Arrowhead? Wait! On June 

 17 there were plenty of migrants taking flight 

 from the plum. A few of these were captured, 

 just as they were about to leave,, and placed 

 on the potted Arrowhead. And the nice little 

 creatures folded their wings and dipped their 

 beaks into the stem of the Arrowhead! 



And five days later that same plant (shown 

 in the accompanying photograph) was crowded 

 with their progeny, *many of them already 

 half grown. If one of them dropped into 

 water it paddled off unconcernedly for the 

 food plant again. They were waxed without 

 and the water did not penetrate. They were, 

 indeed, no other than the "semi-aquatic" — 

 the "lacustrine" aphid. 



CO THE daughters and the granddaughters 

 ^ and the great granddaughters of the aphids 

 that migrated from your plum tree last spring, 

 dwelt on cool water plants for the summer, 

 taking a swim now and then if they happen 

 to drop. And then in the fall a generation 

 with wings appeared and they returned to 

 the plum where the overwintering eggs were 

 left to provide for the messy sticky colony 

 on your plum this spring. 



While you are applying kerosene emulsion 

 or tobacco spray, it may perhaps occur to 

 you that Linnaeus would have been much 

 interested if he had known what you do 

 about that little Pond-lily aphid which hid 

 its secret from mankind for more than one 

 hundred and fifty years. 



Arrowheads and other aquatic plants are often infested 

 like this by plant lice that migrate to the plum in winter 



