36 BULLETIN 965, U. S. DEPAKTMENT OF AGRICULTURE. 



winter colonies in the spring, their instinct to multiply leads to an 

 aggressive search for food at a time when the supply is scarce. Con- 

 sequently, they feed ravenously at this time on ant sirup when offered. 

 It is also during the spring and autumn that the night temperature 

 approaches 50° F., which reduces ant activity, causes the ants to 

 congregate in or about tree crotches of the main limbs, and greatly 

 increases feeding, particularly in the case of trees infested with 

 honeydew-producing insects. During the hot summer months, 

 when the temperature seldom drops to 50° F., ants are active both 

 night and day, and where other food is available they are not greatly 

 attracted by the poisoned sirup. 



This temperature influence, and the further important considera- 

 tion that scale control by fumigation is confined largely to the autumn 

 months, make the fall and spring desirable periods for poisoning the 

 Argentine ant. The writers recommend that control be started 

 between the time ants first appear in the spring and July 1, or from 

 the last of September until congregation into winter colonies takes 

 place. The time of the appearance of ants in the spring varies 

 from year to year, as well as from locality to locality. In 1916 ant 

 activity started the last week in February, whereas in 1920 it was 

 about the middle of April before the winter colonies began to break 

 up. With the approach of cold weather, which normally sets in 

 during December, ants disappear from the trees, although in a very 

 open winter, as that of 1917, complete winter colonizing does not 

 occur. 



Reappearance of Other Ants Following Control of the Argentine Species. 



It has been stated^ by Newell that the Argentine ant will not tolerate 

 the presence of other species of ants within its domains. In southern 

 California the commonest species of ant in citrus groves not entirely 

 overrun with the Argentine ant is Prenolepis imparis Say. Formica 

 cinerea Mayr, var. pilicornis Emery ranks second, and Tapinoma 

 sessile Say, Dorymyrmex pyramicus Roger, and Cremastogaster 

 lineolata Say, var. californica Emery are of lesser importance. 

 Orchards long infested with Argentine ants are entirely freed 

 of these other species. The reappearance of Prenolepis imparis 

 closely following the eradication of the Argentine ant has often been 

 noted, particularly during the autumn and early winter months. 

 One striking instance occurred in 1918 at Pasadena, where ant 

 control was started in April on 258 trees, 4 of which were frequented 

 with Prenolepis. These four infestations of Prenolepis were eradi- 

 cated, and the Argentines on the other trees were brought under 

 control during the summer and totally eradicated during the month 

 of November, with the exception of a few trees in one corner. On 

 October 17 Prenolepis reappeared on 1 marginal tree and by Novem- 



