January, 1919 



THE GARDEN MAGAZINE 



181 



Facts About the Iris Borer 



LOOKING over the November Garden 

 Magazine I read with interest Miss Gil- 

 bert's note on an Iris pest, but if her ob- 

 servations are of the Iri's borer, Macro- 

 noctua onusta, they differ in some respects from 

 mine, inasmuch as I have rarely, if ever, seen a 

 stalk attacked directly, though sometimes one is 

 undermined. The first sign is ordinarily a small 

 round translucent or oily looking spot halfway up 

 on an inner leaf of the sheaf, where the larva com- 

 mences its life work of eating. From this spot 

 it goes downward eating the tenderest parts of 

 the leaves and leaving a slimy trail behind, 

 when near the base so much has been eaten that 

 a slight pull will bring the central leaves away 

 from the outer ones and you find a dingy white 

 larva concealed in them, or the hole into the 

 rhizome, in which case a hatpin is more effectual 

 than a knife in removing it. The whole rhizome 

 may be hollowed out by the borer and if it prove 

 too small it will enter a contiguous one before 

 crawling into the soil to pupate. At the end of 

 this stage, the eating stage of its life, it is a big 

 fat larva, often if inches long, dirty white with 

 more or less of a pink tinge on the back and with 

 a brown face. It buries itself three or four inches 

 from its burrow and about an inch deep in the 

 dirt to pupate. The chrysalis is rather lively 

 for a pupa, is a shining bright brown and about 

 an inch long; from it emerges the moth, about 

 2 inches from tip to tip of the wings when 

 mounted. It flies at night and is a dull velvety 

 brown with lighter areas of a pinkish tan so that 

 it is well camouflaged and seldom is seen. The 

 under surface of the wings look as if they had 

 been singed. The eggs are deposited on the 

 rhizome or on the base of the old leaves. I 

 agree with Miss Gilbert that the remedy is hand 

 picking but I should begin much earlier in its 

 career. It can be killed by pressure when stilly 

 in the leaf. As the signs are unmistakable when 

 once you learn them I could easily keep in check 

 this foe if I had but a few dozen plants to inspect 

 frequently, but as the eggs begin to hatch during 

 the blooming season when there are many other 

 things to be done, and continue over a long period, 

 many of the larvae get full grown among the 

 thousands of my plants, so in September I take 

 up any plant that does not look thrifty and search 

 carefully for borers and pupae. In my garden 

 they are most numerous in shady places, where 

 I imagine the moths find more protection, but 

 in the field plants it is just chance and where 

 there is one egg laid there is likely to be more. 



The borer does not touch Bulbous Irises, but 

 the Siberian, Japanese and Other Apogons are 

 not immune and the pest is far more difficult 

 to eradicate than from among the Bearded irises. 



I am not much troubled by them but I know 

 -of one grower who spreads leaves from the street 

 trees on his Apogons and burns them over in 

 the late fall so as to destroy the eggs. Where 

 a clump is badly infected carbon bi-sulphide 

 can be applied; that, burning over, and lifting 

 and cleaning the plants, are the only effective 

 remedies known to me. If you strip off the old 

 stalks and leaves to keep the garden neat a good 

 many will be removed. But Macronoctua onusta 

 does not confine its depredations to iris in my 

 garden; I have not been able to keep large clumps 

 of Lupins, Columbines, or Bleeding Heart in 

 good condition for a number of years, in fact, in 

 looking back I realize that I lost my pet plants 

 of Aquilegia chrysantha long before I found any 

 trouble in the Iris and I think in my repeated 

 attempts to have a showy clump of Dicentra 

 spectabilis I may have introduced it in the large 

 plants that I bought. 

 Wellesley Farms, Mass. Grace Sturtevant. 



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