THE BIOLOGY OF LETTUCE ROOT APHID

Lettuce root aphid, Pemphigus bursarius L., is a pest which sometimes causes considerable and widespread damage to crops of summer lettuce.Its favourite primary host is Lombardy poplar. On hatching in spring, from eggs that have overwintered on this host, the fundatrices cause the development of hollow, flask‐shaped galls on the leaf petioles. A fundatrix becomes enclosed within a gall, where it matures, and gives rise to between 100 and 250 young. These fundatrigeniae are all potential alatae and on developing wings leave the gall and migrate to lettuce and other secondary hosts; they reproduce rapidly.Lettuce sown between mid‐April and the end of May is most severely attacked, while lettuce sown in July escapes attack. The alternative secondary hosts belong to the Compositae, subfam. Liguliflorae, and although lettuce is preferred by the fundatrigeniae, Lapsana communis L. and Sonchus asper (L.) Hill often have big colonies of Pemphigus bursarius on their roots. The size of the mature apterae varies between hosts and is largest on lettuce and Sonchus asper.All winged alienicolae are sexuparae and fly back to poplar. Thus alternative secondary hosts are not reservoirs for lettuce‐