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-infesting alatae.
The sexuparae begin appearing on lettuce from about the last week in August. They generally come up to the soil surface and settle around the 'collar' of the host plant, to mature and develop wings. Having flown back to the primary host, the sexuparae seek out crevices in the bark where they produce arostrate sexuales. The ovipara contains one egg, and after mating she crawls as far into the bark crevice as possible and oviposits.