60 INSECT PESTS 



to the resistant stage, and the next lot of turnips should 

 be sown at a distance from the site of the previous one. 



Last, but by no means least, there are the moths, of 

 which two, a very small and a fairly large one, will suffice. 



The dainty little Diamond Back Moth {PhUella macu- 

 lipennis) has sometimes been a source of anxiety to the 

 farmer with regard to the turnip crop, and its hordes of 

 caterpillars have been known to devastate whole fields 

 of cabbage, rape and swedes. The insect is about ^ inch 

 long, light brown in colour, with the three white " dia- 

 monds " as shown in the sketch, and the first brood 

 hatches out from pupae which have wintered on charlock, 

 hedge mustard and similar weeds. The females lay their 

 eggs on the undersides of the leaves, and there may be 

 two or more broods in a season. The larvae are green 

 and I inch long when full-grown. The pupa is contained 

 within a pale yellow silken cocoon. Soot and lime 

 dusted on the plants will check these insects, or alter- 

 natively they may be sprayed with carbolic acid and 

 paraffin emulsion. (See Plate 10.) 



The Diamond-Back Moth, however, has many natural 

 enemies among the birds ; starUngs, peewits, plovers and 

 gulls helping to keep them down. Also it has an interest- 

 ing foe among the insects in the shape of a small ichneu- 

 mon fly called Limneria gracilis. This tiny yet beautiful 

 creature, like the rest of its kind, does, unnoticed, work 

 of the highest value. A larva, well grown, for instance, 

 may be feeding away unconcernedly, thinking of nothing 

 but the food in front of it, when our dainty lady ichneu- 

 mon, with glancing gossamer wings, hovers silently above, 

 her ovipositor of needle-like sharpness " trained for 

 action." She ahghts swiftly, and in a single moment 

 has driven it straight into the caterpillar's skin, making 

 off at once without more ado to carry on the good work 

 elsewhere. The caterpillar may start, or twitch itself 

 round, as at a pin-prick, and go on feeding as before, 

 but henceforward it is doomed, for an egg was deftly 



