CABBAGE ROOT FLY
Gardeners find it very difficult to grow brassicas, eg cauliflower and cabbage due to the attacks by the cabbage root fly grub. If you had trouble last year it will be there again this year. The fly is mostly active laying its eggs at the stem of the plant in May, but may appear throughout the year. The eggs hatch and make their way down to the root to feed. The attack usually causes so great a reduction in the roots that the plant collapses especially on a warm day. In dull weather the plant does not grow as robustly as normal.
In the past it was possible to buy chemicals which, when applied in the planting holes near the roots, eventually killed the grub. These have now mainly been withdrawn from sale. We amateurs have now to find other methods of control. Discs may be bought which are put round the stem of the plant at ground level. This very old method is very effective, although more expensive in money and time than the chemical method.
I use polythene, the thicker type used in packing electrical goods, eg radios etc. Cut in strips 1.1/2"-2" and then into squares 1.1/2"-2". These are then double-folded to form a square about 3/4", four layers thick. At the one corner with no cut edges, I snip off the point very slightly with a pair of scissors. When opened up you should have a square with a small hole in the centre. From the middle of the edge on one side cut a slit to the hole.
I use two of these squares to each plant at planting time. Push the polythene along the ground as far as it will go with the stem in the slit. Use the second square on the opposite side of the plant. Place a little soil on the edge of the polythene well clear of the stem to prevent the wind disturbing it. You have now got a plant well protected. The fly will be distracted.
PERMANGANATE OF POTASH - I have recently done a little experimenting. Using permanganate of potash, which is fairly easily obtained from the chemist, I dissolve about one teaspoon in one gallon of water. Permanganate of potash is an irritant, which when applied on lawns infested with worms, causes the worms to come to the surface, so they can be removed. Do this in spring when birds are feeding their young and the blackbirds will be very grateful.
EXPERIMENT - The same principle may also work with the cabbage root fly grub. Applying about one tablespoon of the solution around the stem of the brassica about one week after transplanting and again at 7 day intervals for four weeks or so may deter the fly or irritate the grubs so that they leave the plant. If you try this experiment leave one or two marked plants entirely without treatment as controls to compare results.
The application of the solution may be a problem. One method would be to use a sprayer. Hold the nozzle close to the stem at soil level so that the solution goes directly into the soil where it is needed. I should think that about one tablespoon of the mixture should be adequate at one application. It is a potassium solution so it may also be a plant food. Don't overdo it. This may also have other uses, eg against caterpillars on brassicas, aphids on flower plants and carrot fly control.
If anyone tries out these experiments I should be very grateful to receive your result.