Two generations of the cabbage root fly occurred annually,
the main periods of egg laying being from the last week of May
till the beginning of July, and from the third week of July till
the end of August. The population of the first generation was
higher and so more damaging to the crops.
Predation on the egg stage by Carabid and Staphylinid beetles
was confirmed to be the most important biological controlling
factor of the pest. Bembidion app., T. obtusus, Atheta spp. and
0. rugosus were the principal predators.
In both years predation by beetles reduced significantly
(P = 0.01) the cabbage root fly egg population in the field. The
reduction in the egg numbers of the root fly first generation due
to predation was calculated to be 1l)I and 31 per cent in 1965 and
Diazinon and chlorfenvinphos granules were applied around the
base of cabbage plants on and under the soil surface. Both the
diazinon treatments reduced significantly (P = 0.01) the number
of Carabid predators in the field compared with the control.
These losses were significantly reduced (P = 0.05) in the subsurface treatment compared with the surface treatment.
The Staphylinids in the surface treatment were significantly
reduced (P = 0.05) compared with both the control and the subsurface treatments, the difference between the sub-surface and
the control was non-significant.
The Carabids were found to be remarkably tolerant to the
chlorfenvinphos insecticide. The number of Carabids captured in
the treated plots did not differ from that of the control. On
the other hand the Staphylinids in the surface treatment were
significantly reduced at 1% level, and in the sub- surface treatment at 5, level compared with the controls. The difference
between the surface and the sub- surface treatments was also
significant at 56 level.
Despite the reduction in beetle population by the
insecticide applications, in both years the predation on the egg
stage of the cabbage root fly was not significantly affected by
51 per cent. parasitism by I. rapae and 13 per cent. by
A. bilineata were recorded in 1964. This was reduced to 19 per
cent. all by I. rapae in the same period in 1965 and 17 per cent.
in 1966. This reduction could be due to the use of insecticides
in the field.
Diazinon gave a 100 per cent. control of the cabbage root fly
over a period of about 11 weeks, and gave very little protection
thereafter. The chlorfenvinphos was extremely active against the
pest giving a 100 per cent. control throughout the season.
Weather conditions were noticed to have a marked effect on
the rate of oviposition by the cabbage root fly. Persistently
strong wind caused a large reduction in egg -laying in the field.