White Grub Damage in Soyas
White Grubs have been found to be damaging soyabeans crops in the Harare West area. A soya crop at ART Farm is one of those showing symptoms of attack. The pattern of attack is roughly circular patches of stunted or dried out plants which might look initially like a disease problem. However, the prevailing dry conditions have not favoured disease. Plants dug up in the problem patches were infested with White Grubs, which were found at about 80 mm soil depth. There could be up to about 16 white grubs in about 2 litres of soil, which is quite a large population. White grubs are more commonly found on maize, where the adult beetles may also feed on the crop leaves.
White grubs are the larvae of chafer beetles, a type of scarab beetles. They are “C” – shaped beetle larvae with a hard head capsule and white body or abdomen, which is grey at the end. The beetles lay eggs on vegetable matter during spring or early summer. White grubs feed on organic matter (old crop residue) in the soil but they can also feed on roots. This can result in stunted growth, wilting and eventually death of younger plants. The young soyabean plants examined had poor side root and root hair development.
White grubs generally prefer sandier soils,. However the soils at ART where the white grubs were found were red loams with significant clay. Ploughing exposes white grubs to the sun and to predators, such as storks. White grub problems are arguably an occasional and unfortunate consequence of reduced tillage. Normally white grubs are kept under control by naturally-occurring microbes and fungi, such as Beauvaria sp fungus.
But in a dry year like this, natural control methods are less effective. Beauvaria needs moist, humid conditions to proliferate in the soil, but conditions have not been favourable for fungal control agents. These occur naturally in healthy soils and in fact several white grubs were found which had been killed by (entomopathogenic) fungi. Beauvaria preparations can be also bought as commercial products and applied to soils, an example of which is the locally-available Dynamo. Dynamo is probably best used at planting at 2.5 kg / ha, applied in a high volume of water (500 litres per ha).
There is no really effective chemical control method for white grubs after planting of soyabeans. Since the larvae are feeding at about 80 – 150 mm soil depth or deeper, it is impossible to get chemical insecticides down to the zone where the larvae are feeding. Providing water is a control method in itself, at it makes feeding and movement more difficult for the larvae as well as encouraging insect-killing fungi.
Chlorpirifos (Dursban) is locally registered for use at planting, but the treatment is toxic (red triangle) and no longer in common usage. Carbofuran (Furadan) is used at planting in South Africa, but is even more toxic (purple triangle). Maize seed dressed with imidacloprid (Confidor) or thiamethoxam (Cruiser) is defended against early attack by white grub, but soyabean seed is not usually treated with insecticides.
Certain areas or lands tend to have a history of infestation, so these would be the lands where it may make sense to treat for white grubs in future crops. They have been recorded as damaging a variety of other crops including sorghum, cotton, brassicas, peas, vegetable transplants, tobacco and coffee, so depending on the crop planted it might make sense to include a Beauvaria treatment at planting.
Dynamo is available from Pivotal Agro-Services, phone +242 309800 – 25 or [email protected], [email protected], [email protected] .
The above picture was sourced from https://keys.lucidcentral.org/keys/v3/eafrinet/maize_pests/key/maize_pests/Media/Html/White_Grubs.htm. Further information can be found at this link.
By Stewart Wilson