ABSTRACT

Most commercial products from Bacillus thuringiensis (B.t.) were ineffective for controlling Metisa plana (WZk.). The effectiveness of a B.t. product against an insect depends on its protoxin composition, and the presence of suitable receptor sites and factors activating the toxic crystals within the insect’s midgut. The pH of the midgut must be suitable for solubilizing crystals prior to activation. The selection of a B.t. strain effective against the target pest is a practical approach. A B.t. strain containing the right toxin which binds to receptors sites present in the pest needs to be identified and selected by screening. This paper reports on the progress made in increasing the potency of a local strain of B.t., SRBT1, selcted after a study of its toxicity.

Alkaline and acidic compounds were added to SRBT1 to enhance the effect of its toxins. The addition of 0.5% tannic acid and 1.5% sodium borate showed a 4 fold increase in mortality of M. plana at 4 days afer treatment (DAT). A Three fold increase in mortality resulted from incorporation of 0.5% magnesium chloride and a two fold increase with 0.5% potassium carbonate and 0.5% sodium borate at 4 DAT. At 6 DAT, SRBT1 containing 1.5% sodium borate caused 94% mortality, as against 70% for the best commercial product tested, Florbac. One hundred percent mortality was observed at 9 DAT when 0.5% sodium borate, 0.5% and 1.5% tannic acid were added separately to SRBT1. SRBT1 with no chemical additive produced 100% mortality at 11 DAT. SRBT1 with or without chemical additives, gave a marked reduction, 98 – 100%, in leaf area damaged (LAD) as compared with the control. This reduction is comparable to that produced by methamidophos 99.4%. Florbac gave only 59.7 % reduction in LAD. The reduction in LAD was more obvious when sodium borate and tannic acid were used. SRBT1 must harbour toxins suitable for the control of M. plana, because the effect on feeding behaviour is very pronounced. Determination of SRBT1 cry genes, cry proteins and exploitation of this isolate should be attempted.

KEYWORDS: Bacillus thuringiensis, Metisa plana, local isolate, delta-endotoxin, potency