Storage stability of two liquid formulations of Bacillus thuringiensis subsp israelensis and effect of freezing over time

Over the last two decades, many tests have been performed in the field to i nvestigate the behaviour (persistence, carry, loss of activity, etc.) of di fferent Bacillus thuringiensis subsp. israelensis (Bti) formulations. Depen ding on the experimental protocols, a single container of a formulation cou ld be used more than once over time and field samples containing Bti may ha ve to be frozen to preserve them for bioassays to be performed later. Thus, it is necessary to know how long a formulation could keep its level of eff icacy and also the effects of time on frozen samples. Our results showed th at the efficacy of two commercial liquid formulations of Bti (Teknar HP-D a nd Vectobac 1200L) when tested against Aedes triseriatus behaved differentl y over time when kept at room temperature. Teknar HP-D remained stable for the first two years and its LC50 increased by 20% the third year. For Vecto bac 1200L, although its larvicidal activity was better than that of Teknar HP-D every year, there was an increase in LC50 by 22% the second year and b y another 20% the third year for a total loss of activity of 46% over the t hree-year study. The efficacy of suspensions made with both formulations wa s greatly affected by freezing and the loss of efficacy increased over time . About half of the efficacy of Teknar HP-D was lost after one week of free zing and stayed at that level for three months, while with Vectobac 1200L, no significant effect of freezing was seen after one month, when compared t o fresh material. However, both products showed similar efficacy after thre e or six months of freezing. Overall, the LC(50)s of both products had incr eased by a factor of about 2.5 after six months of freezing.