Potential of Lariophagus distinguendus (Forster) (Hymenoptera: Pteromalidae) to suppress the maize weevil Sitophilus zeamais Motschulsky (Coleoptera: Curculionidae) in bagged and bulk stored maize

Grains are often stored in jute bags in developing countries, especially in Africa, as well as in small quantities in bulk. Parasitoids suitable for biological control of stored-product pests should be able to find their hosts in bulk grain or in jute bags over a certain distance in a warehouse containing stacks of bagged grain. The potential of using Lariophagus distinguendus for the biological control of Sitophilus zeamais was assessed in maize stored in jute bags and bulk grain. The ability of the parasitoid to penetrate the jute cloth and the grain mass and parasitize its host was studied under controlled conditions of 25 +/- 1 degrees C and 65 +/- 5% RH. Experiments were carried out in small 5-kg jute bags containing 28 d old S. zeamais larvae within infested maize kernels, and in cylinders filled with maize grains and containing caged hosts at different depths. L. distinguendus parasitized S. zeamais in the jute bags and in the storage cylinders at various depths. In the jute bag experiment, out of the 60 L. distinguendus adults released, a mean +/- SD of 7.03 +/- 1.78% and 6.34 +/- 1.01% of the 40 females and of the 20 males released, respectively, entered the jute bags. Significantly, no differences were found between the female and male L. distinguendus that entered the bags. Mean reduction of S. zeamais in the jute bags by parasitoids was 81%. The parasitic wasps also significantly reduced the emergence of S. zeamais in bulk maize. At depths of 20-45 cm from the grain surface, mean reduction of S. zeamais was 74%, while from 95 to 100 cm, mean reduction was 34%. When results from depths lower than 50 cm were pooled and compared with pooled data from depths higher than 90 cm, there was a significant reduction in parasitism at depths of more than 90 cm. For depths below 50 cm a mean of 5.3 L. distinguendus adult offspring per cage emerged compared with a mean of 2.6 at depths of more than 90 cm. These results support the approach to utilize L. distinguendus as a component in the integrated control of S. zeamais in bagged or bulk stored maize. (C) 2011 Elsevier Inc. All rights reserved.