Studies on insect pests of Sesbania sesban with special emphasis on Mesoplatys ochroptera (Chrysomelidae: Coleoptera) in Southern Africa

Abstract

There has been considerable interest in Sesbania sesban (L.) Merrill and related species because these have consistently shown high biomass production in agroforestry systems. Damage by insect pests was found to be one of the limiting factors to the wide spread adoption of Sesbania by farmers. The objectives of this study were to (1) determine the distribution and abundance of insects feeding on S sesban, (2) define the biology and population dynamics of M ochroptera and Exosoma sp., (3) identify natural enemies of M. ochroptera, (4) identify accessions of S. sesban resistant to M ochroptera and (5) determine the effect of damage by M ochroptera on the growth and yield of S. sesban.

Over 105 species belonging to 56 families in 13 orders of arthropods were found associated with S. sesban. Herbivores were the most specious guild accounting for more than 50% of all species. They attacked all parts of S sesban and caused varying degrees of damage at different growth stages of the plant. However, the most vulnerable was the seedling stage in the nursery or immediately after transplanting which was severy damaged by beetles and grasshoppers. The beneficial insects included 44 species of natural enemies and 9 species of pollinators. Most of the species were reported for the first time on S sesban in Southern Africa.

M ochroptera and Exosoma sp. were the most serious pests on S sesban, both in terms of numbers and the damage done to the foliage. The annual cycle of activity in M. ochroptera and Exosoma sp. followed the unimodal rainfall pattern of southern Africa. M. ochroptera females laid 2-70 eggs in a masses every day during an oviposition period of up to 56 days. The eggs took 2 to 9 days to hatch. There were three larval instars and larval development took 11 to 34 days. Pupation occurred in the soil and this took 4 to 16 days. The life cycle from egg to adult emergence took 16 to 43 days. The host plants of M. ochroptera in southern Africa included S. bispinosa, S. brevipeduncula, S. leptocacrpa, S macrantha, S. rostrata, S. sericea, S. sesban and S. tetraptera.

Afrius yolofa (Guérin-Ménville), Glypsus conspicuus Westwood, Macrorhaphis acuta Dallas, Mecosoma mensor Germar, Rhinocoris segmentarius (Germar) Deraeocoris ostentans (Stĺl) Cyaneodinodes fasciger (Chaudoir), Tetramorium sericeiventre Emery, Pheidole sp. and Mallada sp. were recoreded for the first time as predators of M. ochroptera. The braconid Perilitus larvicida van Achterberg and the nematode Hexamermis parasitized both the larvae and adults. No egg and pupal parasitoids of M. ochroptera were found.

Thirty accessions of Sesbania were screened for resistance against M. ochroptera and three mechanisms of resistance-antixenosis, antibiosis and tolerance-were found in the species. Sesbania bispinosa, Sesbania leptocacrpa and Sesbania macrantha accesions were more preferred to Sesbania sesban and Sesbania rostrata accesions. Antibiosis was manifested by a significant (P<0.05) reduction in foliage consumption by larvae, reduction in the weight of larvae and adults, reduction in larvae and pupal survivorship and prolonged developmental period of the beetle on resistant Sesbania accessions. Two accessions of S. sesban-Kakamega and Kisii-showed tolerance to M. ochroptera damage.

Artificial infestation of 2-3 month old S. sesban seedlings with up to 30 lavae and adults per seedling produced less than 25% defoliation whereas more than 90 larvae per seedling caused 80-100% defoliation. Removal of up to 100% of the foliage manually at one, two and three months after transplanting did not significantly reduce (P>0.05) height growth, branch number, basal diameter, and leaf and stem fresh weights of Sesbania.