Adele J. Ngi-Song

Volatile infochemicals used in host and host habitat location by Cotesia flavipes Cameron and Cotesia sesamiae (Cameron) (Hymenoptera: Braconidae), larval parasitoids of stemborers on graminae.

FemaleCotesia flavipes Cameron andCotesia sesamiae (Cameron) were attracted to odors in a Y-tube olfactometer from uninfested maize (Zea mays L.), sorghum [Sorghum bicolor (L.)], and napier grass (Pennisetum purpureum Schumach). In dual choice tests,Cotesia flavipes showed a preference for maize over sorghum, while maize and napier grass were equally attractive. In contrast,Cotesia sesamiae preferred volatiles from sorghum and napier grass over those from maize. The two parasitoids were significantly more attracted to maize infested with the stemborers,Chilo partellus (Swinhoe),Chilo orichalcociliellus Strand,Sesamia calamistis Hampson, andBusseola fusca (Fuller), than uninfested maize. In dual choice tests,Cotesia flavipes andCotesia sesamiae were unable to discriminate between odors from plants infested by the different species of stemborers.

A review of the introduction and establishment of Cotesia flavipes Cameron in East Africa for biological control of cereal stemborers.

Chilo partellus Swinhoe (Lepidoptera: Pyralidae) is an exotic stemborer of cereal crops in Africa which invaded the continent from Asia earlier this century. In addition to Ch. partellus, several indigenous stemborers are found in Africa. In 1991, Cotesia flavipes Cameron (Hymenoptera: Braconidae), a gregarious endoparasitoid of stemborers in Asia, was introduced into Kenya for biological control of Ch. partellus. Laboratory studies revealed that the parasitoid could successfully parasitise not only the target stemborer, but also two native stemborers that occur sympatrically with Ch. partellus in some locations. Interbreeding studies demonstrated the Co. flavipes would mate with a native congener, Cotesia sesamiae (Cameron), but no female offspring resulted from these matings. Investigations on competition between the two Cotesia spp. indicated that when Ch. partellus was the host, Co. flavipes appeared to be a superior parasitoid. Releases of Co. flavipes were made in 1993 in Kenya. Recoveries in 1994, 1995 and 1996 demonstrated that the parasitoid was firmly established in two regions of Kenya and in northern Tanzania. Preliminary observations suggest that the parasitoid is causing greater mortality to stemborers in southwestern Kenya than in the coastal area.RésuméChilo partellus Swinhoe (Lepidoptera: Pyralidae) est un exotique foreur de tiges des cultures céréalières en Afrique qui envahit le continent en provenance d’Asie au début de ce siècle. En plus de Ch. partellus, plusieurs foreurs de tiges sont rencontrés en Afrique. En 1991, Cotesia flavipes (Hymenoptera: Braconidae), un grégaire endoparasitoïde des foreurs de tiges en Asie fut introduit au Kenya pour la lutte biologique contre Ch. partellus. Des études en laboratoire révélèrent que le parasitoïde pouvait parasiter avec succès non seulement les foreurs cibles mais aussi deux foreurs en occurrence sympatrique avec Ch. partellus dans quelques localités. Des études sur élevage mixte ont démontré que Co. flavipes pouvait s’accouplert avec un congénère indigène, Cotesia sesamiae (Cameron), mais ces accouplemets ne donnèrent pas naissance à une progéniture femelle. Les investigations sur la compétition entre les deux espèces de Cotesia montrèrent que lorsque Ch. partellus était l’hôte, Co. flavipes était un parasitoïde supérieur. Des lâchers de Co. flavipes furent effectués en 1993 au Kenya. Les recouvrements de 1994, 1995 et 1996 montrèrent que le parasitoïde s’était fermement établi dans deux régions du Kenya et au nord de la Tanzanie. Des observations préliminaires suggèrent que le parasitoïde cause une plus grande mortalité chez les foreurs de tiges au sud-ouest que dans la région côtière du Kenya.

Wolbachia infection in Cotesia sesamiae (Hymenoptera: Braconidae) causes cytoplasmic incompatibility: implications for biological control

Cotesia sesamiae (Hymenoptera: Braconidae) is an indigenous, gregarious, larval endoparasitoid that attacks mid- to late-instar of the stem borer larvae. Although the parasitoid is distributed widely throughout Africa, not all local populations appear to be equally effective in controlling stem borers. Consequently, there is an interest in releasing the more effective strains in areas that already have very low populations of C. sesamiae. Some C. sesamiae populations are infected with bacterial symbionts in the genus Wolbachia, which may induce cytoplasmic incompatibility. Using an antibiotic treatment, we have established that the Wolbachia infection causes cytoplasmic incompatibility of the female mortality type in C. sesamiae. Using a simple recurrent equation model, we showed that mixing infected and uninfected populations that exhibit cytoplasmic incompatibility causes a transient, but possibly long, reduction in population growth rate. Knowledge of the infection status of native populations and populations that would be introduced could be used to avoid the incompatibility problems, by adjusting the population that will be introduced to the infection status of the native population.

Identification of behaviourally active components from maize volatiles for the stemborer parasitoid Cotesia flavipes cameron (hymenoptera: braconidae)

In the present study, Y-tube olfactometric assays confirmed that volatiles from maize seedlings infested with Chilo partellus larvae were more attractive to the borer’s larval endoparasitoid Cotesia flavipes, than volatiles from uninfested maize. Coupled gas chromatography-electroantennographic detector (GC-EAD) analysis of the volatiles from larvae-infested maize revealed six electrophysiologically active compounds on the antennae of the female parasitoid. These compounds were identified by GC-MS as (Z)-3-hexenyl acetate, linalool, (E)-4, 8-dimethyl-1, 3, 7-nonatriene, heptanal, (E)-ß-ocimene and a C-5 aliphatic compound. (E)-4, 8-Dimethyl-1, 3, 7-nonatriene was present in EAG-detectable amounts in the volatiles of uninfested seedlings. In bioassays, a blend comprised of (Z)-3-hexenyl acetate, linalool, (E)-4, 8-dimethyl-1,3,7-nonatriene, (E)-ß-ocimene and heptanal was significantly attractive to the parasitoid. Of the individual compounds, (Z)-3-hexenyl acetate was attractive at the doses tested while (E)-4,8-dimethyl-1,3,7-nonatriene and heptanal showed varying degree of attractiveness to the parasitoid at different doses. Linalool and (E)-ß-ocimene were unattractive at the same doses. The significance of these results is discussed.RésuméDans cette étude, des essais olfactométriques en tubes Y ont confirmé que les matières volatiles de jeunes plants de maïs infestés par des larves du foreur de tige, Chilo partellus attiraient plus de larves de Cotesia flavipes un endoparasitoïde de ce foreur, en comparaison des matières volatiles du maïs non infesté. L’analyse par chromatographie à phase gazeuse couplée avec un détecteur électro-antennographique des matières volatiles du maïs infesté de larves a révélé six composés avec des activités éléctro-physiologiques sur les antennes de la femelle du parasitoïde. Ces composés ont été identifiés par méthodes GC-MS comme (Z)-3-hexényle acétate, linalol, (E)-4, 8-diméthyle-1,3,7-nonatriène, heptanal, (E)-ß-ocimène et un autre composé, le C-5 aliphatique. (E)-4, 8-diméthyle-1,3,7-nonatriène était présent en quantités détectables par méthode EAG, dans les matières volatiles de jeunes plants non infestés. Au cours des essais biologiques, un cocktail fait de (Z)-3-hexényle acétate, de linalool, de (E)-4,8-diméthyle-1,3,7-nonatriène, de (E)-ß-ocimène et d’heptanal, attirait fortement le parasitoïde. Quant aux différents composés pris isolément, (Z)-3-hexényle acétate était attrayant aux doses testées tandis que (E)-4, 8-diméthyle-l, 3, 7-nonatriène et heptanal montraient des degrés variables d’attractivité à différentes doses. Le linalol et (E)-ß-ocimène ne montraient pas d’attrait aux mêmes doses. L’article discute de la signification des résultats obtenus.

Niche overlap and interspecific association between Chilo partellus and Chilo orichalcociliellus on the Kenya coast

Chilo partellus (Swinhoe) and Chilo orichalcociliellus Strand (Lepidoptera: Crambidae) occur sympatrically on the Kenya coast and are injurious to maize and sorghum. Evidence over a period of 30 years indicates that the indigenous stem borer, C. orichalcociliellus, is being gradually displaced by the exotic stem borer, C. partellus. The two species have overlapping niches, but the specific mechanisms driving the displacement are unknown. The distribution and extent of niche overlap and interspecific association between C. partellus and C. orichalcociliellus were investigated in cultivated and wild host plants. No difference in the within‐plant distribution of the two borer species in maize and sorghum was found. A niche overlap of 83.7% between C. partellus and C. orichalcociliellus was found. A weak positive association was found between small‐, medium‐, and large‐sized larvae of C. partellus and C. orichalcociliellus. However, as larvae grew, the association decreased with no significant association at the pupal stage. No negative association was found between C. partellus and C. orichalcociliellus, suggesting that the two species do not avoid or directly harm each other. The partial displacement of C. orichalcociliellus may therefore, be attributed to intrinsically superior attributes of C. partellus that indirectly affect the survival of C. orichalcociliellus.

Multiple Parasitism by Cotesia sesamiae and Cotesia flavipes (Hymenoptera: Braconidae) on Busseola fusca (Lepidoptera: Noctuidae)

Busseola fusca (Fuller) is one of the most important pest of cereals in sub-Saharan Africa. Cotesia sesamiae (Cameron) is the predominant parasitoid attacking B. fusca larvae in many parts of Africa. An exotic parasitoid, Cotesia flavipes Cameron, was introduced into Kenya in 1993 for the control of Chilo partellus (Swinhoe). Laboratory studies indicated that although C. flavipes would search for, and attack B. fusca , it was not able to complete its development in this host. The aim of the present study was to investigate the outcome of multiple parasitism of B. fusca by the two Cotesia species. The study showed that when both parasitoid species stung a B. fusca larva at the same time, both parasitoids emerged from more than half of the host larvae, C. flavipes alone emerged from 17%, and C. sesamiae alone emerged from 9%. When the larvae were parasitized by C. sesamiae first, and then 2 h later by C. flavipes , and vice versa, most of the progeny were C. flavipes . However, when B. fusca larvae were stung by C. sesamiae three days before oviposition by C. flavipes , significantly more C. sesamiae emerged from the larvae. When C. flavipes oviposited first, no larvae produced C. flavipes only. The interaction of parasitoids and the host immune system, and the implications of these results for the biological control of stem borers in East Africa are discussed.

Variation in encapsulation sensitivity of Cotesia sesamiae biotypes to Busseola fusca

Many braconid wasp species inject polydnaviruses to overcome their hosts immune system. In the species Cotesia sesamiae, two biotypes exist that differ in their ability to develop in the host Busseola fusca. The biotype from coastal Kenya is infected with Wolbachia and is not able to develop in larvae of B. fusca, whereas the uninfected inland biotype of this wasp can develop in B. fusca. The genetic transmission of the developmental ability was studied through a series of genetic crosses and superparasitization experiments. The Wolbachia infection of the coastal type did not play a role in the encapsulation response of the host. Experiments show that the polydnaviruses of the wasps could not prevent the encapsulation of the coastal parasitoid eggs. Most likely, larval characteristics such as surface proteins played a more important role in the encapsulation response of the host even in the presence of a functional polydnavirus.

Responses of the stem borer larval endoparasitoid Cotesia flavipes (Hymenoptera: Braconidae) to plant derived synomones: Laboratory and field cage experiments

Abstract Laboratory and field cage experiments investigated the response of females of the stem borer larval endoparasitoid Cotesia flavipes to two synthetic synomone components, the terpenoid (E)-β-farnesene and the green leaf volatile, (Z)-3-hexenyl acetate, both compounds identified previously in headspace volatiles of maize plants damaged by stem borer (Chilo partellus). In dose response tests performed in a Y-tube olfactometer, parasitoids were significantly more attracted to the arms bearing 10 or 15 µg of (Z)-3-hexenyl acetate and (E)-β-farnesene than to the control arm. (E)-β-Farnesene was as attractive as the essential oil from the plant Hemizygia petiolata (Lamiaceae) rich in the same compound (80% relative amount). The plant essential oil elicited responses from females of the parasitoid comparable to those elicited by two positive controls, stem borer larval frass and adult parasitoid diet (20% honey solution), tested in the laboratory assays. In field cage trapping experiments, captures in traps baited with the terpenoid, the plant essential oil, (Z)-3-hexenyl acetate and the control of 20% honey solution, were not significantly different relative to captures in unbaited traps. Addition of the green leaf volatile (Z)-3-hexenyl acetate to the plant essential oil to yield a 1:1 two-component blend captured significantly more female parasitoids than traps baited with either of the two components alone. The results show that blends of green leaf volatiles and sesquiterpenoids may have potential in monitoring C. flavipes populations in the field.

Olfactory responses of Cotesia flavipes (Hymenoptera: Braconidae) to target and non-target Lepidoptera and their host plants

Abstract The attraction of Cotesia flavipes Cameron to volatiles from a range of non-target lepidopteran larvae and their host plants (grasses and trees) or food substrate (honeycomb) was evaluated using a Y-tube olfactometer. The non-target host larvae used in the study included Galleria mellonella (L.), Charaxes cithaeron Felder, Bombyx mori L., and Eldana saccharina Walker. The target insects, Chilo partellus (Swinhoe) and Chilo orichalcociliellus (Strand), were used as controls. Host plants included Afzelia quanzensis Welw., Morus alba L., Cyperus papyrus L., Pennisetum purpureum Schumach, and Zea mays L. The response of C. flavipes to volatiles from the non-target larvae and their food was variable. Attraction to uninfested maize was not significantly different from uninfested plants of non-target hosts or honeycomb. Only maize and honeycomb were preferred over clean air. C. partellus infested maize plants were significantly more attractive than M. alba, A. quanzensis, and honeycomb infested with their herbivores. Infested maize and C. papyrus were more attractive than uninfested ones. When odors from naked larvae were tested, C. flavipes preferred odors from C. partellus larvae over those of E. saccharina and C. cithaeron and larvae of C. partellus and G. mellonella were preferred to clean air. The implications of these findings for biological control and its effect on non-target organisms are discussed.

Variation in Total and Differential Haemocyte Count of Busseola fusca (Lepidoptera: Noctuidae) Parasitized by Two Biotypes of Cotesia sesamiae (Hymenoptera: Braconidae) and Larval Growth Responses

Abstract Cotesia sesamiae (Cameron) is an indigenous larval endoparasitoid that attacks mid- to late-stage gramineous stem borer larvae in Africa. Two biotypes of C. sesamiae have been reported with differential abilities to suppress the immune system of Busseola fusca Fuller. Eggs of a C. sesamiae population from Mombasa were encapsulated, whereas eggs of a population from Kitale were not. Total and differential hemocytes were counted in larvae of B. fusca at six times (2 h, 14 h, 24 h, 72 h, 120 h, 168 h) after being exposed to parasitoids from Kitale and Mombasa. The total numbers of hemocytes in a larva parasitized by the C. sesamiae population from Mombasa were higher as compared with larvae parasitized by the C. sesamiae population from Kitale. Plasmatocytes, in particular, were reduced in larvae parasitized by C. sesamiae from western Kenya from 72 to 168 h after oviposition. Our results suggest that plasmatocytes probably play an important role in the immune response of B. fusca. Significant proportions of the host larvae were still at the larval stage for those parasitized by C. sesamiae from Mombasa and another species of Cotesia, Cotesia flavipes Cameron at day 12. A reduction of B. fusca larval weight was observed on day 12 after oviposition by C. sesamiae from Kitale.