Breeding maize (Zea mays) for Striga resistance: Past, current and prospects in sub-saharan africa.

Striga hermonthica, causes up to 100% yield loss in maize production in Sub-Saharan Africa. Developing Striga-resistant maize cultivars could be a major component of integrated Striga management strategies. This paper presents a comprehensive overview of maize breeding activities related to Striga resistance and its management. Scientific surveys have revealed that conventional breeding strategies have been used more than molecular breeding strategies in maize improvement for Striga resistance. Striga resistance genes are still under study in the International Institute for Tropical Agriculture (IITA) maize breeding programme. There is also a need to discover QTL and molecular markers associated with such genes to improve Striga resistance in maize. Marker Assistance Breeding is expected to increase maize breeding efficiency with complex traits such as resistance towards Striga because of the complex nature of the host-parasite relationship and its intersection with other environmental factors. Conventional alongside molecular tools and technical controls are promising methods to effectively assess Striga in Sub-Saharan Africa.

Single versus multiple enemies and the impact on biological control of spider mites in cassava fields in West-Africa.

To determine whether to use single or multiple predator species for biological pest control requires manipulative field experiments. We performed such tests in Benin (West Africa) in cassava fields infested by the cassava green mite Mononychellus tanajoa, and the cotton red mite Oligonychus gossypii. These fields also harboured the cassava apex-inhabiting predator Typhlodromalus aripo and either the leaf-inhabiting predator Amblydromalus manihoti or Euseius fustis. We manipulated predator species composition on individual plants to determine their effect on prey and predator densities. In fields with T. aripo plus A. manihoti, M. tanajoa densities were reduced by T. aripo alone or together with A. manihoti, but neither of these predators, alone or together, reduced O. gossypii densities. In fields with T. aripo plus E. fustis, T. aripo alone or together with E. fustis exerted significant control over O. gossypii, but weak control over M. tanajoa. Densities of any of the predator species were not affected by co-occurring predator species, suggesting a minor role for intraguild predation in the field, contrary to earlier experiments on small plants in the laboratory. We conclude that (1) T. aripo is the most effective predator species in suppressing M. tanajoa, (2) two predator species, T. aripo and E. fustis, are needed to suppress O. gossypii, and (3) predator species together on the same plant do not negatively affect each other nor the extent to which they control their prey. We argue that intraguild predation is reduced due to partial niche separation among predator species.

Interactions between the predatory mite Typhlodromalus aripo and the entomopathogenic fungus Neozygites tanajoae and consequences for the suppression of their shared prey/host Mononychellus tanajoa.

The predatory mite Typhlodromalus aripo and the entomopathogenic fungus Neozygites tanajoae, both introduced from Brazil for control of the cassava green mite (CGM) Mononychellus tanajoa, now co-occur in cassava fields in Benin. However, studies on interactions between these two natural enemies and how they might affect CGM biological control are lacking. We determined in screenhouse experiments the effects of single and combined releases of N. tanajoae and T. aripo on CGM suppression. In the single natural enemy treatment, both T. aripo and N. tanajoae significantly reduced CGM densities, but the results of the predator (T. aripo) are more quickly measurable than those of the pathogen (N. tanajoae) in our short-term experiment. The level of CGM suppression in the combined natural enemy treatment was reduced considerably compared with T. aripo-alone, but only slightly when compared with N. tanajoae alone, with a simultaneous reduction in T. aripo and N. tanajoae abundance or prevalence. In a laboratory experiment, T. aripo fed more on N. tanajoae-infected CGM than on healthy CGM and its oviposition and survival were reduced when fed on the former compared with the latter, which can help in explaining the reduction in numbers of T. aripo and consequently the considerable loss in suppression of CGM in the combined natural enemy treatment in the screenhouse experiment. Together, the screenhouse and the laboratory experiments predicted negative interactions between the two natural enemies with negative consequences for CGM biological control. Long-term field observations and rigorous field experiments that simultaneously manipulate T. aripo and N. tanajoae abundance and prevalence are needed to validate the prediction of this study.

Potential of the predatory mite, Amblyseius swirskii to suppress the broad mite, Polyphagotarsonemus latus on the gboma eggplant, Solanum macrocarpon.

In Benin, the tarsonemid mite Polyphagotarsonemus latus (Banks) (Prostigmata: Tarsonemidae) is a key pest of gboma eggplant Solanum macrocarpon (L.) (Solanales: Solanaceae), a leafy vegetable on which it causes considerable damage to the plants and substantial reduction in yield. Predatory mites in the family Phytoseiidae have been successfully used in the biological control of numerous agricultural pests worldwide. In that respect, a population of the phytoseiid mite Amblyseius swirskii (Athias-Henriot) (Mesostigmata: Phytoseiidae) has been identified as a potential predator of P. latus, and is now a candidate for release against this pest in Benin. The objective of the present study is to determine, through laboratory experiments, the predation rate and life table parameters of A. swirskii when feeding on P. latus or alternative food such as maize pollen. Under laboratory conditions the mean number of P. latus consumed by A. swirskii, and daily oviposition, significantly increased as the number of prey increased. Total development time of A. swirskii was significantly shorter when it fed on P. latus than on maize pollen. Net reproduction rate, intrinsic rate of increase, mean generation time and the finite rate of increase of A. swirskii were were all significantly lower on P. latus than on maize pollen. However, doubling time was significantly higher on maize pollen. This study shows that A. swirskii is a good predator of P. latus, and that maize pollen can efficiently sustain A. swirskii populations when P. latus densities on plants become low. Consequently, A. swirskii can be used for the biological control of the broad mite P. latus on gboma eggplant, and on other solanaceous crops in Benin and elsewhere.

The predatory mite Typhlodromalus aripo prefers green-mite induced plant odours from pubescent cassava varieties.

It is well known that plant-inhabiting predators use herbivore-induced plant volatiles to locate herbivores being their prey. Much less known, however, is the phenomenon that genotypes of the same host plant species vary in the attractiveness of these induced chemical signals, whereas they also differ in characteristics that affect the predator's foraging success, such as leaf pubescence. In a series of two-choice experiments (using a Y-tube olfactometer) we determined the preference of Typhlodromalus aripo for pubescent versus glabrous cassava cultivars infested with the cassava green mite Mononychellus tanajoa and also the preference for cultivars within each of the two groups. We found that when offered a choice between pubescent and glabrous cassava cultivars (either apex or leaves), T. aripo was significantly more attracted to pubescent cultivars. For each cultivar, M. tanajoa infested leaves and apices were equally attractive to T. aripo. There was however some variation in the response of T. aripo to M. tanajoa-infested plant parts within the group of pubescent cultivars, as well as within the group of glabrous cultivars. Our study confirms not only that T. aripo uses herbivore-induced plant volatiles to search for prey in cassava fields, but it also shows that it can discriminate between glabrous and pubescent cultivars and prefers the latter. This knowledge can be useful in selecting cultivars that are attractive and suitable to T. aripo, which, in turn, may promote biological control of the cassava green mite.

Effects of ultraviolet radiation on predatory mites and the role of refuges in plant structures.

Most studies on ecological impact of solar ultraviolet (UV) radiation generally focus on plants. However, UV radiation can also affect organisms at other trophic levels. Protection against mortality induced by solar UV has, therefore, been hypothesized as one of the reasons why Typhlodromalus aripo hides in the apex of cassava plants during the day and comes out at night to prey on spider mites on leaves. In laboratory experiments using UV lamps, we determined the impact of UVA and UVB radiation on survival and oviposition of two leaf-inhabiting mites (Amblydromalus manihoti, Euseius fustis) and the apex-inhabiting mite (T. aripo), all three species being predators used for controlling the cassava green mite Mononychellus tanajoa in Africa. Whereas on leaf discs UVA has no negative impact on survival of the three predators, UVB is lethal to all of them. In contrast, nearly 85% of T. aripo survived after exposure to UVB inside apex of cassava plants. Exposure of A. manihoti and E. fustis to UVB radiation on the lower surface of a cassava leaf resulted in 36% survival. Oviposition and hatching of eggs laid after exposure to UVB were not affected, but eggs directly exposed to UVB did not hatch. Although caution should be exercised to extrapolate laboratory studies to the field, our results support the hypothesis that lower side of leaves, but especially plant apices, represent refuges that protect predatory mites from UVB. This might explain why T. aripo moves out of the apex to forage on leaves only during the night.

Interactions in an acarine predator guild: impact on Typhlodromalus aripo abundance and biological control of cassava green mite in Benin, West Africa.

To determine the impact of an acarine predator guild on the abundance of a shared herbivorous prey and its principal exotic predator, a series of surveys were conducted in ca. 200 cassava fields in swamp and non-swamp areas in southwestern Benin, West Africa. For each field, the surveys provided data on the density of a pest arthropod, the cassava green mite Mononychellus tanajoa (Bondar), of an introduced and successfully established natural enemy, the apex-inhabiting predatory mite Typhlodromalus aripo DeLeon, and on occurrence of other predator species that inhabit the leaves and share the same prey. These other predators included one exotic species, Typhlodromalus manihoti Moraes, that is successfully established mainly in swamp areas, and two indigenous species, Euseius fustis (Pritchard and Baker) and Typhlodromalus saltus (Denmark and Matthysse), that are commonly found on cassava in Africa. Our aim was to assess the association between the density of M. tanajoa and that of T. aripo, the most successful predator in terms of establishment and abundance, and subsequently determine how this association was affected by the presence or absence of the other predator species. No obvious density-dependent relationship was found by inspecting the scattergrams of T. aripo versus M. tanajoa densities, but high T. aripo densities did not occur when M. tanajoa densities were low and--during the dry season in February--the densities of M. tanajoa steeply increased when T. aripo numbers were low. Given the establishment of T. aripo in all fields, the presence of other species of predatory mites (T. manihoti, both in swamp and non-swamp areas; T. saltus in absence--as well as presence--of T. manihoti in swamp areas; E. fustis in absence of T. manihoti in non-swamp areas) reduced the density of M. tanajoa by a factor 2-3. Thus in all these cases, the presence of an exotic or indigenous species of predatory mite in addition to T. aripo was associated with lower M. tanajoa density. The density of T. aripo was usually positively affected by the presence of other predator species except for T. saltus in presence of T. manihoti that negatively affected the density of T. aripo in swamp areas, an effect likely mediated by either intraguild predation or competition for food. Path analysis showed that indigenous phytoseiid species were more important in suppression of M. tanajoa populations in cassava fields than previously thought. We suggest that the lack of negative effects of the predator species complex is likely due to differential niche use by the various species which reduces interference among the predators. Manipulative experiments are, however, needed to provide details on the relative importance in M. tanajoa suppression by each species within this acarine predator guild.