The production of wheat in Sub-Saharan Africa is threatened by infestations of the Hessian fly, Mayetiola destructor, popularly called gall midges. This insect pest has spread across all major breadbasket areas of Eastern and Southern Africa, and is causing substantial losses of grain and forage yields. Damage to wheat crops is attributed to larvae of the Hessian fly that get dispersed by adult flies which lay eggs in grooves on the upper side of plant leaves. Larvae have the ability to form hardened gall structures which offer protection from natural enemies and other control agents, and make the insect pest a formidable opponent for farmers. On seedlings of wheat the larvae feed on the growth apex (crown), while at later growth stages of the crop larvae establish feeding sites beneath the leaf sheath at the base of the plant. Infestation of wheat plants by Hessian fly larvae is decreasing the number of stems and leaves, and foliar and root biomass, with the largest damage occurring when infested culms break at weakened nodes. Cultivating wheat varieties that possess a natural defence mechanism against the Hessian fly larvae is the most effective control method. Farmers can prevent major infestation and realize bountiful harvests under high occurrence of the insect pest by growing resistant lines of wheat. Crop surveillance and varietal planning is needed to ensure that the use of resistant wheat varieties do not get undermined by new variants of the insect pest.
Resistance of wheat to the Hessian fly is originating from naturally occurring genes that activate a biochemical defence response when larvae feed on the leaf and stem of plants. More than 26 resistance genes have been identified in common wheat, durum wheat and wild wheat that have been incorporated into local high-yielding varieties. Common, non-genetically modified breeding techniques such as phenotyping and backcrossing are used in the development of wheat varieties that withstand Hessian fly attacks. Molecular markers associated with resistance genes for the insect pest have been identified that allow to fast-track varietal selection in breeding programs which reduces the cost and time for release. Hessian fly populations can overcome resistance in wheat over time because variants of the pest emerge through genetic mutation and environmental pressure, which means that the effectiveness of released varieties has to be monitored closely to avoid devastating outbreaks. The speed and risk of loss in resistance from wheat crops is tremendously diminished by combining two different resistance genes since population of Hessian flies are unlikely to overcome both genes at the same time. Marker assisted selection is often use in stacking resistance genes in wheat varieties.
The dispersal of the Hessian fly in Sub-Saharan Africa has not been mapped in detail but the occurrence of this insect pest has been confirmed in wheat production zones of Eritrea, Ethiopia, Kenya, Malawi, Mali, Mozambique, Niger, Nigeria, Sudan, Tanzania, Zambia and Zimbabwe. Especially during rainy seasons in tropical climates there is a large risk for severe infestations of the insect pest because the hatching of eggs and survival of larvae are favoured by high temperatures and moisture in the environment. Genetic resistance to Hessian fly attacks is a suitable approach for protecting wheat crops in all African growing areas since the technology can be embedded into elite lines with a high yield potential and adaptations to other environmental stresses. Warmer temperatures as a result of climate change will enhance yield losses by 20% to 40% due to insect pests across Sub-Saharan Africa.
Hessian fly resistant wheat developed in Morocco, Egypt, Algeria, Syria and South Africa has been released across multiple countries in Sub-Saharan Africa. Globally, more than 200 common wheat and durum wheat varieties were developed that withstand attacks from the insect pest and offer good bread-making quality. There are ongoing efforts by breeders to incorporate resistance genes into wheat cultivars that are commonly grown in the breadbasket regions on the continent.
The multiplication of seed for Hessian fly resistant wheat follows the same procedures as for other cultivars and takes 2-4 growing cycles. In a first stages, early-generation or basic seed is produced by agricultural research centers, and in a second stage this is passed on to private farms, farmer cooperative unions, farmer seed producer associations and model farmers for multiplying large volumes of certified seed. In-field techniques, such as the ear-to-row method where best-performing ears are selected and replanted in single lines, can be used by farmers to maintain stocks of planting material. The planting density and mineral fertilizer input for cultivation of Hessian fly resistant wheat varieties follows local recommendations like for other cultivars. Hessian fly control in farming systems is most effective when pest surveillance is carried out in connection to thresholds for yield and economic impacts. To ensure high yields and cost-effectiveness of resistant wheat there is need to adopt complementary practices such adherence to optimum planting dates, destruction of reservoir plants or ‘green bridges’, and use of insecticides.
Raised bed furrow irrigation and supplemental sprinklers, Conservation agriculture and zero tillage, Integrated pest, disease and weed management