12. Future research
Several factors affect the plant responses to M. sacchari injury such as the time of infestation (season and plant growth stage), duration of the infestation, environmental stresses (especially drought) and nutritional status of the host plant, which cumulatively affect the yield loss. The sugarcane aphid feeding at the basal leaves had a strong influence on mobility of mineral nutrients, amino compounds, and carbohydrates in the phloem. By feeding at these sites, M. sacchari alters the carbohydrate-partitioning patterns of sorghum, suggesting that infestation might alter sink-source relationships within the plant. Very few studies have investigated the impact of environmental factors on M. sacchari damage symptom development, which mostly depends on the initial aphid density colonized as well as the inherent plant resistance. Thus, the impact of abiotic factors such as temperature and photoperiod on the life history traits of M. sacchari needs to be investigated. Further, the contribution of weeds and the native vegetation to the sugarcane aphid population dynamics is not clearly known, although considered as a major factor influencing sorghum infestation. Diversity of natural enemies may vary within and among cultivar communities depending on a variety of factors including microclimate, prey density, annual changes in host plant species abundance, the timing and rate of migration and dispersal, and geographic location. The epidemiology of transmission of three persistent viruses transmitted by the sugarcane aphid has not been substantially investigated. Emphasis should be given on the population colonization at the seedling stage of sorghum because they may have greater impact affecting the growth and development of the crop.
The impact of natural enemies on sugarcane aphid populations in sorghum is not well understood. Quantification and predicting the impact of natural enemies, as a group or as individuals, has been difficult because of the many species involved. The natural enemies have a greater effect early in the season, since the increase per generation of the sugarcane aphid population early in the season is slow compared with late season increases. Comprehensive information needs to be generated on the population dynamics of M. sacchari and the factors that lead to rapid population buildup. Environmental factors are largely responsible for the initial decline in aphid levels following a summer outbreak ( Hall, 1987). Quantification of development rates of immatures, and potential population growth rates of the sugarcane aphid under different temperature regimes similar to those occurring in the post-rainy season sorghum niche in India would greatly aid in forecasting models. Drought and sugarcane aphid infestation are two factors that individually cause yield reduction in sorghum, but when they occur in succession results in a synergistic interaction between aphid infestation and predisposing the crop to drought stress. Field observations have indicated that aphid damage is greater when drought also occurs as a stress factor. The magnitude of plant damage to sorghum in relation to the density of M. sacchari as well as the stage of plant growth is not known. It may be that sorghum can recover and compensate for damage due to M. sacchari as has been observed by the feeding injury caused by S. flava in sorghum. Insect density relationships with yield loss are not available. It is not known, whether the sugarcane aphid injects a phytotoxin during feeding that degrades chloroplasts or the damage results from the plant's response to mechanical injury. As little is known about how aphids elicit these changes and the physiological mechanisms associated with injury, improving our understanding of how M. sacchari affects the sorghum plant physiology will be important in identifying new targets for aphid resistance in sorghum.
Cultural practices such as elimination of volunteer sorghum plants from the previous crop and the destruction of susceptible weedy hosts needs greater emphasis. Search for new sources of resistance is essential to diversify the genetic background. Identification of resistant sources in elite background as well as in parental lines in the development of hybrids, and knowledge on the mechanisms would greatly assist in developing cultivars with stable sources of resistance to the sugarcane aphid, under high yield background. Combinations of different categories have effects that may be more beneficial and compatible in trophic interactions than the effect of individual components of resistance.