How climate change affects crop pests and diseases
The rising levels of CO2 and temperatures have a direct effect on pests and diseases in crops. But is the overall effect negative or positive? This is a question yet to be answered.
More CO2, increase in pests and diseases
Elevated CO2 can increase levels of simple sugars in leaves and lower their nitrogen content. These can increase the damage caused by many insects, who will consume more leaves to meet their metabolic requirements of nitrogen. Thus, any attack will be more severe. Higher temperatures from global warming will mean that more numbers of pests will survive the winter season. Elevated CO2 will help in easier over-wintering of pathogens while higher temperatures will favour thermophilic fungi. Higher temperatures will lead to a poleward spread of many pests and diseases in both hemispheres. This will lead to more attacks over longer periods in the temperate climatic zone
Other possible effects of climate change need to be taken into account. On one hand, warmer temperature lowers the effectiveness of some pesticides but on the other hand, it favours natural enemies of pests and diseases. Thus, depending on the pest or pathogen, elevated CO2 may act in a synergic or opposing manner with higher temperatures. Results of such interactions are difficult to be anticipated. Thus, one is obliged to wait for visual signs of the appearance of a pest or disease for initiating action.
Elevated CO2 levels and higher temperatures will keep changing the composition and duration of infective stages of pests and diseases. The current agromet models for anticipation and control of crop pests and diseases will thus be ineffective.
When to initiate action?
Elevated carbon dioxide and higher temperature may act in a synergic or opposing manner depending on the pest or pathogen concerned. The result of these changes cannot be foreseen as yet and waiting for the visual appearance of a pest or disease to initiate action, is the only remaining option. Organising manual surveys to cover all major crop pests and diseases will be a very costly and nearly impossible.
Under these circumstances, spore and insect trap data can assist in anticipating incidences of pests and diseases. A network spore and insect trapping centres to cover all major irrigated and rainfed crops and their pests and diseases need to be set up. To ensure the network covers major irrigated crops with minimum stations, delineated homogeneous rainfall zones and crop-climate zones need to be taken into account.
Summation of the mean air temperatures above a specified base value, known as accumulated degree-days, has been useful for anticipating the incidence of many crop pests and diseases. To overcome the unsuitability of calendar dates to begin calculations, the concept of Biofix is used. For a pest or a disease, Biofix date can mark the beginning of sustained trapping of insects or disease spores.
Similarly in case of diseases, it is important to note the Critical Disease Level (CDL)—the point in time before which application of fungicides is not required and after which will be ineffective. Thus, for integrated management of pests and diseases using spore and insect trap data, it is necessary to lay down ETL and CDL criteria for important crop pests and diseases respectively.
Even within a season, there are variations in severity of pests and diseases. Thus, mid-seasonal advisories for resuming control action may be needed. In the case of some diseases, the above problem is sought to be overcome through the concept of Disease Severity Value, DSV. In this, after a specified initial period, DSVs are commenced to be accumulated as per a formula and each day a DSV is calculated. The ratings are accumulated to a threshold value that calls for initiation of action.
After the action, DSV is set to zero and the accumulations commenced till they reach an assigned value calling for another spray and the process is repeated till the end of the crop period. If adequate data is available, similar concept of Pest Severity Value, PSV can be developed. PSV values for important crop pests need to be developed. In perennial orchards, the trees are attacked by more than one generation of a pest. Thus, after fixing the first Biofix date and criterion for initial spraying of pesticide, a watch must be kept for increase in number of trap catches to fix subsequent Biofix dates and spraying criteria.
The increase in temperatures will be more at night than during daytime. The higher nocturnal temperature will reduce the duration of Leaf-Wetness and result in lesser disease incidence. Biological control of a pest or disease through an introduction of their natural enemies from other regions will become more effective. Warm temperatures will favour their quick establishment and development.
Forewarnings based on current crop position and expected weather are most effective when there is a time lag between the onset of favourable conditions and the manifestation of pest or disease affliction. Secondly, an initial inoculum in insect or spore traps and the organism's phenological development is amenable to calculation by the accumulated degree days approach above a base value. The effectiveness of forewarnings of pests and diseases is maximal over large mono-cropped areas and minimal in situations of variegated cropping.
In India, most of the farm holdings are not only small but are also fragmented. Formation of farming cooperatives is the only way to consolidate the small holdings to facilitate integrated management of pests and diseases. The governments should act as facilitators in providing incentives for the formation of farming cooperatives. It should also remove legal bottlenecks that hamper the formation of cooperative farms and train farmers in cooperative crop management.
(S Venkataraman is a renowned agrometeorologist. The views are personal.)