ABSTRACT
The agricultural sky is abode to a wide range of biotic factors that potentially affect global crop production trends. Among them, our discussions in this chapter confine to aerosol microbes, insects, and Aves. They are important to farming enterprises. The focus here is on biotic aspects of lower atmo-sphere close to crop’s canopy. Microbes traced in the atmosphere influence crops in several ways. Microbes occur on the surface of soil/crop’s canopy. Investigation of air samples in the atmosphere indicates that diverse species of microbes are distributed at different population densities, at altitudes up to 13–15 km in the troposphere. Microbes are also found in the stratosphere up to 41–74 km altitude from the crop’s canopy. The atmosphere with its harsh environment is said to be home to microbes that are versatile. They withstand relatively harsh environment. Molecular methods of detection of microbes utilizing rRNA suggest that microbes are traceable even in space. Microbes may descend on earth’s surface (on crops) periodically. Like soil, atmosphere, particularly, at low altitudes above crop’s canopy too harbors diverse flora of microbes. We need to decipher in detail their influence on crops. We should aim at manipulating aerosol microbes to our advantage if they are beneficial. We should curb those microbes that cause diseases, epidemics/pandemics on staple crops. Of course, several microbial species could be commensals. Bioaerosol microbes could include bacteria, fungi, viruses, archaea, algal cells, lichen fragments, protozoans, etc. Clouds may be a good medium for aerosol microbes to survive, perpetuate, and get dispersed. Microbes have a role in enucleation and formation of snow, frost, and fog. Microbes found on smog/organic particulate matter may aid 212biological transformation of nutrients. Airborne microbial species transit long distances along high-altitude winds and jet streams. Microbes are capable of intercontinental dispersal. Microbes transit via dust storms, clouds, precipitation, and wind in the atmosphere above agrarian regions. For example, aeolian dust that moves from Sahara/Sahel into North American agrarian carries microbes. The agricultural sky is an important region with regard to the initiation and spread of crop disease pandemics. Airborne microbes include several species that cause devastating crop diseases. The propagules such as spores, hyphae, and cells may get disseminated via intercontinental dust movement. The turbulence of low wind distributes pathogenic microbes only for shorter distance. Whereas high altitude jet streams carry spores of disease-causing fungi, such as Puccinia graminis tritici (PgT), to long distances of over 15,000 km. Often such winds are responsible for the initiation and dispersal of pandemics. Several examples pertaining to epidemic/pandemics on crops. We may note that agrarian sky has been devastating farms and distributing catastrophic disease, periodically. Perhaps agrarian sky has been more dangerous to crops than the subsoil region of cropped fields. Pathogenic microbes are distributed by vectors. Particularly, insect species like aphids, flies, and bugs that hover above crop canopy. Long-distance transport of disease microbes is possible. Climate change is an added factor that farmers have to negotiate. The climate change effects could affect crops plus the disease-causing aerosol microbe to different extents. It could enhance virulence of epidemic/pandemic causing fungal pathogen or may reduce it. Phyllosphere is a large aerial surface. It supports microbes of diverse physiological capabilities. Phyllosphere microbes aid biochemical transformations nutrients traced of foliage. Phyllosphere microbes may also interact with microbial pathogens that invade crops via the leaves. Several microbes are known to reduce disease spread by suppressing crop pathogens. Phyllosphere microbes may also include asymbiotic biological nitrogen fixers and plant growth-promoting bacteria. Studies on insect distribution in the atmosphere above crops began during the 1920s. Aerospace from 200 to 16,000 m altitude supported insect species and their eggs floating in the air. The agricultural sky supports several insect pests that attack crops. Cereal aphids such as Russian aphid (Diuraphis noxia), green bugs, pod borers (Helicoverpa armigera), leaf eaters, cut worms, boll eating bugs, etc., are commonly traced in the atmosphere above crops. Locusts are migratory pests. Locust (short-horned grasshoppers) spread is an important aerial phenomenon in certain parts of the world. Locusts move as clouds of billions of insects and initiate epidemics/pandemics. Locusts destroy large areas of 213natural vegetation and crops in a short period. Aerial insects may act as predators of insect pests. They are useful biological control agents. Insects like honeybees, moths, butterflies, vasps, bugs act as important pollinators and induce good seed set. Honeybees in particular produce honey. Aves (birds) are important components of agrarian sky. There are several bird species that could be pests of crops (e.g., weaver birds, red-winged blackbird, finches, sparrows, rock pigeons, parakeets, etc.). The predatory species (e.g., falcons, raptors) are helpful in reducing insect pest and other pest bird species. Birds are important biological control agents. Birds are also efficient pollinator species. A few crops like sunflower depend on bird-mediated pollination for good seed set. Bird scaring using aerial vehicles like Helikites is new to agrarian sky. Eradication of bird pests through chemicals is generally not an option. Diversion of birds by removing nesting zones near crop fields is practiced. Birds aid in dispersal of seeds. So, they could induce volunteers, weeds, and other undesirable plant species in farms. Birds that flock the aerospace in flocks also offer “bird refuse” called as “Guanos.” Guanos are rich in nutrients needed by crops.
