Fertilization and pest treatment
Pests in irrigated areas
Pest problems are generally more serious in irrigated areas than in the other environments because of dense planting, high fertilizer use, and high cropping intensity. Disease epidemics and insect pest outbreaks occur in irrigated areas, whereas in other environments endemic problems are more common. Especially the use of nitrogen fertilizers and broad-spectrum insecticides can affect the intensity of pest problems. The key insect pests of irrigated rice are (among others) brown planthopper (Nilaparvata lugens), green leafhopper (Nephotettix virescens), white backed planthopper (Sogatella furcifera), green rice leafhopper (Nephotettix cincticeps) and gall midge (Orselia oryzae) (Bonman et al.).
Weeds in irrigated fields
The floating water fern Azolla is a potent source of organic N fertilizer for increasing rice production. In transplanted rice fields, Azolla pinnata was found to control the growth of some weeds: Cyperus difformis, Cynodon dactylon, Echinochloa crus-galli, and others. Azolla rice dual cropping alone reduced the weed quantity by 50%, green manuring plus dual cropping reduced weeds by 60%. This reduction may be due primarily to the dense mat of Azolla which effectively reduced light available for weed growth. On the other hand, the use of chemical fertilizer increased weed growth (Satapathy and Singh).
Weeds in fertilised fields
The effect of broadcast or band-placed application of fertilizer on the growth of weeds was studied at two locations on low-fertility acid soils in Suriname. Band placement of fertilizers considerably reduced weed growth compared to fertilizer-applied broadcast. Weed growth was also stimulated in planter wheel-tracks (Everaarts). Another field study was conducted to determine the effects of various timings and application methods of nitrogen (N) fertilizer on weed growth and spring wheat yield. Nitrogen application method generally had larger and more consistent effects than application timing on weed growth and wheat yield. Depending on the weed species, the weed seed bank at the conclusion of the 4-year study was reduced by 25 to 63% with point-injected compared with broadcast N application (Blackshaw, R.E., Molnar, L.J. and Janzen, H.)
Tillage practices and pests
Conservation tillage and minimum-tillage practices may increase, decrease, or have no effect on plant diseases. Tillage practices directly influence the properties of the soil, soil moisture and temperature, populations of vectors of plant pathogens etc., and may indirectly influence plant diseases by causing changes in the kind, rate, and time of fertilizer application; pesticide use; plant spacing, irrigation; and other cultural practices. Some tillage practices may have a similar influence on certain pathogens all over the world while other practices may have a quite variable influence depending on the climate and the cropping sequence (Sumner et al.).
Soil, fertilization and pests
Cultural methods such as crop fertilization can affect susceptibility of plants to insect pests by altering plant tissue nutrient levels. The ability of a crop plant to resist or tolerate insect pests and diseases is tied to optimal physical, chemical and mainly biological properties of soils. Soils with high organic matter and active soil biology generally exhibit good soil fertility. Crops grown in such soils generally exhibit lower abundance of several insect herbivores, reductions that may be attributed to a lower nitrogen content in organically grown crops. On the other hand, farming practices, such as excessive use of inorganic fertilizers, can cause nutrient imbalances and lower pest resistance (Altieri and Nicholls). Integrated soil fertility management is a basis for integrated pest management.
The effect of nitrogenous-fertilization on the population dynamics and natural control of rice leaffolders was studied in an irrigated rice area in the Philippines. Herbivores, predators, and parasitoids increased in abundance with nitrogenous-fertilization level. The average density of rice leaffolder larvae at the highest nitrogen level was eight times the density at zero nitrogen level, and the peak percentage of injured leaves increased from 5 to 35%. The strong increase in larval density was due to the positive effect of nitrogenous-fertilization on egg recruitment and survival of medium-sized larvae. The percentage of parasitism of eggs and larvae was not affected by nitrogenous-fertilization. The increase in survival of medium-sized larvae with nitrogen levels was associated with lower predator to leaffolder ratios (Kraker et al.). Pest populations have increased in corn crops since farmers in the highlands of Guatemala abandoned organic fertilization and adopted synthetic fertilizers. Corn in fields treated with organic fertilizer hosted fewer aphids (Rhopalosiphum maidis) than corn treated with synthetic fertilizer. The difference seems attributable to high concentration and total content of foliar nitrogen in corn in the synthetic fertilizer plots (Morales et al.).
References
Altieri, M. A. and Nicholls, C. I., 2003: Soil fertility management and insect pests: harmonizing soil plant health in agroecosystems. Soil and Tillage Research, 72: 203-211
Blackshaw, R.E., Molnar, L.J. and Janzen, H., 2004: Nitrogen fertilizer timing and application method affect weed growth and competition with spring wheat. Weed Science: July 2004, Vol. 52, No. 4, pp. 614-622. http://dx.doi.org/10.1614/WS-03-104R Bonman, J. M., Khush, G. S. and Nelson, R. J., 1992: Breeding rice for resistance to pests. Ann. Rev. Phytopathology, 30: 507-528 Everaarts, A. P. (1992), Response of weeds to the method of fertilizer application on low-fertility acid soils in Suriname. Weed Research, 32: 391–397. http://dx.doi:10.1111/j.1365-3180.1992.tb01900.x Kraker, J. de; Rabbinge, R., Huis, A. van, Lenteren, J. C. van and Heong, K. L., 2000: Impact of nitrogenous-fertilization on the population dynamics and natural control of rice leaffolders (Lep.: Pyralidae). International Journal of Pest Management, 46 (3), 225-235 Morales, H., Perfecto, I., and Ferguson, B., 2001: Traditional fertilization and its effect on corn insect populations in the Guatemalan highlands. Agriculture, Ecosystems and Environment, 84: 145-155 Satapathy, K.B. and Singh, P.K., 1985: Control of weeds by Azolla in rice. J. Aquat. Plant Manage, 23: 40 – 42 http://www.apms.org/japm/vol23/v23p40.pdf Sumner, D. R., Doupnik, B. (jr.) and Boosalis, M. G., 1981: Effects of reduced tillage and multiple cropping on plant diseases. Ann. Rev. Phytopathology, 19: 167-187
