Integrated pest management in the small farmer's maize crop in Nicaragua.

Maize, the main food crop in Nicaragua, is produced by a large group of small landowners, who farm under constraints of land tenure, marginal soils, poor infrastructure and inadequate production services (credit, technical assistance, marketing). Rural development plans, designed to raise the peasant above his low subsistence level, encourage the use of new varieties, fertilizer and pesticides to increase the low yields. However, as pesticides can have severe health, socioeconomical and ecological implications, particularly for the small farmer, they should be used judiciously. This is attempted by a program of integrated pest management. Among the destructive maize insects two moth species, viz. the whorl defoliator Spodoptera frugiperda and the stalk borer Diatraea lineolata, are the most important, but a well-considered control strategy is lacking. An experimental approach and an extensive literature review aimed at developing an integrated pest management program, taking into consideration that the biophysical and socio- economical conditions, and the agronomic practices of the small farmer often differ greatly from those at research stations. In various experiments S. frugiperda reduced yields by 30 to 60 per cent. However during the two to three weeks after emergence, plants proved to be almost insensitive to whorl injury by either S. frugiperda or artificial defoliation. The reason for the reported losses of yield as a result of damage at this stage of development is because plants are eliminated by larvae feeding on the meristematic tissue of the bud. This loss can be compensated for by sowing at higher densities and thinning the infested and least vigorous plants two to three weeks after emergence. Therefore chemical control during early plant development can generally be avoided, giving full scope to natural mortality factors. These are for S. frugiperda braconid parasites, mainly Rogas laphygmae and Chelonus insularis, the predacious earwig Doru taeniatum and heavy rainfall, and for D. lineolata the egg parasite Trichogramma pretiosum. During later growth stages of maize S. frugiperda is heavily attacked by tachinids, mainly Lespesia archippivora, the parasitic nematode Hexamermis sp. and several species of insect predators. These natural enemies however, are often not able to keep the pest below the level of economic injury. Control measures should be taken when 20 per cent of the whorls are injured by S. frugiperda. To preserve the beneficial fauna, the spraying of liquid insecticides should be avoided and granules or insecticide baits (e.g. mixtures with sawdust) should be applied instead, at low concentrations, to the whorls or to the injured whorls only (the latter therapeutic method, favoured by the small farmer, seems promising). To preserve the parasitic nematode of S. frugiperda soil treatments with insecticides which are also nematicides should be prevented. No information was available of the damage to maize by D. lineolata. The assessment of field crop losses by the borer is complicated, as the infestation coincides with that of S. frugiperda and the separation of both types of damage is extremely difficult. The control of S. frugiperda for example, can cause a heavier attack of D. lineolata, possibly by oviposition of the moth on the enlarged leaf area of treated plants (oviposition by D. lineolata on maize up to silking, was proportional to the green leaf area). Therefore maize was artificially infested in cages. Results were statistically analysed by two independent methods: per cage and per plant. Yield losses ranged between three and six per cent per borer per plant. The plant was most sensitive to injury of the lowest internodes. Chemical control of D. lineolata seems not to be justified because 1. field infestation is generally low, 2. egg masses are small and too inconspicuous for easy pest scouting and 3. insecticide applications are mostly ineffective. Therefore other control methods need to be emphasized such as stalk and stubble burning in the dry season. Droughts are frequent in Nicaragua and under this soil moisture stress the control of S. frugiperda did not increase yields. The application of NPK fertilizer stimulated the attack by S. frugiperda and D. lineolata. Yield was only increased by fertilizer use, when combined with chemical protection of the whorl. Maize-bean intercropping is common practice in Latin America and has many agronomic and socio-economic advantages for the small farmer. It lowers the injury by both S. frugiperda and D. lineolata when compared with a monoculture. Less plants were colonized by first instar larvae of S. frugiperda, as the larvae, when dispersed by the wind, are probably trapped by bean plants; oviposition by D. lineolata is probably reduced. Grass weeds, mainly Digitaria sp. and Eleusine indica, may contain high numbers of S. frugiperda larvae forming a potential threat to maize. Oviposition on maize was significantly larger in plots with weeds than in weeded plots. The introduction of exotic parasites should be considered i.e. for S. frugiperda the egg parasite Telenomus remus, and for D. lineolata the braconid Apanteles flavipes, and the tachinids Paratheresia claripalpis (Peruvian strain) and Lixophaga diatraeae. In the Central American region a resistance breeding program in maize should be established, that concentrates on S. frugiperda, utilizing the identified resistant material of CIMMYT. Also for the development of other integrated pest management methods the Central American countries could greatly benefit from an interinstitutional coordination of efforts. An integrated pest management program is best adapted to the small farmer's needs if it is a part of an interdisciplinary crop strategy and if research and extension closely cooperate.