Modifying traditional and high-input agroecosystems for optimization of microbial symbioses: a case study of dry beans in Costa Rica

Optimization of symbioses between plants and microbes has been suggested as method for enhancing nutrient uptake in low-input agroecosystems. In generall symbioses may be maximized through: (1) the selection of plant cultivars responsive to symbiontss; (2) ionoculation with highly efficient strains of symbionts; (3) habitat modification to encourage symbioses. These methods were tested under a traditional, slash mulch, ‘frijol tapado’ en agroecosystem and/or a high-input (‘espequeado’) system to increase nodulation and vesicular-arbuscular mycorrbizae (VAM) of beans with the following results: (1) Under the espequeado system (modified by the exclusion of fertilizer application), four cultivars of indeterminate, vining dry beans (Phaseolus vulgaris) nodulated more than two determinate, bush varieties, especially under low available soil phosphorus (P). VAM levels were not statistically different under these conditions. (2) Under the tapado system, pelletization with rock phosphate and Rhizobium inoculum did not significantly change nodule biomass, dinitrogen (N₂)-fixation or yield. (3) Without P application, neither nodulation nor % VAM were significantly different between the two systems of bean production. However, as applied P increased, differences in %VAM of the two systems became significant, decreasing more sharply with increasing P in the tapado system.The tripartite symbiosis of beans can be influenced by various factors such as cultivar, nutrient application and production system. Although traditional varieties and systems may optimize certain microbial symbioses, the quantity of the symbiosis appears to depend on a number of complex factors. An intermediate agricultural technology, the mixing of traditional and modern, is exemplified here by the application of moderate levels of fertilizer to the tradional, tapado systems. This intermediate approach promises to decrease inputs in comparison to the espequedo system, as well as the need for shift cultivation, but long-term testing is needed.     

Red legs and golden gasters: Batesian mimicry in australian ants

 There are numerous reports of invertebrates that are visual mimics of ants, but no formal reports of mimicry of an ant, by an ant. Two endemic Australian ants, Myrmecia fulvipes and Camponotus bendigensis are remarkably similar in colour and size; both are generally black but have red legs and golden gasters. The density and hue of the pubescence of each ant's gaster are relatively uncommon in ants, but are very rare when combined with the black forebody and red legs. The ants are similarly sized but are smaller than other species closely related to M. fulvipes. The range of C. bendigensis lies entirely within that of M. fulvipes, and both species excavate ground nests in open woodland. Finally, workers of both species are crepuscular and forage solitarily. These data suggest that the relatively benign formicine C. bendigensis is a Batesian mimic of the formidable myrmeciine M. fulvipes. Received: 9 August 1999 / Accepted in revised form: 22 December 1999     

Mechanics and Morphology of Silk Drawn from Anesthetized Spiders

 CO₂ and N₂ anesthetized Nephila spiders produced dragline silk with mechanical properties that differed from control silk as a function of time under anesthesia. Silk from CO₂ spiders had a significantly lower breaking strain and breaking energy, significantly higher initial modulus, and marginally lower breaking stress. At the onset of anesthesia the silk diameter became highly variable. During deep anesthesia silk either became thinner or retained cross-section but fibrillated. Received: 26 August 1999 / Accepted in revised form: 22 December 1999