By mid-April I was near the end of my 8-week stay in Argentina. One of the more enjoyable tasks during this time was to go back out and visit some of the soybean fields that I had seen earlier in the season. I enjoy watching the progression of soybeans over time—both in plant phenology and in the guilds of insects present. Defoliating caterpillars like Rachiplusia nu (oruga medidora) and Anticarsia gemmatalis (oruga de las leguminosas) abound during the late vegetative and early to mid-reproductive stages of growth, feeding day and night on the lush, green foliage. As the days grow shorter and cooler, the soybean fields slowly morph from dark green to tawny-yellow, and leaf-feeding guilds give way to seed-feeding stink bugs like Piezodorus guildinii (chinche de las leguminosas) and Nezara viridula (chinche verde).
Ever present amongst the plant-feeding insects are their natural enemies, with spiders being among the more numerous predators. This small (~10 mm length) crab spider (family Thomisidae) was seen in a soybean field in Santa Fe Province with the plants at R6 stage of growth (pods completely filled). I’m fairly certain it represents Misumenops pallidus based on its close resemblance to the spider in this photo. Piezodorus guildinii stink bugs were especially abundant, and just as the crop of newly hatched nymphs was poised to take advantage of the fat, juicy seed pods, this spider seemed poised and ready to take advantage of the fat, juicy nymphs. In fact, M. pallidus is the most abundant crab spider in soybean agroecosystems in the Humid Pampas of Argentina (Liljesthröm et al. 2002), which as a group comprise nearly half of all spiders in those systems (González et al. 2009). Perhaps one reason for this is their generalist prey selection tendencies, feeding on prey species such as R. nu and P. guildenii when they are abundant and switching to non-pest prey species (except the heavily sclerotized weevils and the large and noxious adults of N. viridula) when they are absent (González et al. 2009).
González, A., G. Liljesthröm, E. Minervino, D. Castro, S. González & A. Armendano. 2009. Predation by Misumenops pallidus (Araneae: Thomisidae) on insect pests of soybean cultures in Buenos Aires Province, Argentina. The Journal of Arachnology 37:282–286.
Liljesthröm, G., E. Minervino, D. Castro & A. González. 2002. La comunidad de arañas del cultivo de soja en la provincia de Buenos Aires, Argentina. Neotropical Entomology 31:197–209.
Copyright © Ted C. MacRae 2012