The seasonal occurrence and spatial distribution of the crambine stem borer, Chilo partellus (Swinhoe) were investigated in maize, Zea mays, and sorghum, Sorghum bicolor, fields at the Kenya coast. During the vegetative growth stage, small-sized larvae were typically found behind leaf sheaths and in whorls in maize and sorghum. In reproductive and senescent maize, small-sized larvae were also found feeding in ears. Later immature life stages were located predominantly in stems in sorghum, and in the stems and ears in maize. Density and variance estimates of small, medium-, and large-sized larvae and pupae in maize and sorghum were fitted to Taylor's power law which provided significant regressions in all cases. The between-plant distribution was aggregated in all life stages. In maize, the distribution remained aggregated through the larval and pupal life stages, but in sorghum the distribution became progressively less aggregated as insects aged. The relationship of C. partellus density to the proportion of plants infested also suggested aggregation. Optimal sample sizes were determined for both binomial and enumerative sampling plans at two levels of precision, and a discussion is included on the relationship of optimal sample sizes to economic injury levels. Additionally, a presence/absence sequential sampling plan is proposed.
Uscana lariophaga Steffan is an egg parasitoid of bruchid species in West Africa. The hosts are Callosobrucbus maculatus (Fabricius) and Bruchidius atrolineatus Pic, insect pests of stored cowpea. Experiments were carried out to study the selection between hosts of different ages and of different species in choice and no-choice situations. Results were obtained through parasitization studies and by direct behavioural observations. A description of the parasitization behaviour of Uscana lariophaga is given. At 30°C, C. maculatus eggs of 0–2 day old are readily accepted for parasitization, but in a choice situation the 0 day old eggs are preferred. Of the older eggs only those of 3 days old are parasitized but significantly less than those of 0–2 days. At 30°C, B. atrolineatus eggs older than 24 hours are significantly less parasitized than younger eggs also in the no-choice situation. When eggs are older than one day, the time needed for penetrating the egg increases with host age. Both bruchid species are suitable hosts for U. lariophaga. In a choice situation B. atrolineatus is the preferred host and especially so when U. lariophaga has been reared on this species. However, in a no-choice situation the highest rate of successful parasitization is obtained on hosts of the species from which they have been reared. The higher parasitization rate of C. maculatus in the field in Niger can be explained by the difference in the susceptible period between the two host species.
and Implications This project examined the feasibility of producing USDA Choice beef-without grain-based finishingthrough genetic selection and pasture management. Purebred Angus heifers with high-marbling potential and small/medium frame size were born spring 2011. Heifers were allotted to either pasture (grass-finishing) or feedlot (grain-fed) treatments based on liveweight and intramuscular fat content. When reaching market weight (±1,000 lb) heifers were harvested and carcass data was collected. Feedlot cattle were marketed on August 27, 2012. The grass-finished cattle were marketed on November 1, 2012. Starting and end weights were similar for both treatments but feedlot cattle reached market weight 50% faster (3.9 vs 2.1 lb/day). The average intramuscular fat percent was not different at the start of the trial but there was a trend of grain-fed cattle to have greater intramuscular fat. Rib eye area, yield grade, and number grading Choice were numerically higher for feedlot cattle. This project demonstrated that it is feasible to combine high marbling genetics with pasture management to produce Choice beef in Iowa. Grass-finished cattle were able to achieve 2.1 pounds of gain per day and 60% of the grass-finished cattle ultimately graded Choice. Selection of small-framed, highmarbling potential beef cattle is essential because of the relatively low-energy density of the grass-based diet and the limited grazing season. Efforts to improve pasture quality and extend the grazing season would be beneficial to meet this goal.
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