A. P. Gutierrez

Pests as a Common Property Resource: A Case Study of Alfalfa Weevil Control

Recently, problems in pest management have attracted considerable attention in economic literature. This paper describes an economic optimization model that incorporates detailed biological input specific to the alfalfa weevil. The problem is examined from a societal point of view and recognizes specific common property characteristics of the pest. The major discrepancy between the results obtained here and current practices involves the timing of pesticide application. These results indicate the advantage of applying pesticide prior to the growth season, specifically the time period in which the number of adult pests reaches its peak. In the long run each farmer is affected by the cumulative effects of individual decisions. The pest constitutes a common property resource, and a nonregulated market would not yield the optimal solution. The optimal societal solution could be implemented by an information agency, which would enhance cooperation between farmers.

A demographic model of assimilation and allocation of carbon and nitrogen in grapevines

Wermelinger, B., Baumg~rtner, J. and Gutierrez, A.P., 1991. A demographic model of assimilation and allocation of carbon and nitrogen in grapevines. Ecol. Modelling, 53: 1-26. A dynamic crop model is presented for grapevine (Vitis vinifera L.) dry matter and nitrogen (N) assimilation and allocation with distributed developmental times and age structures of the plant subunits. The model forms a basis for future analyses in the vineyard ecosystem. A flowchart of the daily computations is presented and the corresponding mathematical structure is described. In the model, the plant is divided into annual populations of fruit, leaves, shoots and roots which develop on a perennial frame. In general, these populations are age-structured and have the attributes of numbers, dry matter and N mass, and their dynamics are simulated as a time-invariant distributed delay process with attrition. Growth occurs per degree-day above the developmental threshold of 10°C. The seasonal N dynamics is the net result of the processes of new tissue formation with high N concentrations and the constant proportional export of N from ageing parts to reserves. The latter process commences immediately after tissue formation. The mean concentration of N in a population of plant subunits is determined by the ratio between young N-rich zones and ageing N-exporting tissues. Simulations were used to assess the patterns and magnitudes of photoassimilate allocation to the three sinks: maintenance respiration, reproductive growth and vegetative growth. In summer, roughly one-third of the assimilate available daily is allocated to each.

Why Do Some Bt-Cotton Farmers in China Continue to Use High Levels of Pesticides?

China was the first developing country to introduce Bt cotton on a large scale. This paper provides an indepth economic analysis of Bt cotton production by small-scale farmers in China. Data were collected in 2002 in Linqing County, in Shandong Province and comprised a season-long cotton production monitoring with 150 farmers and complementary household interviews. For quality assessment, the Bt toxin concentration of the various Bt varieties used by the farmers was determined for each plot. All farmers were growing insect resistant Bt cotton varieties. Yet, they sprayed high amounts of chemical insecticides, out of which 40% were extremely or highly hazardous. The paper reviews methodological issues inherent to impact assessment of crop biotechnology and identifies market and institutional failure as possible reasons for continued high pesticide use. Using the damage function methodology the coefficients for both damage control inputs, i.e., Bt varieties (measured as toxin concentration), and insecticide quantity were not significantly different from zero. Results show that absence of enabling institutions and lack of farmer knowledge can considerably limit the benefits of Bt cotton for small-scale farmers. The paper points out the importance to include the institutional conditions in the evaluation of agricultural biotechnology in developing countries.

On the optimal allocation of pesticides with increasing resistance: The case of alfalfa weevil

Abstract The problem of pesticide application under increasing pesticide resistance is explored. A theoretical model is developed to determine optimal pesticide use. This allocation is compared to the laissez-faire solution and to the centralized solution with incomplete information about pesticide resistance. The methodology is then applied in a case study on the Egyptian alfalfa weevil in California.

Effect of Bt-toxin (Cry1Ac) in Transgenic Cotton on the Adult Longevity of Four Heteropteran Predators

Abstract The management of agroecosystems affects intricately linked assemblages of organisms, and nontarget species are not necessarily unimpacted. We examined the effect of Bt-cotton and of lepidopteran prey (Spodoptera exigua Hübner) that had ingested it on the adult survivorship of four important heteropteran predators of cotton pests. Longevity significantly decreased for Orius tristicolor White and Geocoris punctipes Say (by 28 and 27% of the control value, respectively), whereas no effect was found for Nabis sp. and Zelus renardii Kolenati. This finding contrasts with the results of previous studies in which Orius spp. and G. punctipes were either fed only plant material or nonlepidopteran prey. S. exigua is a lepidopteran with low susceptibility to the Bt toxin expressed in cotton and therefore exemplifies the possible effect on predators of lepidopteran pests that would become resistant to Bt. The importance of Bt toxin type, the difference between plants and prey and between different prey species as routes of ingestion of Bt toxins, and the need for studies assessing the population and ecosystem-level effects of Bt cotton are discussed.

Analysis of Biological Control of Cassava Pests in Africa. I. Simulation of Carbon, Nitrogen and Water Dynamics in Cassava

SUMMARY (1) A model for the growth and development of cassava, Manihot esculenta Crantz, as modified by weather, soil nitrogen and water is described, and used in later papers to evaluate damage by exotic cassava pests and the effectiveness of control by natural enemies. (2) Analogous relationships between trophic level models at the population and per capita levels were exploited to develop a model that has a common mathematical form across trophic levels. (3) The distributed delay model of Manetsch (1976) was used as the conceptual shell in describing the population dynamics of populations and of organism organ levels (e.g. leaves of plants, ova in animals). (4) The Frazer-Gilbert functional response model from animal ecology was modified to predict rates of photosynthesis and nitrogen and water uptake in plants. (5) Dry-matter partitioning in cassava as affected by solar radiation, temperature, and water and nitrogen stress was simulated and compared with three sets of field data.

Biological and economic foundations of renewable resource exploitation

Abstract A physiologically based population dynamics model of a renewable resource is used as the basis to develop a model of human harvesting. The model incorporates developing technology and the effects of market forces on the sustainability of common property resources. The bases of the model are analogies between the economics of resource harvesting and allocation by firms and adapted organisms in nature. Specifically, the paper makes the following points: (1) it shows how economic and ecological theories may be unified; (2) it punctuates the importance of time frame in the two systems (evolutionary versus market); (3) it shows, contrary to prevailing economic wisdom, how technological progress may be detrimental to resource preservation; (4) it shows how the anticipated effects of high discount rates on resource use can be catastrophic when synergized by progress in harvesting technology; (5) it suggests that increases in efficiency of utilization of the harvest encourages higher levels of resource exploitation; and (6) it shows the effects of environmental degradation on consumer and resource dynamics. The model leads to global implications on the relationship between economic growth and the ability of modern societies to maintain the environment at a sustainable level.

Effects of crop diversification levels and fertilization regimes on abundance of Brevicoryne brassicae (L.) and its parasitization by Diaeretiella rapae (M'Intosh) in broccoli.

1 The effects of intercropping via competition on crop yields, pest [cabbage aphid Brevicoryne brassicae (L.)] abundance, and natural enemy efficacy were studied in the Brassica oleracea L. var. italica system.

Analysis of biological control of cassava pests in Africa. III: Cassava green mite Mononychellus tanajoa

SUMMARY (1) The interactions of cassava green mite, Mononychellus tanajoa (Bondar) s.l., and cassava, Manihot esculenta Crantz, under West African weather and soil conditions were examined using a simulation model. (2) Field studies had demonstrated the effects of plant and leaf age and rainfall-induced mortality on M. tanajoa population growth. In the absence of effective natural enemies, the model explains, in order of importance, the effects of rainfall, drought stress via the host-plant, food availability (production and persistence of new foliage) and leaf quality (N concentration) on M. tanajoa population growth. (3) Rainfall-induced mortality greatly reduced M. tanajoa populations in the rainy season, drought and N stresses acting indirectly via food availability being most important in the dry season. (4) The combined effects of M, tanajoa feeding and water and N stress on cassava tuber yield were assessed.

Limits to the potential distribution of light brown apple moth in Arizona–California based on climate suitability and host plant availability

The highly polyphagous light brown apple moth (LBAM) (Epiphyas postvittana (Walk.): Tortricidae) is indigenous to Australia and was first found in California in 2006. It is currently found in 15 coastal counties in California, but nowhere has it reached outbreak status. The USDA projects the geographic range of LBAM will include much of Arizona and California and the southern half of the US, which together with economic estimates of potential crop losses have been used as the rationale for an eradication program in California. We report a temperature-driven demographic model to predict the likely distribution and relative abundance of LBAM using the detailed biology reported by Danthanarayana and colleagues, and climate data from 151 locations in California and Arizona for the period 1995–2006. The predictions of our model suggest that the near coastal regions of California are most favorable for LBAM, the northern Central Valley of California being less favorable, and the desert regions of Arizona and California being unfavorable. The model also predicts that LBAM populations can develop at two of the hottest locations in SE Australia where it is has long been known to occur. This reassessment of the potential distribution of LBAM in California and Arizona suggests that its likely ecological and economic impacts would be less than previously assessed by USDA and that its current pest status warrants re-evaluation.