Jon C. Allen

A model of the daily photosynthetic rate in a tree

Abstract A theoretical model of the daily net photosynthetic rate (kg glucose/day) of a tree is formulated as an analytical function of certain agricultural and meteorological variables. The experimental, single-leaf rate (kg glucose/m 2 leaf area - day) is integrated over both the tree canopy and the photoperiod for spherically and cubically shaped canopies. Consideration is given to cloud cover and mutual shading, one tree by another. Results are analyzed with respect to the efficiency of light energy conversion by a tree and variations in tree canopy height and shape. Quantitative comparisons with experimental data are made, primarily for the case of orange trees. Proportions of leaf photosynthate allocated to various tree parts are generated.

The thermal environment of the citrus rust mite

Abstract The distribution of citrus rust mite on individual fruit and in the whole citrus tree suggests an avoidance of solar exposure. A model of fruit temperatures as affected by solar radiation in different parts of the tree was constructed, and its predictions are compared with field temperatures and rust mite distribution patterns. The fruit temperature model is a function of latitude, time of year, time of day, fruit diameter and reflectance, position in the tree, atmospheric transmissivity and wind velocity. Where model predictions indicate unfavorable conditions, rust mite population levels were low. When the tree was divided into north-south, top-bottom quadrants, the north bottom quadrant had the most favorable temperatures and usually the most rust mites; the south bottom was also favorable and had high mite densities. The south top quadrant was least favorable often having temperatures in the lethal range, and it also had the lowest rust mite populations.

Principles for Conduct of Pest Risk Analyses: Report of an Expert Workshop

The North American Free Trade Agreement (NAFTA) and the General Agreement on Tariffs and Trade (GATT) have focused attention on risk assessment of potential insect, weed, and animal pests and diseases of livestock. These risks have traditionally been addressed through quarantine protocols ranging from limits on the geographical areas from which a product may originate, postharvest disinfestation procedures like fumigation, and inspections at points of export and import, to outright bans. To ensure that plant and animal protection measures are not used as nontariff trade barriers, GATT and NAFTA require pest risk analysis (PRA) to support quarantine decisions. The increased emphasis on PRA has spurred multiple efforts at the national and international level to design frameworks for the conduct of these analyses. As approaches to pest risk analysis proliferate, and the importance of the analyses grows, concerns have arisen about the scientific and technical conduct of pest risk analysis. In January of 1997, the Harvard Center for Risk Analysis (HCRA) held an invitation-only workshop in Washington, D.C. to bring experts in risk analysis and pest characterization together to develop general principles for pest risk analysis. Workshop participants examined current frameworks for PRA, discussed strengths and weaknesses of the approaches, and formulated principles, based on years of experience with risk analysis in other setting and knowledge of the issues specific to analysis of pests. The principles developed highlight the both the similarities of pest risk analysis to other forms of risk analysis, and its unique attributes.

Dislodgeable residues of ethion in Florida citrus and relationships to weather variables

Five different treatments of ethion on Valencia orange trees were compared for decay rates and for ethion monoxon and ethion dioxon production under different environmental conditions. The oxon metabolite levels observed were low and always below the residue level of ethion itself. There were no significant differences in the decay rates of ethion between treatments.A model of ethion decay utilizing environmental variables as a time base is presented. This model explains 94% of the variation observed in ethion decay during very wet and very dry periods in Florida.The application of these results and general experimental approach to worker safety reentry standards is discussed.

An age-structured population model of citrus rust mite: a fruit–mite–fungal pathogen system

An age-structured model of a fruit‐mite‐fungal pathogen system was developed to study interactions between the ‘Hamlin’ orange fruit, the citrus rust mite (CRM) Phyllocoptruta olei6ora, and its fungal pathogen Hirsutella thompsonii. The model consists of a set of difference equations incorporating the age and stage change of CRM and its fungal pathogen. The abiotic factors included in the model were daily mean temperature and daily dew period the biotic factors were mite density (egg, protonymph, deutonymph, and adult), pathogen density (latent pathogen, and infectious pathogen), and undamaged fruit surface area. One data set for the fruit‐mite‐pathogen dynamics was used to estimate parameters related to density-dependent CRM egg production and pathogen transmission rate. Parameters were estimated by choosing the parameter combinations which gave the least combined error sum of squares between observed and simulated populations for CRM and pathogen. For the data set used for parameter estimation, the model captured 92.2% of variation in CRM population dynamics, and 61.9% of variation in the density curve pattern of the pathogen, over a period of 5 months. Three additional data sets were used to partially test model predictability. For one data set, the model captured 87.1% of variation in CRM population dynamics, and 94.4% of variation in the density curve pattern of the pathogen, over a period of 5 months. The model, however, did not make accurate predictions for low CRM density level-pathogen interactions. The corresponding cumulative mite days and fruit surface damage were also generated by the model for each data set. With further modification and refinement, the model could be developed into a useful tool for CRM management.

FUNCTIONAL RESPONSE, REPRODUCTIVE FUNCTION AND MOVEMENT RATE OF A GRAZING HERBIVORE: THE CITRUS RUST MITE ON THE ORANGE

A model of the attack rate for a grazing herbivore, the citrus rust mite, is obtained as a Type 2 functional response analogous to Hollings disc equation. The attack rate is given by A@? = cx@? 1 + ct@