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84 Current Perspective to Predict Actual Evapotranspiration

Springer, New York, USA, 1998.

Gabino Alberto Martínez-Gutiérrez

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.68519

#### Abstract

The moisture evaporation process from granular biopesticides (GBs) containing entomopathogenic nematodes (EPNs) has influence in the shelf-life of these biological products, but the approach to design GBs with desired transport properties lacks of theoretical support to get closer in a better way to formulations design of long-term storage. In this chapter we review the state of art in theoretical studies about the physics of the moisture evaporation to elucidate what are the mechanisms of drying of GBs. We found that several external and internal factors influence the transport process of moisture exchange among others phenomenon that happened in a porous media such as GBs; consequently, complex and highly dynamic interactions between medium properties, transport processes, and boundary conditions result in a wide range of evaporation behaviors. The theory of drying process in two stages for porous materials with high moisture content seems to be a good starting point to explore further the drying of GBs at different scales and mechanistic and correlative models of evaporation are available to analyze the desiccation in different stages of the elaboration process, which is also of interest in the subject area of science and technology of the formulation of EPNs.

Keywords: evaporation, entomopathogenic nematodes, storage stability, reservoir

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

## 1. Introduction

Moisture evaporation from porous media is studied by its importance in the drying of foods, building materials and biological products such as biopesticides. The formulation of biopesticides is a process to transform biocontrol agents in a product to exploit their pathogenicity against insect pests. The functions of the formulation oriented towards the biocontrol agents are to improve their storage stability (increased survival time and maintained infectivity), protect them of adverse conditions and potentiate their pathogenicity against insect pests in different developmental stages, which facilitate their transport and use as functions oriented towards the final users [1]. Entomopathogenic nematodes (EPNs) are natural regulators of insect populations and are also applied as biocontrol agents of insect pests in agricultural crops. The infective juvenile (IJ) is the unique free-living stage of EPN; typically, they dwell in the soil, their natural reservoir, until they are able to infect an insect to resume their development and reproduction [2]. The IJs have many strategies to survive when they are subjected to adverse abiotic factors. Particularly, under low moisture conditions in soil, as in drought, they experiment a desiccation process that, when happens as a gradual moisture reduction, allows them to diminish their metabolism gradually until they reach an anhydrobiosis state (life without water) in which they are capable to survive for many years waiting for better moisture conditions to continue their life cycle [2]. Regarding this survival strategy, the granular biopesticides (GBs) as granules or pellets were designed to replicate the desiccation regime of IJs, to gradually arrest their metabolism and to extend their storage stability at room temperature [1].

Each GB can contain up to 3 105 IJs and compared with the conventional aqueous suspension it is a better media for long-term storage of IJs at moderate temperature. The main phenomenon that governs the functional performance of the GBs is that if the rate of water reduction from their structure is optimal, the IJs are properly desiccated at a suitable rate and their storage stability is increased [1]. In laboratory, it has been found that the removal of moisture content from pellets by evaporation is well related to the survival of the IJs (Pearson r = 0.725) and infectivity on Galleria mellonella larvae (Pearson r = 0.904), suggesting that the relationship between the pellet drying rate and the storage stability may be an important factor to improve the shelf life of GBs containing Steinernema glaseri IJs [3].

On the one hand, the rate of water reduction of IJs to survive to desiccation is influenced by their size, energy reserves, metabolism, genetic, historical adaptations, origin and other characteristics that are unique to each EPN species and even strains. Also, as a living entity, they receive stimulus from the ambient biotic and abiotic factors. Knowledge of EPNs biological fitness is needed for their optimal formulation, particularly their desiccation tolerance (survival under water evaporation at low relative humidity or hypertonic osmotic conditions) and also to evaluate the possibility of improvement through pre-adaptation, selective breeding, genetic engineering or others methods [4]. On the other hand, the drying process of the formulated must be optimal to induce the IJs into an anhydrobiotic state. To produce an EPN product with best performance, a proper selection of carrier's materials, adjuvants, elaboration process to produce them and a certain combination of ambient storage conditions that allow the evaporation of enough moisture from the GB at a rate reduction particularly suitable to gradually remove the water from the IJ's body is required [1, 5]. However, it is worth mentioning that the materials selection and its combination to achieve the desired drying characteristics of the GBs are carried out by the formulator in an iterative way until the proper solution is found.

1. Introduction

86 Current Perspective to Predict Actual Evapotranspiration

temperature [1].

Moisture evaporation from porous media is studied by its importance in the drying of foods, building materials and biological products such as biopesticides. The formulation of biopesticides is a process to transform biocontrol agents in a product to exploit their pathogenicity against insect pests. The functions of the formulation oriented towards the biocontrol agents are to improve their storage stability (increased survival time and maintained infectivity), protect them of adverse conditions and potentiate their pathogenicity against insect pests in different developmental stages, which facilitate their transport and use as functions oriented towards the final users [1]. Entomopathogenic nematodes (EPNs) are natural regulators of insect populations and are also applied as biocontrol agents of insect pests in agricultural crops. The infective juvenile (IJ) is the unique free-living stage of EPN; typically, they dwell in the soil, their natural reservoir, until they are able to infect an insect to resume their development and reproduction [2]. The IJs have many strategies to survive when they are subjected to adverse abiotic factors. Particularly, under low moisture conditions in soil, as in drought, they experiment a desiccation process that, when happens as a gradual moisture reduction, allows them to diminish their metabolism gradually until they reach an anhydrobiosis state (life without water) in which they are capable to survive for many years waiting for better moisture conditions to continue their life cycle [2]. Regarding this survival strategy, the granular biopesticides (GBs) as granules or pellets were designed to replicate the desiccation regime of IJs, to gradually arrest their metabolism and to extend their storage stability at room

Each GB can contain up to 3 105 IJs and compared with the conventional aqueous suspension it is a better media for long-term storage of IJs at moderate temperature. The main phenomenon that governs the functional performance of the GBs is that if the rate of water reduction from their structure is optimal, the IJs are properly desiccated at a suitable rate and their storage stability is increased [1]. In laboratory, it has been found that the removal of moisture content from pellets by evaporation is well related to the survival of the IJs (Pearson r = 0.725) and infectivity on Galleria mellonella larvae (Pearson r = 0.904), suggesting that the relationship between the pellet drying rate and the storage stability may be an important factor

On the one hand, the rate of water reduction of IJs to survive to desiccation is influenced by their size, energy reserves, metabolism, genetic, historical adaptations, origin and other characteristics that are unique to each EPN species and even strains. Also, as a living entity, they receive stimulus from the ambient biotic and abiotic factors. Knowledge of EPNs biological fitness is needed for their optimal formulation, particularly their desiccation tolerance (survival under water evaporation at low relative humidity or hypertonic osmotic conditions) and also to evaluate the possibility of improvement through pre-adaptation, selective breeding, genetic engineering or others methods [4]. On the other hand, the drying process of the formulated must be optimal to induce the IJs into an anhydrobiotic state. To produce an EPN product with best performance, a proper selection of carrier's materials, adjuvants, elaboration process to produce them and a certain combination of ambient storage conditions that allow

to improve the shelf life of GBs containing Steinernema glaseri IJs [3].

Actually, both research lines (EPN biological fitness and optimal drying of GBs) are ongoing, but the second is less common, although the major factor to enhance EPN longevity and, perhaps, increase the range of applications, given the inherent limitations of EPNs survival ability, it is likely to be the improvement of the formulation [4] and theoretical literature concerning energy and mass transfer is extensive. In this regard, if the mechanisms of transport of moisture, temperature and oxygen in these GB were understood and focused on the transport properties of the materials formulation based on the materials science approach, the extensive empirical knowledge in formulation of EPNs would be complemented with theoretical support. It is suitable for the design of these biological products because it can help to build mathematical models that describe the migration patterns of moisture, the thermodynamic equilibrium and exchange of oxygen [6], and would serve for analytical solving of design problems. Afterwards, several methods could be tested to optimize the moisture removal in the formulated product. Also, modelling could help to avoid too long, unnecessary or expensive experiments on the EPN-GB system.

Recently, it has been found that the drying kinetics of pellets is reproduced by a surface evaporation model [7] with a percent relative average deviation value of 21.95%; consequently, there is room for improvement through the proposed simplifying assumptions, but they need to be determined experimentally and expressed in mathematical form to feedback the model [3]. But, to improve this theoretical approach, it is necessary as first step to understand the water migration by evaporation from GBs towards the surrounding atmosphere and its variation factors. Hence, the objective of the present work is to make a review and discuss the state of the art in theoretical studies about the physics of moisture evaporation process from porous media to elucidate what are the basic mechanisms of moisture migration from GB, which is of paramount interest in the subject area of science and technology for the formulation of EPNs.
