2. Problem statement of moisture evaporation from granular biopesticides

The pellets are solid particles in the form of spherical granules of 10–20 mm in diameter (Figure 1). Drops of a suspension of nematodes are dropped on a powdery material to entrap the IJs into a solid matrix, which results in the formation of the granules or pellets. The powder material may be one, or a combination of, several different carrier materials and adjuvants to produce GBs with the desired characteristics [1, 5]. Experimental wettable powder formulations have been reported to include inert powders such as talc, sand, diatomaceous earth and various clays. Natural ingredients are often used in formulations in order to maintain the

Figure 1. Various diatomaceous earth pellets containing S. glaseri IJs.

environmentally "green" concept associated with biopesticides. Polymers are a usual material for pelletisation of EPNs and often include natural carbohydrate and/or protein polymers such as starch, sodium alginate, acacia gum, lignin and gelling agents amongst others [6, 8, 9].

The nematodes can be divided into two groups, slow-dehydration strategists and fastdehydration strategists, depending upon the rates of water loss in the environment that they will survive. S. carpocapsae IJs survive well at high rate of water removal and S. glaseri IJs survive better at low rate of water removal [10, 11]. Therefore, the major cause of nematode survival to desiccation in anhydrobiosis is the controlled rate of water removal from the IJs [12]. Therefore, the quiescent state of IJs and their shelf life in GB are strongly influenced by moisture content [12–14].

In laboratory, the GBs are stored in closed room where evaporation occurs in calm air, and they are subject to artificial variations to replicate the shelf storage conditions and to test their storage stability over time (temperature from 20 to 30C, relative humidity of 0–100% and no wind-flow present). It has been observed that the initial moisture content of GBs containing EPNs is usually between 40 and 100%, from which a part can be removed faster in few days or slowly in various months. In the most successful case, S. carpocapsae IJs survived up to 7 months (Figure 2) and maintained their infectivity on G. mellonella above 70% in water dispersible granules (WDG) stored at 25C, whose moisture is reduced at a rate of 8.26 <sup>10</sup><sup>7</sup> % s<sup>1</sup> [13, 15]. In the less successful case S. glaseri IJs survived 8 days and maintained its infectivity on G. mellonella above 80% in diatomaceous earth pellets (Figure 3) stored at 23 <sup>3</sup>C, whose rate of pellet moisture reduction is 6.134 <sup>10</sup><sup>5</sup> % s<sup>1</sup> [3]. This reduced shelf life at room temperature limits their commercial exploitation, especially if GBs contain S. glaseri IJs and if these are compared with the long shelf life of commercial chemical pesticides.

Moisture Evaporation from Granular Biopesticides Containing Quiescent Entomopathogenic Nematodes http://dx.doi.org/10.5772/intechopen.68519 89

Figure 2. Mean survival of S. carpocapsae IJs and moisture content of the WDG formulation stored at 25C, reported in [13].

environmentally "green" concept associated with biopesticides. Polymers are a usual material for pelletisation of EPNs and often include natural carbohydrate and/or protein polymers such as starch, sodium alginate, acacia gum, lignin and gelling agents amongst others [6, 8, 9].

Figure 1. Various diatomaceous earth pellets containing S. glaseri IJs.

88 Current Perspective to Predict Actual Evapotranspiration

The nematodes can be divided into two groups, slow-dehydration strategists and fastdehydration strategists, depending upon the rates of water loss in the environment that they will survive. S. carpocapsae IJs survive well at high rate of water removal and S. glaseri IJs survive better at low rate of water removal [10, 11]. Therefore, the major cause of nematode survival to desiccation in anhydrobiosis is the controlled rate of water removal from the IJs [12]. Therefore, the quiescent state of IJs and their shelf life in GB are strongly influenced by

In laboratory, the GBs are stored in closed room where evaporation occurs in calm air, and they are subject to artificial variations to replicate the shelf storage conditions and to test their storage stability over time (temperature from 20 to 30C, relative humidity of 0–100% and no wind-flow present). It has been observed that the initial moisture content of GBs containing EPNs is usually between 40 and 100%, from which a part can be removed faster in few days or slowly in various months. In the most successful case, S. carpocapsae IJs survived up to 7 months (Figure 2) and maintained their infectivity on G. mellonella above 70% in water dispersible granules (WDG) stored at 25C, whose moisture is reduced at a rate of 8.26

% s<sup>1</sup> [13, 15]. In the less successful case S. glaseri IJs survived 8 days and maintained

% s<sup>1</sup> [3]. This reduced

its infectivity on G. mellonella above 80% in diatomaceous earth pellets (Figure 3) stored

shelf life at room temperature limits their commercial exploitation, especially if GBs contain S. glaseri IJs and if these are compared with the long shelf life of commercial chemical

at 23 <sup>3</sup>C, whose rate of pellet moisture reduction is 6.134 <sup>10</sup><sup>5</sup>

moisture content [12–14].

<sup>10</sup><sup>7</sup>

pesticides.

Figure 3. Mean survival of S. glaseri IJs and moisture content of the diatomaceous earth pellet formulation stored at 23 3C and high RH (96%), reported in [3].

It is thought that the desiccation process of IJs happened by slow absorption of the aqueous suspension containing them for the carrier material, starting a reduction process of moisture, which developed at an appropriate rate, and diminished the metabolism of the IJs [5]. In fact, 52% of the variation in its survival rate is explained by the behaviour of the moisture content of the DE pellet, whereas 84% of the variation in infectivity on G. mellonella is explained by the survival of S. glaseri IJs in diatomaceous earth pellets [3]. The hypothesis is that the sudden death of the IJs formulated under these conditions is due to the diffusional migration of water molecules surrounding the IJs to the DE pellet surface followed by the contact of the DE particles with the nematode cuticle which absorbs moisture faster from the IJ's cells [3]. Also as it can be observed in Figure 4, the behaviours of drying kinetics of diatomaceous earth pellets are different with or without S. glaseri IJs.

Recent inspection of cross-sectional area of diatomaceous earth pellets using scanning electron microscopy showed particles as plate-form and non-uniform pore distribution that form a complex and disordered microstructure [6], probably dominated by a double porosity due to two distinct distributions, one for the region of macroscopic porosity between particles, and another for the region of microscopic porosity within particles [16]. Due to the above-mentioned facts, in next sections, we will be dealing with theories applied to understand the moisture evaporation from porous media to understand how the evaporation from pellets happened, which can be useful to set design criteria to elaborate GB reservoirs for the optimum storage of EPNs.

Figure 4. Drying kinetics of diatomaceous earth pellets without EPNs with several initial moisture contents, stored in quiescent surrounding at room temperature (23 3C).
