Formulation of Biological Insectides - ACS Symposium Series (ACS

13 Formulation of Biological Insectides Surfactant and Diluent Selection 1

MICHAEL G. WARD

Downloaded by CORNELL UNIV on October 7, 2016 | http://pubs.acs.org Publication Date: June 5, 1984 | doi: 10.1021/bk-1984-0254.ch013

U.S. Department of Agriculture, Agricultural Research Station, Insect Pathology Research Unit, Boyce Thompson Institute, Ithaca, NY 14853

The improper selection of surfactants and diluents decreases shelf life and the effectiveness of biological insecticides. The diluent's physical structure, buffering capacity, surface charge and particle size are a l l factors that will be reviewed as influencing physical and chemical incompatibilities. Storage tests up to 2½ years with mycoinsecticide/diluent mixtures demonstrate that differences can occur. Surfactant selection is also critical in maintaining viability and virulence of active ingredient. Mycoinsecticides exposed to surfactants of differing hydrophilic-lipophilic values, concentrations, and chemical type may be sensitive to surfactant chemical structure.

In a p e s t i c i d e formulation everyone i s q u i t e aware of the chemical or b i o l o g i c a l a c t i v i t y of the a c t i v e ingredients. However, the success of the formulation i s dependent on the c o r r e c t s e l e c t i o n of the i n e r t components. The formulation o f a m i c r o b i a l p e s t i c i d e presents unique problems that are not encountered i n a standard chemical formulation. The f i r s t and obvious one i s the maintenance of b i o l o g i c a l a c t i v i t y during storage and a p p l i c a t i o n . In order to avoid l o s s o f a c t i v i t y o f the m i c r o b i a l organism, a c a r e f u l assessment of the chemical and physical parameters must be completed. Once the requirements of the m i c r o b i a l organism are d e f i n e d , s e l e c t i o n of the formulation type and i n g r e d i e n t s can be undertaken. The i n e r t components, even though termed " i n e r t " , are q u i t e chemically a c t i v e . They are i n e r t toward the t a r g e t pest

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Current address: Mycogen Corp., 5451 Oberlin Dr., San Diego, CA 92121 This chapter not subject to U.S. copyright. Published 1984, American Chemical Society

Scher; Advances in Pesticide Formulation Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

ADVANCES IN PESTICIDE FORMULATION TECHNOLOGY

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but possess q u a l i t i e s that enhance storage, handling, and application. The major i n e r t component found i n dusts, wettable powders and g r a n u l a r s i s the d i l u e n t , a l s o termed "carrier", in some formulation types, depending on the m a t e r i a l ' s absorbent c a p a c i t y . The d i l u e n t o f a formulation can be any m a t e r i a l , organic or i n o r g a n i c , that d i l u t e s the a c t i v e i n g r e d i e n t to a c o n c e n t r a t i o n that can be managed and applied by accepted a g r i c u l t u r a l p r a c t i c e s (1)· Table I e x e m p l i f i e s some of the types of m a t e r i a l s that can be used as diluents.

Table I.

Types of D i l u e n t s and C a r r i e r s Used i n A g r i c u l t u r a l Formulations

Inert Type

Examples

Classification Botanical

ORGANIC Synthetics

Silicates

Corn cobs, soybean-wheat flour, rice hulls, walnut s h e l l , wood f l o u r Cellulose

Palygorskite Kaolinite Montmorillonite Illite Synthetic

Attapulgite Kaolin Montmorillonite Bentonite Vermiculite Ca S i l i c a t e

INORGANIC

Oxides

Calcium Carbonates Sulfates Silicon Synthetics

Lime Dolomite Gypsum Diatomite Ca carbonate/ S i l i c o n dioxide

Phyllosilicates (clay) are the most commonly used d i l u e n t s because they are relatively inert, available in large q u a n t i t i e s a t a low c o s t , and are e a s i l y handled during manufacturing and a p p l i c a t i o n . Van Olphen (2) d e f i n e s c l a y s or clay materials as fine-grained, crystalline, hydrous silicates. The s t r u c t u r e s o f c l a y s impart a c e r t a i n degree o f r e a c t i v i t y upon the m i c r o b i a l formulated with them. Several


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different methods f o r c l a s s i f i c a t i o n o f c l a y s have been developed. A g e n e r a l i z e d d i s c u s s i o n o f the a g r i c u l t u r a l l y important c l a y s w i l l be reviewed.


Clay

Structure

Three major types of clays are predominantly used f o r p e s t i c i d e formulations. These, Grim (2) c l a s s i f i e d by t h e i r crystalline s t r u c t u r e : two l a y e r , three l a y e r , and c h a i n clays. The l a y e r s are sheet s t r u c t u r e s composed o f minerals c o n s i s t i n g o f c a t i o n s and anions that group t o form the b a s i c u n i t s that make up a sheet. The two-layer c l a y s (1:1) are composed o f one l a y e r o f s i l i c o n oxide tetrahedrons and one l a y e r o f aluminum oxide octahedrons. T h i s group i s termed the k a o l i n i t e group, with k a o l i n being the important d i l u e n t member. The montmorillonite group, a t h r e e - l a y e r type (2:1), i s composed o f two l a y e r s o f s i l i c o n oxide tetrahedrons and one c e n t r a l aluminum oxide d i o c t a h e d r a l or t r i o c t a h e d r a l layer. The sheets or l a t t i c e s o f montmorillonite c l a y are l o o s e l y held together by van der Waal's f o r c e . T h i s weak a s s o c i a t i o n o f l a y e r s permits the c r y s t a l l a t t i c e t o expand or c o n t r a c t i n the presence or absence o f water. T h i s group contains the m o n t m o r i l i o n i t e s prophybitite, t a l c , bentonite, v e r m i c u l i t e and mica. The t h i r d commercially important group, i n terms o f a g r i c u l t u r a l formulations, forms chains instead o f a l t e r n a t e l y stacked l a y e r s . The chains form sheets o f s i l i c o n oxide tetrahedrons l i n k e d together by o c t a h e d r a l groups o f oxygens and hydroxyls containing aluminum and magnesium cations. The major r e p r e s e n t a t i v e o f t h i s type i s a t t a p u l g i t e clay. A g r e a t v a r i a t i o n occurs w i t h i n each type o f c l a y . If the c a t i o n , which i s the c e n t r a l p o r t i o n o f the tetrahedron and octahedron, i s i n the pure s t a t e , i t w i l l be s i l i c o n or aluminum. However, these elements can be replaced by sodium, calcium, magnesium, or any other elements a c t i n g as a c a t i o n . The anions o f the s t r u c t u r e s are oxygen and/or hydroxyl. T h i s replacement (isomorphic substitution) i s the mechanism r e s p o n s i b l e f o r the d i f f e r e n t i a l charge a t the l a y e r ' s s u r f a c e (4t). I t should be noted that due t o t h e i r s t r u c t u r e t a l c , p y r o p h y l l i t e , and k a o l i n i t e do not undergo isomorphic substitution. The d i f f e r e n c e s i n the k a o l i n i t e group a r e , t h e r e f o r e , the r e s u l t o f hydration and v a r i a t i o n s i n the stacking o f l a y e r s . The variations i n structures o f these clays are r e s p o n s i b l e f o r the d i f f e r e n c e s i n b u f f e r i n g c a p a c i t y , surface charge, and other physico-chemical p r o p e r t i e s . f


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Physico-chemical Compatability

Properties

that

Influence

Microbial/Diluent

The physico-chemical p r o p e r t i e s o f c l a y s are derived from t h e i r s t r u c t u r e s , and are r e s p o n s i b l e f o r the performance o f the formulation. The s u r v i v a l o f m i c r o b i a l s can be d i r e c t l y a f f e c t e d by these physico-chemical p r o p e r t i e s . The importance o f c l a y s t o microorganisms was summarized by Stotzky (5), "The c l a y or c l a y mineral probably exerts the g r e a t e s t i n f l u e n c e on microorganisms because o f high surface a c t i v i t y of clays r e s u l t i n g from t h e i r l a r g e surface area and chemical and minerological composition." The proper s e l e c t i o n o f the d i l u e n t c l a y w i l l increase the s h e l f l i f e and the o v e r a l l compatability of the b i o l o g i c a l insecticide by three mechanisms: maintenance o f favorable pH, absorption of suppressive chemicals, and m o d i f i c a t i o n o f the microclimate. Maintenance o f a Favorable pH. The f l u c t u a t i o n s o f pH a r e considered t o be a major environmental s t r e s s t o m i c r o b i a l populations (6). The c o m p a t a b i l i t y and, u l t i m a t e l y , the s u r v i v a l o f an organism i n a formulation i s dependent upon the i n i t i a l pH and the b u f f e r i n g c a p a c i t y o f the c l a y d i l u e n t . The i n i t i a l pH o f c l a y s v a r i e s widely as shown i n Table II. I t i s q u i t e obvious how i n c o m p a t a b i l i t i e s between the biological

Table I I .

Physico-chemical P r o p e r t i e s o f S i l i c a t e Groups

Clay Type

pH

Attapulgite Kaolinite Montmor i l i o n i te Vermiculite Talc S i l i c a Gel

7.5 - 8 4-7 6 - 9.5 6.5 - 7.5 8-10 see f o o t n o t e

(10% S l u r r y )

CEC

(meg/100g) 120 2 80 130

2

- 130 -10 - 120 - 170 2-6 62

1 Data given as a range o f the commonly used a g r i c u l t u r a l d i l u e n t s . Source: ( 4 ) , (6), ( 2 ) · Dependent upon treatment by manufacturer. 2

organism and the d i l u e n t can occur when the i n i t i a l pH ranges from 10 t o 4 . The b u f f e r i n g c a p a c i t y i s a f u n c t i o n o f the s t r u c t u r e and i o n complex o f the c l a y . The b u f f e r i n g c a p a c i t y



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is expressed numerically as the c a t i o n exchange capacity (CEC). Clays which possess a high CEC can replace a g r e a t e r number of hydrogen ions with b a s i c c a t i o n s than c l a y s with a low CEC. The type o f c a t i o n present a l s o a f f e c t s the buffering capacity. In decreasing order, the c a t i o n s with the g r e a t e s t exchange c a p a c i t y are sodium, calcium, magnesium, potassium, and hydrogen (8)· Montmorillonite c l a y ' s degree of isomorphic s u b s t i t u t i o n and the a b i l i t y to expand and c o n t r a c t i n the presence o f water impart a high CEC. The expanding of the c r y s t a l l a t t i c e allows the ions between the c r y s t a l edges to become a v a i l a b l e for exchange. T h i s give., montmorillonite c l a y a 17 to 20 times higher CEC than that o f k a o l i n i t e c l a y (5). Therefore, montmorillonite c l a y s have a much greater p o t e n t i a l to b u f f e r the pH o f organisms during storage. Examples o f cation exchange c a p a c i t y f o r d i f f e r e n t types o f c l a y and s i l i c a g e l are shown i n Table I I . The e f f e c t of the buffering capacity of c l a y s with microorganisms was shown i n the study o f Fusarium wilt Fusarium oxysporum ( f . sp. cubense) o f the banana, and the human pathogenic fungi, Histoplasma capsulation (Darling)(9)(10). Both fungi spread and were found in geographical areas that contained non-swelling clays, predominantly k a o l i n i t e . Stotzky (j*) explained that areas with high b u f f e r i n g c a p a c i t y were able to maintain a healthy flora of bacteria, thereby preventing the fungi from establishing. The importance of b u f f e r i n g c a p a c i t y was e x h i b i t e d i n the entomopathogen formulations o f Metarhizium a n i s o p l i a e (11) (12). The c o n i d i a of M. a n i s o p l i a e were stored f o r s i x months at 20° C as mixtures of conidia/silica gel, c o n i d i a / s i l i c a g e l A a o l i n i t e c l a y , and c o n i d i a with the s i l i c a g e l separate. The c o n i d i a mixed with s i l i c a g e l decreased i n v i a b i l i t y r a p i d l y , probably as a r e s u l t o f the low i n i t i a l pH o f the s i l i c a g e l . However, the mixture o f c o n i d i a / s i l i c a g e l A a o l i n i t e c l a y r e t a i n e d 20% greater v i a b i l i t y than the mixture where the s i l i c a g e l was not present to b u f f e r the pH. Storage t e s t s conducted f o r 2 years with the mycoins e c t i c i d e Beauveria bassiana (Bals.) and c l a y d i l u e n t s c l e a r l y demonstrate the d i f f e r e n c e s i n v i a b i l i t y b e l i e v e d to be due to the b u f f e r i n g c a p a c i t y o f the d i l u e n t . Table I I I is a summation of the storage of B. bassiana c o n i d i a mixed with commercial d i l u e n t c l a y s (Ward, unpublished)·



180

Table I I I .

The E f f e c t o f Clav D i l u e n t s on B. bassiana Stored Two Years a t 4 ° C and 26° C PERCENT VIABILITY

Clay Type

4°C

Attapulgite Bentonite Vermiculite Kaolinite Control

82% 86% 65% 25%

Formulation of Biological Insectides - ACS Symposium Series (ACS

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