The physical action of three diatomaceous earths against the cuticle of the flour mite Acarus siro L. (Acari: Acaridae)

Article abstract of DOI:10.1002/ps.1484

Experiments examined the accepted modes of action of the desiccant dust, diatomaceous earth (DE), against the flour mite, Acarus siro (L.) (Astigmata). Adult mites were exposed to three DE products for periods of 3, 18 and 72 h under conditions designed to allow partial desiccation of the mites without causing mortality. After exposure, the DE dust particles were washed off the mite bodies, and both the DE and the mites were examined for presence of cuticular hydrocarbons by gas chromatography-mass spectroscopy (GC-MS) analysis. GC-MS identified seven major cuticular lipids of chain length C ₁₃-C₂₆ that may have a role in the waterproofing of A. siro. After 18 h exposure, n-tridecane and several different long-chain fatty acid ethyl esters were detected on one of the DE products. After 72 h, n-tridecane was detected on all three DE products. Mite samples retained after removal of the DEs were examined by microscopy. Tentative evidence was observed by conventional low-power microscopy that might indicate uniform removal of the epicuticle. However, a detailed examination by scanning electron microscopy showed no signs of abrasion. Crown copyright 2007. Reproduced with the permission of Her Majesty's Stationery Office. Published by John Wiley & Sons, Ltd.

Full text of DOI:10.1002/ps.1484

Pest Management Science
Pest Manag Sci 64:141–146 (2008)
The physical action of three
diatomaceous earths against the cuticle
of the flour mite Acarus siro L. (Acari: Acaridae)
Dean A Cook,∗ Maureen E Wakefield and Gareth P Bryning
Central Science Laboratory, Defra, Sand Hutton, York YO41 1LZ, UK
Abstract: Experiments examined the accepted modes of action of the desiccant dust, diatomaceous earth (DE),
against the flour mite, Acarus siro (L.) (Astigmata). Adult mites were exposed to three DE products for periods of
3, 18 and 72 h under conditions designed to allow partial desiccation of the mites without causing mortality. After
exposure, the DE dust particles were washed off the mite bodies, and both the DE and the mites were examined
for presence of cuticular hydrocarbons by gas chromatography–mass spectroscopy (GC-MS) analysis. GC-MS
identified seven major cuticular lipids of chain length C13 –C26 that may have a role in the waterproofing of A.
siro. After 18 h exposure, n-tridecane and several different long-chain fatty acid ethyl esters were detected on one
of the DE products. After 72 h, n-tridecane was detected on all three DE products. Mite samples retained after
removal of the DEs were examined by microscopy. Tentative evidence was observed by conventional low-power
microscopy that might indicate uniform removal of the epicuticle. However, a detailed examination by scanning
electron microscopy showed no signs of abrasion.
 Crown copyright 2007. Reproduced with the permission of Her Majesty’s Stationery Office. Published by John
Wiley & Sons, Ltd.
Keywords: astigmatid mites; Acarus siro; inert dust; diatomaceous earth; lipid; cuticle; mode of action
through the Home Grown Cereals Authority’s grain
storage guide3 and topic sheets,4–6 which are based on
field-validated research for use as a grain top-dressing
when combined with aeration for cooling,7 and for
structural treatments.8
1
INTRODUCTION
Over the last decade there has been a resurgence
of worldwide interest in using diatomaceous earth
(DE) for management of arthropods associated with
stored products. This naturally occurring material is
mined from geological deposits of fossilised diatoms
and is composed of over 90% silica. DE works against
invertebrate pests as a desiccant and, owing to its inert
nature, is seen as a benign alternative to chemical
insecticides. Benefits include low mammalian toxicity,
no harmful residues, persistence and stability at high
and low temperatures.1,2 DE products are registered
for storage use in a number of countries including the
USA, Canada, Australia, Japan, Indonesia, Germany,
Saudi Arabia and Brazil. Since 2001, DE has received
industry uptake in the UK for grain storage protection,
with the launch of several products. Under UK
national guidelines it is permissible to market DE
as a plant protection product without the need for
formal registration owing to its accepted mode of
action, under an exemption as set out in the Control
of Pesticides Regulations, UK, COPR 1986 regulation
3.2 Nevertheless, the UK Pesticide Safety Directorate
still requires notification of the intended use. However,
this will change as the EU Plant Protection Products
Directive (PPPD), 91/414/EEC, supersedes national
regulations and harmonises registration across the EU.
In the UK, industry guidelines for the use of DEs
under damp maritime climate conditions are available
With regard to mode of action, it is widely
accepted that DE causes desiccation by removing the
waterproofing components of the epicuticle. Ebeling9
stated ‘Diatomaceous earths are both abrasive and
slightly sorptive, and both types of action can result
in desiccation when insects crawl among dust-coated
kernels, thus rubbing all parts of their bodies against
films of dust’. The original formulations of DE
were believed to work primarily by abrasion, since
the specific surface area of dust particles was small
compared with the more sorptive silica aerogels.
Modern formulations, based on better selection of
particles and new sources of DE deposits, are believed
to offer much greater adsorption of lipids, with workers
considering oil-carrying capacity as a key predictor
of dust efficacy.10 Some formulations, referred to as
enhanced DEs, are coated with silica aerogels, or have
silica added, in order to increase this.11 However, the
present authors can find little work to demonstrate this
absorptive action through direct measurement of lipids
transferred directly from the cuticle onto DE particles,
with virtually all previous studies concentrating on
indirect indicators as aforementioned. There has been
no full study at all on the mode of action of DE
∗ Correspondence to: Dean A Cook, Central Science Laboratory, Defra, Sand Hutton, York YO41 1LZ, UK
E-mail: dean.cook@csl.gov.uk
(Received 17 March 2006; revised version received 15 May 2007; accepted 21 August 2007)
Published online 30 October 2007; DOI: 10.1002/ps.1484
 Crown copyright 2007. Reproduced with the permission of Her Majesty’s Stationery Office.
Published by John Wiley & Sons, Ltd. Pest Manag Sci 1526–498X/2007/$30.00

DA Cook, ME Wakefield, GP Bryning
against stored product mites (order Astigmata), which
are the primary target of these treatments in the UK.
Preliminary mode of action work at this laboratory with
the DE ‘Dryacide’ had not shown any abrasion to the
cuticle, or ingestion of the dust, for the mites Acarus siro
(L.) or Lepidoglyphus destructor (formerly Glycyphagus
destructor) (Schrank) Cook and Armitage.12 It was
therefore proposed that DE probably works against
these mites by cuticular lipid absorption.
Building upon this, in the present study, lipid anal-
ysis by gas chromatography–mass spectroscopy (GC-
MS) and imaging by scanning electron microscopy
(SEM) have been used further to elucidate the effects
of DE on the cuticle of A. siro. In addition, a secondary
aim of the study was to develop a better understanding
of the mite cuticle which in turn can be exploited as a
target for future generations of acaracides. The study
comprised method development as well as the results
reported here.
several times until all adult mites were removed. Visual
examination immediately afterwards confirmed that
the specimens were clean and undamaged.
2.3 DE treatment
For each experiment, 9 cm diameter glass petri dishes
were treated by hand sieving either Dryacide, Protect-
It or Silico-Sec DE at 10 g m−2 through a 250 µm wire
mesh sieve onto the base of the dish. An additional
dish was left untreated as a control. Adult A. siro
that had been carefully removed from culture as
described in Section 2.2 were added at 0.1 g per dish.
An estimate of the number of mites contained in
each 0.1 g aliquot was made by counting the number
of adults in a 0.01 g sample under a low-powered
binocular microscope. This gave an estimation of
approximately 10 500 mites per replicate (n = 3 per
run). The quantity of mites needed to allow detection
of cuticular hydrocarbons had been determined from
earlier experiments. The experiments were carried out
under conditions designed to allow partial desiccation
of the mites without causing mortality. Owing to the
considerable time required to set up each experiment,
a single replicate each of control and DE products
for each exposure time was assessed on a separate
occasion. This gave a total of nine experiments,
consisting of three replicates each for either 3, 18
or 72 h exposure at constant conditions of 15 ◦C and
80% RH.
2
MATERIALS AND METHODS
2.1 Diatomaceous earths
Three DEs were tested:
1. Silico-Sec – >90% SiO2, Biofa GmbH, Germany.
2. Protect-It – >90% SiO2 ‘enhanced’ by addition of
silica gel (10% by weight), Hedley Technologies
Inc., Canada.
3. Dryacide – 97% SiO2 ‘enhanced’ by coating with
a silica aerogel (3% by weight) and 2% fluoride
adjuvant, Dryacide Australia Pty Ltd.
2.4 Separation of DE from mites
After exposure, the contents of each petri dish were
rinsed onto a 100 µm stainless steel mesh placed in a
Buchner funnel, using deionised water. The water was
removed under vacuum. The mesh retained the mites
while allowing the DE particles to pass through. The
mesh was then removed from the funnel and placed
into a clean petri dish. Immediately afterwards, the
mites were checked using a low-powered binocular
microscope to ensure that the fragile bodies had not
been damaged during washing and that little, if any, of
the DE remained. The mesh and retained mites were
then subjected to solvent extractions. The DE/water
suspension was collected during the mite washing
and then filtered through a preweighed 9 cm diameter
Whatman No. 6 filter paper (3 µm) to collect the DE.
Solvent extractions were also carried out on the filter
papers/retained DE. The mite replicates that had not
been exposed to DEs were subjected to the same
separation procedure for comparison.
All had been obtained from the manufacturers
during earlier studies in 2001, 1998 and 1994
respectively, therefore allowing direct comparison with
previous work. However, they do not necessarily
represent current formulations (source of DE, particle
size, etc.). Of these, only Silico-Sec is marketed in
the UK. The precise oil-carrying capacity of these
products was not available, although, in view of the
added silica,1 it is likely that they rank Protect-It >
Dryacide > Silico-Sec.
2.2 Preparation of mites
Adults of an insecticide-susceptible strain of A. siro
(strain 9528/2) were reared in large quantities using
existing CSL methodologies (unpublished). Adult
mites were 0–2 weeks old when used and were
separated from the known aged cultures by sieving
gently over a 500 µm mesh sieve. These mites were
then tipped gently onto a 15 cm diameter Whatman
No. 1 filter paper and left for sufficient time for the
mites to cling to the paper. This paper was then
inverted at a 45◦ angle above a second paper and
gently tapped so that food and frass contaminants
were removed from the mites. The mites were then
examined under a low-powered binocular microscope
and, if visibly clean, transferred into a holding flask
with a sharp tap to the underside of the paper. This
procedure was repeated on the food and frass fraction
2.5 Solvent extractions
Preliminary experiments had shown that extraction
using chloroform gave larger amounts of the cuticular
hydrocarbons than pentane, and chloroform was
therefore used for the rest of this study. Typical
chromatographic profiles with both extraction solvents
are shown in Fig. 1. It was also found that all of
the hydrocarbons could be extracted using a 15 min
extraction period.
142
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DOI: 10.1002/ps

Action of diatomaceous earth on cuticle of A. siro
482
2
552
4
100
%
Chloroform extract
C
20-C26 fatty acid ethyl esters
544
7
1323
9
3
559
1
329
8
6
1494
5
10
1375
1233
599
1590
1406
1222
663
0
554
100
480
Pentane extract
%
0
544
559
601
665
326
1325
1376
1495
30.000
1234
10.000
15.000
20.000
25.000
Retention time (minutes)
Figure 1. Comparisons of GC profiles for 0.1 g untreated Acarus siro extracts after 5 min soaking in either chloroform or pentane. Peak 1,
phenylacetaldehyde; peak 2, 2-hydroxy-6-methylbenzaldehyde; peak 3, unsaturated hydrocarbon; peak 4, n-tridecane; peak 5, unidentified
aromatic aldehyde; peak 6, C20:1 FAEE; peak 7, C22:2 FAEE; peak 8, C23:2 FAEE; peak 9, C25:2 FAEE; peak 10, C26:2 FAEE.
One day prior to extraction, the chloroform was
purified by passing through a 20 cm column of neutral
alumina (100–250 mesh, 30 mm internal diameter).
Three-stage 15 min extractions were made for each
sample by soaking samples in successive 2 mL aliquots
of fresh solvent for 5 min at a time. For each sample
the three solvent extracts were then combined and
filtered through a Whatman No. 1 filter paper into
a glass microreaction vessel (Supelco), to ensure no
carryover of mite bodies or DE particles. This gave
single 6 mL solvent extracts for each sample. The filter
papers used to collect the DE were then air dried and
reweighed to allow the amount of DE recovered to
be calculated. A clean stainless steel mesh, clean filter
papers, 0.001g of mite food, solvent and samples of
each DE were also subjected to extraction, to act as
controls. Each sample was then concentrated to 50 µL
using a stream of dry argon prior to analysis.
The concentrations of the cuticular hydrocarbons
present in the samples were calculated by comparison
with injections of external standard solutions of n-
docosane, C22H46 (100 ng µL), in chloroform.
Standards of tridecane, ethyl hexadecanoate, ethyl
octadecanoate, ethyl (Z)-9-octadecenoate and ethyl
eicosanoate (Aldrich Chemical Co.) were also analysed
by GC-MS to confirm their presence in chloroform
extract of A. siro.
Ethyl (Z)-11-eicosenoate (C20:1 FAEE) was syn-
thesised from (Z)-11-eicosenoic acid using excess of
ethanol and a trace of concentrated sulfuric acid.
Ethyl (Z, Z)-13,16-docosadienoate was also synthe-
sised from (Z, Z)-13,16-docosadienoic acid using the
same method. Both esters were analysed by GC-MS
to determine if they were present in A. siro.
2.6 Analysis of abrasion using SEM
Treated and untreated replicates were set up and the
DE washed off the mites as described in Section 2.4.
A minimum of ten mites from each replicate were
examined for signs of abrasion using cryo-SEM at
−180 ◦C on a Philips XL20 SEM.
The cuticular hydrocarbons were analysed by GC-
MS on a Hewlett Packard 5890 series II GC coupled to
a VG Trio-1 mass spectrometer. Injections (1 µL) were
made using a Leap Combi-Pal autosampler in splitless
mode (purge on after 1 min) at 280 ◦C onto a 50 m
Chrompack CP-Sil 8CB capillary column (0.25 mm
ID, 0.25 µm film thickness). The GC was operated
in constant-flow mode at 15 psi at 50 ◦C. The oven
temperature was initially 50 ◦C (1 min hold), then
programmed to 310 ◦C at 10 ◦C min−1 and held at this
temperature for 30 min. The mass spectrometer was
operated in electron impact mode (EI+) at 70 eV and
scanned from 33–550 amu once every second. The
mass spectrometer source and interface temperatures
were held at 200 ◦C and 310 ◦C respectively.
3
RESULTS
3.1 Lipid absorption after 3 and 18 h exposure
Examination of the control samples showed no
contaminating peaks in any of the areas of interest. The
cuticular hydrocarbon n-tridecane, and both saturated
and unsaturated fatty acid ethyl esters (FAEEs) with
fatty acid chain lengths of C16 –C28, were identified
from extracts of the bodies of the treated and untreated
mites (Table 1). None of these cuticular hydrocarbons
was detected from DE removed from the mites after
3 h exposure. After 18 h, tridecane and several of the
The treated mites, untreated mites and DE
extractions were compared to show whether the lipids
were absorbed from the cuticle.
Pest Manag Sci 64:141–146 (2008)
143
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DA Cook, ME Wakefield, GP Bryning
Table 1. Comparison of cuticular hydrocarbons identified for Acarus
siro compared with those found in Acarus immobilis
the DE products after being washed off the treated
mites (Table 2). Quantities of n-tridecane from these
samples compared with the DE controls were ca
4 times greater, indicating that this was a genuine
artefact of the experiment. After adjustment for the
controls, these quantities varied between 0.2 and 0.3%
w/w compared with that found on the untreated mites.
Comparison of filter-paper weights before and after
collection of the DE showed a mean DE recovery from
the washings of 93% (range 74–100%) for the three
exposures (n = 27).
Acarus siro (n = 6)
Acarus immobilisa
C13; n-tridecane
C13; n-tridecaneb
C13; trideceneb
C14; n-tetradecane
C14; n-tetradecane
C
15; n-pentadecane
C15; n-pentadecane
C17; n-heptadecane
(Z)-8-C17; (Z)-8-heptadecene
C17; n-heptadecane
(Z, Z)-6, 9-C17
;
(Z, Z)-6,9-heptadecadiene
25; n-pentacosane
26; n-hexacosane
Ethyl (Z)-11-eicosenoate (C20:1 FAEE) was found
to be present in mite extracts as a major component,
peak 6 (Fig. 1), but ethyl (Z, Z)-13,16-docosadienoate
was not found in mite extracts. Ethyl (Z, Z)-13,16-
docosadienoate was considered to be the most likely
candidate compound for peak 7.
C
C
C27; n-heptacosane
C
C
28; n-octacosane
29; n-nonacosane
Ethyl hexadecanoate (C16:0
FAEE)
Ethyl octadecanoate (C18:0
FAEE)
Ethyl (Z)-9-octadecenoate
(C18:1 FAEE)
Ethyl icosanoate (C20:0
FAEE)
C20:1;b C22:2;b C23:2;b
C25:2;b C26:2;b C28:2b
FAEEs
Ethyl hexadecanoate, ethyl octadecanoate, ethyl Z-
9-octadecenoate and ethyl eicosanoate (C16:0, C18:0,
C18:1 and C20:0 FAEEs respectively) were also found
to be present in the mite extracts, but only as minor
components (Table 1).
3.3 Analysis of abrasion using SEM
SEM analysis of 200 images did not show the DEs
causing any obvious signs of abrasion. However,
when additional mites from the same samples were
viewed using conventional low-powered microscopy,
the DE appeared to have removed the shiny coating
normally observed on untreated mites. This may have
been indicative of uniform removal of the epicuticle
containing the lipids.
a From reference 15.
b Major cuticular hydrocarbon components of Acarus siro.
main FAEEs from the mites were detected from two
of the three Protect-It replicates. The amounts of
tridecane from the various DE treatments were only
approximately 0.2% w/w compared with that found
on the control mites, and there was not enough of the
other two FAEEs to allow quantification.
4
DISCUSSION
A comprehensive summary of the cuticular structure
for Acari can be found in Evans,13 although there
is little detail available regarding the cuticle of
astigmatid mites. For astigmatid mites the cuticle
is the main respiratory surface and plays a major
role in osmoregulation, since these mites lack an
organised respiratory system, and transpiration occurs
by diffusion directly through this surface. Hughes14
states that the cuticle is composed of a thin exocuticle,
covered by an epicuticle, lying directly above the
epidermis. A distinct chitinous endocuticle is missing,
except where the exocuticle is thickened to form
the ring-shaped sockets of sensory setae. Previously,
only n-tridecane had been identified as a cuticular
3.2 Lipid absorption after 72 h exposure
Lengthening of the exposure time to 72 h increased
the amount of cuticular lipids extracted. Again,
examination of the control samples revealed no
contaminating peaks in the areas of interest, with the
exception of the DE controls where small quantities
of tridecane were found on each of the three DE
products. Again, cuticular hydrocarbons of carbon
chain length C13 –C28 were identified from the bodies
of the mites. However, amounts of tridecane along
with three other compounds (two aromatic aldehydes
and one aromatic alcohol) were found on all of
Table 2. Concentrations (ng µL−1) of compounds identified on DE washed off treated mites after 72 h exposure
(a) Tridecane
from washings
(b) Tridecane
on DE controlsa
Tridecane
(a − b)
2-Hydroxy-6-
methyl-benzaldehyde
Phenyl-
acetaldehyde
2-Phenyl-
ethanol
Control mitesb
Dryacide
Silico-Sec
Protect-It
430.2
2
1.57
1.22
n/a
430.2
1.43
1.32
0.64
587.4
0.13
0.02
33.4
0.06
0.035
0.017
18.4
0.57
0.25
0.58
0.012
0.001
0.037
0.012
a Solvent extractions on DE with no exposure to mites.
b Solvent extractions from untreated mites.
144
Pest Manag Sci 64:141–146 (2008)
DOI: 10.1002/ps

Action of diatomaceous earth on cuticle of A. siro
hydrocarbon of A. siro.15 Although the function of this
compound is unknown, it has been suggested that it
may serve as a solvent for the semiochemicals such
as neral and neryl formate that are produced by some
mite species.16
As in a previous study,12 there were no obvious
signs of puncture marks, gouges or other signs of overt
damage caused by the DE. However, it is possible
that the thin epicuticle may have been uniformly worn
down or removed, and this would not have been
obvious unless viewed as a section. Further work is
needed to confirm any abrasive action of the DE,
using thin-layer electron microscopy (TEM).
However, this study does raise questions about the
role of tridecane. With the lack of other evidence to the
contrary, and regardless of its volatile nature, it may
play a role as a waterproofing agent. This is further
supported by the fact that the pheromone 2-hydroxy-
6-methylbenzaldehyde,16,20 which was present in
greater quantities in the mites, was only absorbed
at 1.5–10% as much as tridecane. If tridecane does
have this role, then it could be concluded that DE has
a similar mode of action against mites as hypothesised
for insects. More in-depth studies are needed to
support these results.
In the present study, three major volatile, non-lipid-
like components were identified: phenylacetaldehyde,
2-hydroxy-6-methylbenzaldehyde and an unidentified
aromatic aldehyde. Five of the seven major cutic-
ular hydrocarbon components were also putatively
identified and two positively identified: tridecane and
ethyl Z-11-eicosenoate, peaks 4 and 6 (Fig. 1). Two of
these identified major components (n-tridecane and an
unsaturated hydrocarbon of the same chain length) are
of identical chain length to one found in Acarus immo-
bilis Griffiths.15 The other five were esters of long-chain
fatty acids >C20. ‘Conventional wisdom’ dictates that
these longer-chain lipids (>C20) are the most likely
to be responsible for waterproofing of the cuticle,
as longer-chain hydrocarbons give more protection
against water loss and are non-volatile, so are not
constantly lost through evaporation, as is n-tridecane.
However, all the major cuticular components detected
are liquids at room temperature.
The results reported here are important additions
to current knowledge. The cuticle has been identified
as one of several potential physiological targets for the
development of new acaracides. Not only is the cuticle
an important site for osmoregulation/respiration but it
is also an anchorage for muscles and protects against
physical damage and penetration of pathogens.17 For
example, a current study at this laboratory is looking
to develop an entopathogenic fungal-based biocontrol
agent, and such information is useful when deciding
upon formulation.
The only cuticular lipid detected in significant
amounts on all of the DE products that had been
rinsed off the mites was tridecane (Table 2). This was
perhaps not surprising, as this is the most abundant
lipid available for uptake (Fig. 1). Only in one case did
DE appear to absorb a wider range of A. siro lipids. It
is interesting that these were only found on Protect-It,
which was marginally the most effective DE product
against mites in an earlier study.18 It was surprising
that these two longer-chain lipids (C23:2 and C25:2
FAEEs) were not subsequently redetected at the
longest exposure, but this may indicate that recovery
of these was near the limit of detection of the method.
In contrast, medium chain-free fatty acids – palmitic
acid, stearic acid, oleic acid and linoleic acid – have
been detected by other workers on DE removed
from the bruchids Callosobruchus maculatus (F.) and
Acanthoscelides obtectus (Say) after only 6.5 h exposure
at 25 ◦C, extracted in hexane and analysed by gas
chromatography.19 However, the amount of lipids
available for extraction from the bruchids would have
been very much higher, and it may have been that the
higher temperature in this study also had an effect,
increasing the rate at which the lipids were removed.
ACKNOWLEDGEMENTS
The authors are very grateful to their retired
colleague (and research leader) Dr John Chambers
for his encouragement and guidance and his helpful
comments on this manuscript. This work was funded
by the Sustainable and Food Science Group (formerly
the Arable Crops Division) of the Department for
Environment, Food and Rural Affairs (DEFRA).
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DOI: 10.1002/ps