Short communication

Effect of plant essential oils as acaricides against the poultry red mite,Dermanyssus gallinae, with special focus on exposure time

D.R. George a,*, G. Olatunji b, J.H. Guy a, O.A.E. Sparagano a

a School of Agriculture, Food and Rural Development, Newcastle University, Newcastle upon Tyne, NE1 7RU, England, UKb School of Medical Sciences, Newcastle University, Newcastle upon Tyne, NE1 7RU, England, UK

Veterinary Parasitology 169 (2010) 222–225

A R T I C L E I N F O

Article history:

Received 27 October 2009

Received in revised form 15 December 2009

Accepted 21 December 2009

Keywords:

Dermanyssus gallinae

Essential oil

Toxicity

Knock-down

A B S T R A C T

Essential oils from thyme and cade have been shown to be effective acaricides against the

poultry red mite, Dermanyssus gallinae (De Geer) when tested over a 24 h period. Data on

the actual rate of knock-down achieved with these products is lacking and potentially

important as essential oils are likely to display only short-term toxicity.

When tested over periods of less than 24 h, thyme essential oil killed D. gallinae

relatively quickly and so may make for an effective acaricide even if the residual toxicity of

this product is low. However, cade essential oil did not display such a high level of mite

knock-down, suggesting it may hold less promise in D. gallinae management.

Comparison of the results with those obtained elsewhere using alternative D. gallinae

products further confirms the possibility that thyme essential may be useful in control of

this pest. This might be especially true if thyme essential oil were employed as part of an

integrated pest management approach.

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1. Introduction

Pest management by conventional means (the use ofsynthetic pesticides) has become increasingly hamperedby issues including stricter legislation, pest resistance andan increase in consumer demand for foodstuffs producedin an environmentally responsible or even organic manner.As a result, increased interest is being shown in developingalternative methods of pest control to reduce or eliminatereliance on synthetic pesticides. One such method involvesthe use of plant-derived-products, such as plant essentialoils, as pesticides (Isman, 2000).

The poultry red mite, Dermanyssus gallinae (De Geer),is a serious ectoparasite of laying hens in the UK andEurope (Chauve, 1998; Sparagano, 2009). In taking ablood-meal from its host, D. gallinae can cause stress and

* Corresponding author. Tel.: +44 0 1912228893;

fax: +44 0 1912226720.

E-mail address: D.R.George@ncl.ac.uk (D.R. George).

0304-4017/$ – see front matter � 2010 Elsevier B.V. All rights reserved.

doi:10.1016/j.vetpar.2009.12.038

even anaemia in the birds it feeds from, and may serve asa vector for numerous poultry pathogens (Kim et al.,2004). Several plant extracts and essential oils havealready been identified as being toxic to D. gallinae (Kimet al., 2004; George et al., in press, 2009a,b; Maurer et al.,2009).

Research suggests that plant essential oils exert theirtoxic effect against D. gallinae by fumigant action (Kimet al., 2004; George et al., 2009b). As might be expected,the volatile/aromatic compounds (primarily terpenes)responsible for exerting this toxic effect often display lowresidual toxicities. For example, mortality of adult D.

gallinae exposed to a range of lavender essential oils wasgreatly reduced after the oils had been exposed to theenvironment for a period of just 24 h (George et al., 2008).Such a lack of residual toxicity could be viewed asbeneficial, where environmental persistence of anybioactive compounds will be low (Isman, 2000). Never-theless, if a product with a low residual toxicity is to beefficacious against D. gallinae at the same time, a rapidknock-down of the target pest is required. To date, work

D.R. George et al. / Veterinary Parasitology 169 (2010) 222–225 223

with essential oils and D. gallinae has only considered mitemortality following product exposure over 24 h (e.g. Kimet al., 2004; George et al., in press). With the possibilitythat essential oils may not remain efficacious for morethan 24 h, high D. gallinae knock-down in a shorter periodmay be required for these products to work effectively asacaricides.

The aim of this experiment was to test the toxicity oftwo plant essential oils found in previous work to beeffective as acaricides for D. gallinae over a 24 h period,over shorter periods. A general comparison of this datawith similar figures for other alternative D. gallinae controlproducts was also made.

2. Methods

The essential oils of thyme (Thymus vulgaris L.) and cade(Juniperus oxycedrus L.) were selected for study on the basisthat both oils had been found to kill 100% of D. gallinae at0.21 mg/cm2 over 24 h in previous work (George et al., inpress). Essential oils were sourced from New Directions(Southampton, UK) and applied to Whatman No. 2 filterpapers (4.25 cm diameter) at this same concentration byspot treatment. The experiment described was conductedat Newcastle University (UK).

Treated (or untreated control filter papers) wereplaced into Petri-dishes with approximately 23 adultfemale D. gallinae collected from a free-range laying henfarm in Northumberland (UK) no more than 6 days priorto use. Dishes were then sealed using a layer of Clingfilmand Parafilm and placed in a growth room at 22 8C (16:8L:D cycle). Mite mortality in dishes was assessed aftervarying exposure periods (EP) of 0 (mites placed intodishes as above then removed immediately), 2, 4, 8, 12and 24 h. Mortality was assessed under magnificationwhere a mite was deemed dead if it exhibited nomovement following repeated agitation with an ento-mological pin.

Sixteen replicates were undertaken over 16 runs of theexperiment, with two runs completed per week. Each runtested one replicate of both essential oils against all EP

Fig. 1. Mean mortality of Dermanyssus gallinae subjected to thyme and cade esse

letter are significantly different (P< 0.05). n = 16 for all means except ‘Thyme-2 h

outliers; see methodology).

treatments. For each EP treatment, a control (spottreatment with distilled water only) was used. Freshessential oils were used each week.

Mortality in dishes treated with essential oil wascorrected to take account of control mortality usingAbbott’s correction. The data was then subjected to atwo-way analysis of variance (ANOVA) with oil and EP asthe main factors. Post hoc testing was done using TukeyTests. Prior to analysis it was necessary to arcsin square-root transform the data and remove three data points asoutliers (>3 SD from the mean) to fit the residuals from theANOVA to a normal distribution. Probit analysis was alsoconducted to allow general comparison of Lethal Time (LT)values with those previously published for other alterna-tive D. gallinae products.

3. Results

Overall, significantly higher D. gallinae mortality wasachieved with thyme essential oil than with cadeessential oil (F(1,177) = 384.49, P< 0.001). The effect ofEP was also significant overall (F(5,177) = 88.19, P< 0.001),where higher mortality (P< 0.05) was achieved after alonger EP except when comparing data collected after a4 h EP to that collected after either a 2 h or 8 h EP. Therewas also a significant interaction between oil and EP(F(5,177) = 9.27, P< 0.001). As can be seen in Fig. 1, wheredifferences between means as identified by Tukey’s Testsare displayed, thyme essential oil was both more toxic tomites, and faster in exerting this toxicity than cadeessential oil. After only a 2 h EP to thyme essentialoil significant D. gallinae mortality, as compared to the0 h EP treatment, was achieved. The same was not seenwith cade until mites had been exposed for four timesas long.

LT values confirmed that thyme was the more acaricidalessential oil where, at the concentration used, 50% D.

gallinae mortality could be expected after less than 5 h ofexposure. This figure compares favourably with workconducted elsewhere on alternative D. gallinae controlproducts (Table 1).

ntial oils for varying exposure periods (EP). Means not sharing a common

EP’ where n = 15 and ‘Cade-24 h EP’ where n = 14 (data points removed as

Table 1

LT values (h) for alternative products (i.e. non-synthetic or non-chemical) proposed as acaricides and tested against adult female Dermanyssus gallinae in the

present study and work elsewhere. Values (means) are presented with �standard errors.

Product type Product name Concentration LT50 LT90 LT99

Essential oil Thyme1 0.21 mg/cm2 4.1� 0.3 14.3� 0.5 22.6� 0.9

Essential oil Cade1 0.21 mg/cm2 19.5� 0.5 33.8� 1.1 45.5� 1.6

Diatomaceous earth Insecto2 Immersion 96.0� 8.2 – –

Diatomaceous earth Diamol3 Immersion 117.6� 21.2 – –

Diatomaceous earth Diamol3 0.085–0.85 g/m2a 322.3� 43.0–161.0� 7.0 – –

Diatomaceous earth SilicoSec4 Immersion 75.8� 20.5 – –

Diatomaceous earth SilicoSec4 0.085–0.85 g/m2a 202.4� 47.3–32.8� 8.8 – –

Diatomaceous earth ProtectIt5 Immersionb 63.4� 11.1 – –

Diatomaceous earth Fossil Shield 90.06 Immersionb 28.3� 6.6 – –

Synthetic amorphous silica RID7 Immersion 138.9� 50.3 – –

Synthetic amorphous silica ID8 Immersion 13.9� 4.9 – –

Synthetic amorphous silica ID8 0.085–0.85 g/m2a 27.8� 7.1–14.6� 4.0 – –

Entomopathogenic fungi Beauveria bassiana9 Unknown c. 96 c. 384 –

No action (70% RH, 20 8C) –10 – 816 – –

1current study; 2–8Kilpinen & Steenberg (2009) at 75% RH, 25 8C; 9Steenberg et al. (2006); 10Nordenfors et al. (1999). 2Natural Insecto Products, Costa Mesa,

CA, USA; 3O.W.A. Kemi, Skanderborg, Denmark; 4MiljøXuen, Gandrup, Denmark; 5Hedley Technologies, ON, Canada; 6FS 90.0, Bein, Eiterfeld, Germany;7Rentokil, Glostrup, Denmark; 8confidential formulation under test.

a Product applied to filter paper which then exposed to mites for 24 h.b Product modified with synthetic amorphous silica.

D.R. George et al. / Veterinary Parasitology 169 (2010) 222–225224

4. Discussion

As expected, at a concentration of 0.21 mg/cm2 thymeessential oil proved to be highly toxic to D. gallinae over24 h. Cade essential oil was significantly less toxic over thesame period. Results from George et al. (in press) suggestthat both oils should have been highly toxic at theconcentration used, although related work (George et al.,2009a,b) highlights the potential variation in D. gallinae

toxicity that may occur between seemingly similar oils, oreven different batches of single oil. Such variation mayresult from the composition of an essential oil beingaffected by factors such as plant harvesting regime andplant/oil storage and could explain why cade essential oilwas found to be less toxic than expected. This lack ofconsistency suggests that cade essential oil may not be asuitable candidate for D. gallinae control.

In addition to displaying a high level of toxicityconsistent with previous research, thyme essential oil alsodisplayed a favourable rate of D. gallinae knock-down inthe current study in comparison to both cade essential oiland the other ‘alternative’ control methods reported in theliterature. Kilpinen and Steenberg (2009) observed rela-tively fast D. gallinae knock-down with ID, a syntheticsilica, where a LT50 of 0.6 days was reported. Although notas rapidly acting as thyme oil, this rate of knock-downwould still seem sufficient to achieve rapid D. gallinae

control. Nevertheless, mites need to contact ID in order forit to have an effect. This may delay the knock-downobserved with these products in the field where D. gallinae

naturally sequesters itself in hard-to-treat refugia for dayson end without emerging to feed. As essential oils,including thyme, kill D. gallinae by fumigant action (Georgeet al., 2009b), penetration of these refugia is achievable inthe field and knock-down should consequently remain asrapid as suggested by laboratory study.

The above suggests that any short residual toxicitydisplayed by thyme essential oil could be compensated forby its rapid and fumigant acaricidal action. It remains to be

shown that thyme essential oil displays residual toxicitiesto D. gallinae as low as those found with other essential oils(George et al., 2008), but it is nevertheless likely that otheralternative control strategies benefit from lengthierresidual toxicities. Diatomaceous earths (DEs), for exam-ple, can remain effective against D. gallinae for up to 10days (Maurer and Perler, 2006) and perhaps longer undermore favourable conditions (Fields, 1999). Biologicalcontrol products such as entomopathogenic fungi andnatural enemies, which are also being researched for D.

gallinae control (Lesna et al., 2009), may even reproducethemselves in situ. In theory such ‘products’ could thusprovide indefinite control, although this assumes thatconditions are suited to the survival and reproduction ofthe organism in question. As biological organisms arestrongly affected by abiotic conditions this will not alwaysbe the case (entomopathogenic fungi, for example, requirehigh humidity to proliferate).

In conclusion, the results of the current study suggestthat on account of its fast knock-down, thyme essential oilcould be used as a stand-alone treatment for D. gallinae.Nevertheless, longer-term D. gallinae control may beachieved by combining application of thyme essential oilwith other control strategies in an integrated pestmanagement approach (Harrington et al., 2009), thoughcombining control options may not always lead toimproved overall efficacy (Maurer et al., 2009, re: DEand pyrethrum extract). It seems logical to suggest thatrapid D. gallinae knock-down via the application of areliable essential oil such as thyme would lend itself wellto combination with less fast-acting and less environmen-tally stable, but longer-term alternative managementoptions.

Acknowledgements

This study was funded by a Welcome Trust VacationScholarship. The work described formed part of a larger

D.R. George et al. / Veterinary Parasitology 169 (2010) 222–225 225

study funded by the Department for Environment, Foodand Rural Affairs (UK) investigating the use of plant-derived products for management of D. gallinae (MITEe-HEN). The authors would also like to thank Dr Ole Kilpinenand Dr Tove Steenberg for providing specific values forinclusion in the table presented.

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