Journal of Insect Behavior, Vol. 10, No. 6, 1997

Host Plant Acceptance by Redlegged Earth Mite,Halotydeus destructor (Tucker) (Acarina:Penthaleidae)

Kylie R. Gaull1,2 and James Ridsdill-Smith1,3

Accepted April 21, 1997; revised August 10, 1997

Host plant acceptance behavior of redlegged earth mites (Halotydeus destructor)on cotyledons of subterranean clover (subclover) lines was investigated in thelaboratory. Sustained feeding consisted of a series of short bouts of feeding;rather than one long period of feeding. H. destructor preferred to feed at mite-damaged rather than undamaged sites on cotyledons. With time the numbers ofmites feeding in aggregations increased, as mites were attracted to damagedpatches. Most feeding occurred in aggregations, and mites in such groups ben-efited by greater weight gain. The sequence of foraging leading to feeding wassimilar between subclover lines, however, feeding and aggregation activitieswere markedly reduced on a resistant line. Host plant acceptance occurredduring probing but only after some feeding had occurred.

INTRODUCTION

The redlegged earth mite, Halotydeus destructor (Tucker) (Acari: Penthaleidae),is a major pest of pasture legumes in Australia but feeds on a wide range ofother plant species (Swan, 1934; Ridsdill-Smith, 1997). H. destructor distin-guishes between plant species and feeds relatively more on subterranean clover(subclover), Trifolium subterraneum L. (Leguminosae), than on other species

1 CSIRO Entomology, Private Bag, Wembley, W.A. 6014, Australia.2 Present address: Department of Zoology, University of Western Australia, Nedlands, W.A. 6907,Australia.

3 To whom correspondence should be addressed. Fax: +61 8 9333 6646. e-mail: jamesr@cc-mar.csiro.au.

KEY WORDS: redlegged earth mite; feeding; aggregation; host acceptance; subclover plants;cotyledons.

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in annual pastures (Gaull and Ridsdill-Smith, 1996). Host plant selection involvesboth host acceptance and host suitability (Klinghauf, 1987; Bernays and Chap-man, 1994). Host suitability, as measured by the rate of H. destructor multi-plication, varies considerably on different plant species, probably due to theirnutritional values (Annells and Ridsdill-Smith, 1994) and, in some cases, tosecondary compounds. Host suitability is not considered in this paper.

Host acceptance is based on a sequence of selection steps: testing the plantsurface (palpating), penetration of the cell or tissue (probing or biting), andcontinuous feeding (feeding) (Miller and Strickler, 1984; Klinghauf, 1987). Forexample, larvae of diamondback moth can detect a resistant plant at the palpatingstage from the surface waxes (Eigenbrode et al., 1991). Locusta migratorianymphs reject sorghum seedlings at palpation unless the wax is removed (Wood-head, 1983). Rejection of nonhosts by locust nymphs occurs on the first encoun-ter at the biting stage, while in subsequent encounters nonhosts are rejectedduring the palpation stage (Blaney and Simmonds, 1985). Feeding damage byH. destructor is positively correlated with the number of mites on a subclovercotyledon (Jiang and Ridsdill-Smith, 1996a). Antifeeding effects of a numberof water-soluble and water-insoluble plant compounds have been demonstratedin bioassays with H. destructor (Ridsdill-Smith et al., 1995). Secondary com-pounds in the sap and cells of plants have been identified as feeding deterrentsto a range of insect species (Miller and Strickler, 1984; Bernays and Chapman,1994). The insect may respond to secondary compounds before or after feeding.

Host acceptance in insects is usually studied on a plant taxonomic basisbetween species, with much less information available about factors used byinsects to distinguish between lines of the same species (Bernays and Chapman,1994). The latter are utilized in the development of host plant resistant to pests.Host plant resistance to H. destructor has been detected in subclover seedlings(Gillespie, 1991), associated with antifeeding mechanisms (Jiang and Ridsdill-Smith, 1996a). In this paper we investigate host acceptance behavior of H.destructor on subclover seedlings and compare it with nonacceptance on a resis-tant subclover line.

MATERIALS AND METHODS

Mites

H. destructor were collected from grazed subclover dominant pastures nearToodyay (31 "OS'S, 116°04'E) and Quindanning (33°03'S, 116°34'E) (600-700mm rainfall per annum) in southwestern Australia. In the laboratory, mites werefed with cut subclover foliage at 15°C until required for use (up to 3 daysmaximum). Alternatively, if not available in the field, mites were obtained froma H. destructor culture maintained on Vicia sativa cv. Blanchfleur in a controlled

860 Gaull and Ridsdill-Smith

temperature cabinet (10:14-h light:dark, 18:11°C). The young adult stage wasused because they feed readily and are relatively easy to observe (1 mm long).Mites of this stage, determined visually on the basis of body size and shape,length, and color of legs, were sorted with a compressed air pooler under adissecting microscope and counted for experiments. Mites were starved over1-2 h for experiments using detached cotyledons, and over 18 h for experimentsusing entire plants, in humidified vials (separated with fine mesh from moistsponge) at 15°C. The objective of starving was to induce mite feeding, whichwas the target for observation. Mites had further to move to reach foliage ofentire plants and were starved longer.

H. destructor were marked to allow the technique of focal animal sampling.One individual is observed in the presence of conspecifics, recording all instancesof several different categories of behavior for a predetermined period of time(Martin and Bateson, 1993). The focal individual was chosen before the obser-vation period, marked with a coating of fine fluorescent powder by rotating ina vial of powder for approximately 1 min. Marked mites were observed underfluorescent light and were identifiable for up to 8 h. The fluorescent powder hadno adverse effect on mite behavior. Experiments were performed in a constant-temperature room at 16 ± 2°C, a typical field temperature for this species. Therepertoire of H. destructor foraging behavior included palpating, probing, feed-ing, and mites frequently feed in aggregations (Gaull and Ridsdill-Smith, 1996).Behavior was recorded only when mites were on the cotyledons. The time thatmites were on the soil was omitted.

Plants in Pots

Plastic pots were filled with sand:loam mix (4:1), fertilized with 0.13%superphosphate and 0.04% Richgro Complete Trace Elements. Seeds of sub-clover cultivar Dalkeith (the most widely recommended cultivar in WesternAustralia) were sown at a depth of 1 cm, covered with a mix of soil and groupC legume inoculant to assist nodulation, and kept at 15°C until germination.When seedlings emerged, the pots were moved to a glasshouse. H. destructorforaging behavior on Dalkeith (control) was compared with that on SE020 (aresistant line). Mite feeding damage is reduced on this line (Ridsdill-Smith,1995; Jiang and Ridsdill-Smith, 1996a). Mites were retained on plants in potseither using Vaseline smeared around the opening edge or with a clear plastictop.

Foraging and Aggregating Behavior

Observations were recorded by one observer using either continuous record-ing or time sampling methods. Continuous recording commenced when the focalindividual moved from the soil to a plant and ended either when the individual

Host Plant Acceptance by Redlegged Earth Mite 861

moved back to the soil or when the predetermined observation period ended.Initially manual methods were used to record the movements of focal individ-uals. Subsequently a computer program was designed to mimic an event recorder(L. Lebel, personal communication): one keypress started the timer for a des-ignated behavior and a second stroke of the same key stopped the timer for thatbehavior. The data readout gave the code, the start time, and the duration ofthe behavior assigned to each key. Time sampling involved periodical obser-vations of mite behavior and was carried out on a marked focal individual oron several unmarked individuals together. At the instant of each sample, thebehavior of the individual(s) under observation on cotyledons was recorded.

Foraging on Cotyledons

Foraging behavior on entire subclover plants was measured over 70 min.Mites were starved for 18 h before each trial. Mites were placed in 6-cm-diameter vials, mesh bottomed, half-filled with brown sand/loam, with a toplayer of white sand. A single plant was grown in each pot, and 7-10 days afterseedling emergence, 1 marked (focal) and 30 unmarked individuals were added,equivalent to 11,000 mites/m2, a common density in pastures. Observationscommenced when the marked mite moved onto a plant. Eleven replicates wereobtained over 4 days. Mite behavior was recorded continuously and manually,noting the time in minutes and seconds at the commencement and cessation ofeach activity.

Foraging in Aggregations

The mean weight gain per mite over 2 h was compared at different densitiesof mites on one cotyledon to determine if aggregations benefit H. destructorfeeding. Seedlings were grown in plastic pots of 13-cm diameter (15 seeds perpot). At 7-10 days after emergence a cotyledon was cut from a seedling andplaced in a clip cage (Adams and van Emden, 1972). The cage was made ofPerspex rings 14 mm in internal diameter and 9 mm high, with a mesh size of200 /itm. Mites were starved for 1 h, then placed on a cotyledon for 2 h at adensity of 1, 5, 10, or 15 mites per cage. Treatments were replicated seventimes, and two experiments were run on consecutive days. Each replicate (mitesin one cage) was weighed collectively before and after the 2-h feeding period.Mean gain in weight per mite was expressed as a function of mite density foreach experiment, fitting linear regression lines to the means.

Cotyledons damaged by H. destructor feeding attract further mites (Jiangand Ridsdill-Smith, 1996a; Jiang et al., 1997), which could be causing H.destructor to aggregate. Behavior of single H. destructor was compared oncotyledons which had fresh mite-feeding damage and undamaged cotyledons.Cotyledons were damaged by placing 300 adult H. destructor in pots withseveral seedlings, enclosed with small bottomless plastic bottles. Mites were

Gaull and Ridsdill-Smith862

left for approximately 2 h when damage patches were ~30% of the cotyledonupper surface, then removed. The damaged cotyledons were cut off immediately,and undamaged cotyledons were cut from seedlings of similar age not exposedto mites. One young adult H. destructor was starved for 2 h, then added to asmall petri dish (3.5-cm diameter), with one mite-damaged cotyledon and oneundamaged cotyledon, laid nearly touching adaxial surface up on damp soil.Observations commenced when the mite moved from soil onto a cotyledon, andbehavior was recorded continuously with a computer event recorder for 30 min.Mites were discarded if no choice was made within 15 min. There were 17replicates.

Foraging Compared on Cotyledons of Control and Resistant Subclover Lines

Foraging behavior of focal mites on seedlings was compared in a choiceexperiment on control and resistant lines. Four seedlings, two each line, weregrown in soil in plastic pots 10 cm in diameter. The resistant plants were markedwith colored pins in the soil. Young adult mites were starved for 18 h, and 1marked and 40 unmarked mites, equivalent to 5,000 mites/m2, were added 3days after emergence. The behavior of the focal mite was recorded continuouslyfor 2 h, using the computer event recorder, commencing each time the mitemoved onto one of the subclover plants, ending when it left that seedling. Thetime spent in each activity was estimated. The sequence of events was recordedby counting the number of transitions from one behavior to the one followingit, to determine if the sequence changed on the resistant plants. There were 16replicates.

The numbers of mites forming aggregations on the control and the resistantline were compared in a nonchoice experiment. Each pot (13-cm diameter)contained six seedlings of one variety, with 10 replicates of each variety. Miteswere starved for 18 h before use. One hundred young adult H. destructor,equivalent to 7,500 mites/m2, were added to each pot 2 days after seedlingemergence. The number and size of aggregations were recorded by time sam-pling, at intervals of 30 min over 7 h. Twenty-four hours after the experimentcommenced all cotyledons were removed and scored for mite feeding damage.The proportion of the adaxial surface area damaged, determined as the size ofthe silvery patches, was estimated on a scale of 1-10, where 1 = 10% or lessdamage and 10 = 100%.

RESULTS

Host Acceptance of Cotyledons

H. destructor spent 77% of the time on plants feeding, 20% probing, and2% palpating during the 70-min observation period. However, 66% of the time

863Host Plant Acceptance by Redlegged Earth Mite

864 Gaull and Ridsdill-Smith

Fig. 1. Mean duration (±SE) of each foraging behavior forindividuals when solitary/paired or within aggregations; 70-mincontinuous observations of 11 focal mites on seedlings in soil.

was in aggregations, and mites in aggregations spent 83% of their time feeding,while in solo/pairs only 65% of the time was spent feeding (Fig. 1). Mites inaggregations were probing and feeding, while single mites or pairs were pal-pating for 9% of their time, as well as probing and feeding (Fig. 1).

Intake, measured as mean weight gain per mite over 2 h, was significantlygreater at higher mite densities. The correlation with density in Experiment 1was r2 = 0.962 (P < 0.05), and that in Experiment 2 was r2 = 0.989 (P <0.01) (Fig. 2). The standard errors on the estimates were lower at higher den-sities, because at these densities each replicate measure was determined from alarger number of mites. Most of the mites within each cage fed in groups (i.e.,

Fig. 2. Effect of mite density on mean weight gain/mite (±SE) over 2 h in twoexperiments. Seven replicates for each density on cut cotyledons in clip cages. Linearregressions fitted to mean weight gain at each density as a function of mite density.

aggregated) on the cotyledon. The mean gain in weight per mite over 2 h at thehighest density averaged 20% of the initial weight (70 jig).

When presented with a choice, 11 of the individual mites moved first tothe mite-damaged cotyledon, and 6 moved first to the undamaged cotyledon. Inthe 30-min period, mites spent twice as much time feeding, and twice as longwas involved in total activities, on damaged cotyledons as on undamaged coty-ledons (Table I).

Host Acceptance on Control and Resistant Lines

Mites initially tapped the surface of the cotyledon with palps (palpation),and then tested the surface with their mouthparts (probing), before starting tofeed. In a choice situation the sequences of foraging behavior were similar onboth control and resistant lines. Over 2 h, there was no difference in the timespent probing by H. destructor on the two lines, but the time spent feeding bymites on the resistant line was 46% of that on the control (Table II). The timespent in aggregations on the resistant line was 17% of that on the control (TableII). Each separate probing event and feeding event took the same amount oftime on both lines (Table II). There was a high frequency of transitions between

865Host Plant Acceptance by Redlegged Earth Mite

Table I. Mean Time (±SE) Spent Foraging by Individual H. destructor Given a Choice BetweenMite-Damaged and Undamaged Cotyledons of the Control Subclover Line Over 30 min

Treatment

DamagedUndamaged

Duration of feeding (min)

13.92 ± 1.955.57 ± 2.82

*

Total time on cotyledons (min)

24.22 ± 2.0712.08 ± 4.53

*

*P < 0.05 (rank sum test).

Table II. Mean Time (±SE) Spent Foraging by H. destructor Given a Choice BetweenCotyledons of Control and Those of Resistant Lines Over 2 h

Line

ControlResistant

Total activity (min)

Probing

18.72 ± 4.8415.14 ±3.99

NS

Feeding

32.16 ± 5.3314.90 ± 4.76

*

Aggregating

7.57 ± 2.551.30 ± 0.98

**

Mean duration/event (min)

Probing

0.78 ± 0.100.95 ± 0.19

NS

Feeding

2.09 ± 0.311.41 ± 0.23

NS

*P < 0.05 (rank sum test).**P < 0.01 (rank sum test).

866 Gaull and Ridsdill-Smith

Fig. 3. Total number of transitions (±SE) between mite foraging behaviors on seedlings ofcontrol and resistant lines in a choice experiment. Continuous observations of 16 individuals,each over 2 h.

behaviors, which averaged 83 per mite over the 2-h observation period. Tran-sitions were from palpating to probing and back, and from probing to feedingand back, but not between palpating and feeding (Fig. 3). After mites startedfeeding, there were frequent transitions from feeding to probing and back onboth lines, but there was a greater number of feeding events on the control. Thenumber of transitions from palpating to probing and back on the resistant linewas 79% of that on the control, whereas the transitions from probing to feedingand back on the resistant line were 62% of those on the control (Fig. 3).

In the nonchoice experiment the numbers of mites varied on both subcloverlines from sample to sample, perhaps in response to shading when clouds passedin front of the sun over the glasshouse. To smooth out the pattern, averageswere taken for observations from each pot between 0 and 2, 2 and 4, and 4 and7 h. Analyses of variance were carried out on these data. The total number ofmites aggregating was similar between lines for the first 4 h, but from 4 to 7 hthe number of mites was significantly greater on the control (the number on theresistant line was 63% on the control) (Table III). About 10% of mites wereaggregating on both lines in the first 2 h, which increased on the control to 20%after 4-7 h. Feeding damage to the resistant cotyledons after 24 h was 64% ofthat on the control (Table III). Numbers of mites per aggregation were similaron control (6.4) and resistant (6.2) lines.

DISCUSSION

The normal pattern of host acceptance by H. destructor is that whichoccurred on seedlings of the control subclover line. There was a testing phaseof palpating and probing before the feeding phase. Each individual foragingevent was short, with probing taking about 1 min and feeding about 2 min.Thus over a period of time there were frequent transitions between palpatingand probing and back and between probing and feeding and back. Host accep-

tance is taken to occur when sustained feeding occurs (Miller and Strickler,1984), but H. destructor continued to feed for short periods after host accep-tance, instead of settling down to a single long period of feeding.

Aggregation was a major factor in H. destructor feeding behavior and wasa key component of host acceptance. Aggregations on subclover cotyledons inthis study averaged six mites, but much greater numbers have been observed inpastures (Gaull and Ridsdill-Smith, 1996). The resultant feeding damage isvisible as silvery patches (Swan, 1934), which attract searching individuals(Jiang and Ridsdill-Smith, 1996a; Jiang et al., 1997). In this study we haveshown that H. destructor engaged in more feeding at damaged sites, in prefer-ence to undamaged sites. When H. destructor feed on subclover cotyledons,damage-induced metabolitess, in particular, the green leaf volatile 2-(£)-hex-enal, are produced, which enhance mite aggregation at low concentrations (Jianget al., 1997). With time more mites were attracted and were feeding in aggre-gations. There were clear feeding benefits for individuals feeding in such groups.High physical strength of the upper epidermis of cotyledons has been associatedwith reduced feeding damage by H. destructor on resistant subclover lines (Jiangand Ridsdill-Smith, 1996a,b). Therefore it is hypothesized that it may be phys-ically easier for individuals to penetrate the surface cells, and obtain sap, whenthe surrounding cells have already been damaged by mite feeding. The phenom-enon of aggregation is well-known to provide feeding benefits in other insectspecies. The formation of aggregations causes increased feeding in green Junebeetle (Domek and Johnson, 1988), in Pieris brassicae larvae (Blackwell, 1988),in aphids (Way and Cammell, 1970), and in mosquito larvae (Piper, 1988).

Host acceptance behavior was interrupted by resistance at the feeding stageof foraging. The sequence of foraging leading to this stage was similar betweenlines. However, feeding and, especially, aggregation activities were markedly

Host Plant Acceptance by Redlegged Earth Mite 867

Table III. Mean Number of H. destructor Aggregating (±SE) on Cotyledons of Control andResistant Subclover Lines Over 7 h" in a Nonchoice Experiment and Mean Damage Score (±SE)

Resulting from Mite Feeding Over 24 h: 10 Replicate Pots for Each Line

Line

ControlResistant

"Mean for several*P < 0.05 (t test)**P < 0.001 (i te

Number of mites aggregating

0-2 h

11.9 ± 2.810.5 ± 1.3

NS

observations of

st).

2-4 h

15.3 ± 2.310.0 ± 2.0

NS

individual mites in eacl

4-7 h

20.4 ± 2.212.5 ± 1.3

*

i time period.

Damage score,24 h

5.5 ± 0.43.5 ± 0.2

**

reduced on the resistant line, resulting in lower feeding damage. High concen-trations of another damage-induced volatile metabolite, octenone, are producedfrom damaged cotyledons of resistant subclover lines, which deter the mitesfrom the cotyledons (Jiang et a/., 1996, 1997). Thus, feeding must commencefor mites to detect resistance.

In conclusion, it is hypothesized that host acceptance by H. destructoroccurs after the commencement of feeding and that, if the green leaf volatilehexenal is detected at low concentrations, aggregations will form and increaseover time. However, if the green leaf volatile octenone is produced at highconcentrations, there will be no increase in the number of mites aggregatingand feeding, which constitutes nonacceptance. The host plant is accepted prob-ably when the mite is probing and fails to move on to a feeding event. Hostplant acceptance by insects is more like a series of take-or-leave it situationsthan comparison shopping (Miller and Strickler, 1984). Other sap sucking insectsalso appear to accept their host during the probing stage. Aphids tend to accepttheir hosts during the probing phase (Klinghauf, 1987). Spider mites are oftenblown onto plants, and host acceptance probably occurs during initial piercingor probing, but no detailed observations have been made to confirm this (Sabelis,1985).

H. destructor feeds on a wide range of host plants, so that it is unlikelythat the same factors will be involved in host acceptance of all of them. Thegreen leaf volatiles affect mite feeding and aggregation, but further investigationsare required to determine if the same volatile compounds are involved in hostacceptance of other plants by this polyphagous mite.

ACKNOWLEDGMENTS

Ms. T. Picen and Ms. J. Darcy-Evans are thanked for assistance in theglasshouse. Australian wool growers provided financial support through theInternational Wool Corporation. Dr. L. Lebel made available a design for aprogram event recorder and Mr. J. Barton helped with modifications. Mr. D.Gillespie is thanked for providing subclover seed.

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