J.

Med. EDt.

31 October 1975

Vol. 12, no. 4: 409-412

RELATIONSHIPS OF IMMATURE DERMACENTOR VARIABILIS (SAY) (ACARI: IXODIDAE) WITH THE WHITE-}~OOTED MOUSE, PEROMYSCUS LEUCOPUS, IN SOUTHWESTERN WISCONSIN! By John O. Jackson and Gene R. DeFoliart2

Acari other than ixodids associated with the White-footed Mouse, Peromyscus leucopus, and its nests in southwestern Wisconsin were reported by Jackson & DeFoliart (1975). The only ticks associated with the mouse-nest system were larvae and nymphs of Dermacentor variabilis (Say). Various aspects of the ecology of this species have previously been assessed from live-trapping studies (Smith et al. 1946, Sonenshine et al. 1966, Sonenshine & Stout 1968b, Dodds et al. 1969) and from nestbox studies (Drummond 1957). In the present study, the tick-mouse relationships with respect to seasonal occurrence and mouse nesting populations were examined. MATERIALS

AND METHODS

Two field study sites, I located on the south slope, the other on the north slope of Dunbar Hill, at Clyde (section 12) in Iowa County, Wisconsin were described by Jackson & DeFoliart (1975). Results of vegetational analysis were detailed by Jackson (1973). In general, the vegetational composition of the south-slope oak woods indicated xeric conditions, while north-slope oak woods were characterized as dry-mesic, as defined by Curtis (1959) . The procedure for collecting Acari, including immature D. variabilis, from mice and their nests and design of the nestboxes deployed at both study sites were described by Jackson & DeFoliart (1975). Mouse nesting activity (mouse "time in residence") was grouped into 6 classes: (1) nonchambered (NC) characterized by absence of nestchamber, but with 'Supported in part by NIH Grant AI-07453. Approved for publication by the Director, Research Division, College of Agricultural and Life Sciences, University of Wisconsin, Madison, 53706, U.S.A. 'Department of Entomology.

edges of cotton slightly worked and surface dotted with fecal pellets; (2) platform feeding type (PF); (3) chambered nests (CHU), with nesting chamber formed by a mouse between weekly observations and occupants not observed and, therefore, not specified. When mice were observed in nest boxes, nests with chambers were grouped as follows: (4) "single adult," male or female (CHSA); (5) "multiple adult" (CHMA), aggregations of 2 to several adults occupying a nestbox from I to several weeks or I adult residing in a nestbox through several weeks; (6) "multiple juvenile" (CHMJ), juvenile mice and adult females in lactating condition or nests occupied by 2-6 juveniles less than 40-50 days old with or without the female. To collect ticks the cotton nests were exposed for 8 hI' in a Berlese funnel (length = 43.2 cm, sample chamber volume = 2880 cm3). Following this the nests were handsorted in search of entrapped ticks. The amber to maroon color of ticks which contrasted with the gray color of nests simplified collection of desiccated ticks. Unengorged larvae were mounted on glass slides in Hoyer's medium for identification, while nymphs in ethanol were identified using a dissecting microscope. Dried engorged larvae were expanded in lactic acid and examined in depression slides.

Analysis procedure: To examine the relationship between ticks per nest and mouse nesting, nests collected were grouped according to nesting class and location (hill slope). Data were transformed (log X 1), and tick means were reported as geometric means (Williams 1937). Confidence limits (P = 0.05) around these means were reported by Jackson (1973). Differences among ranked observations (tick numbers) for mouse nesting classes were tested by the Kruskal-Wallis ranking test (adj. H) (Siegal 1956). One-way analysis of variance was used to test differences between logarithmic means of each study site within same mouse nesting class. Variance homogeneity was tested by Fmax-tests(Sokal & Rohlf 1969) and where log (X I) transformation did not make variance independent the Wilcoxon 2-sample test (t,) was used.

+

+

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Abstract: Derl1lacelltor mriabilis (Say) larvae and nymphs were collected from P. leucopus and from cotton nests formed by them in nestboxes at Clyde, ·Wisconsin.D. variabilis larvae were seasonally distributed in 2 peaks occurring on both north- and south-facing hill slopes in late April and early June. Numbl~rs increased with increased mouse nesting, and significant relationships were detected among nesting classes.No differences were found between tick means and the 2 slopes studied. Berlese fUlUwlswen~ found to be inefficient in expelling engorged D. utlriabilis from nest material.

J.

4]0

Vol. ]2, no. 4

Med. Ent. 260

RESULTS AND DISCUSSION

Seasonal occurrence: A total of 7706 immature D. variabilis was collected from 337 of 851 nests collected when ticks were active. The occurrence of immature D. variabilis both on mice and in nests was seasonal. Engorged and unengorged larvae were first observed in warmer south-slope nests in mid-March and onset of larval activity probably preceded the observations by 4 or 5 days. In 1969 seasonal distribution of D. variabilis larvae in mouse nests occurred in 2 peaks, I at the end of April and the other in early June; these peaks were on both south (FIG. 2) and north hill slopes. The seasonal curve for the north slope closely resembled that of

f~

225

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175

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Larvae: engorged.

m%97 ____

unengorged m% 3 " ••••••••••••••••••••••

I

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#

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75

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'4

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7

Mar.

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May

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9

16 23

Jun.

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I

7

14 21

J.1.

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r

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28

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11

Aug.

FIG. 2.

Seasonal occurrence of Dermacentor variabi/is larvae in Peromyscus leucopus nests on south slope of Dunbar Hill, spring 1969; 2-point moving means.

the south slope. In 1968 an April peak had been observed, but if a 2nd occurred it was not observed because collections were not made from 10 June to 9 July. A fall larval peak as reported by Smith et al. (1946) in Massachusetts and Sonenshine et aI. (1966) in Virginia was not observed in either 1967 or 1968 in southwestern Wisconsin. Only a few larvae were found in nests after mid-August (e.g., 2 engorged larvae from each of 3 nests were collected on 2 September and 7 and 24 October 1968, respec-

TABLE I.

Total D. variabi/is collected directly from the Whitefooted Mouse, Peromyscus /eucopus, during summer 1967, Dunbar Hill, Wisconsin.

260

P. leucopus

,\

:,I ~ Ir,~

225

175

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,

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I

75

25

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Mar. FIG. I.

l.

Apr.

fnguged

Total

larue:

hand-sort

m" 85.8

-------

funnel 11%

11.2

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112 ,19 2~

May

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June 6 13 20 27 July 4 II

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D. mriabilis

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.11

Aug.

The number of engorged Dermacentor variabilis larvae in Peromyscus leucopus nests as determined by handsorting versus the number expelled from nests by the BerleseTullgren funnel procedure; 2-point moving means.

8 ]5 22 Sept. 12 Total

infested

Larvae

7 6 12

100.0 ]00.0 66.6 75.0

28 34 12

6 7 8 ]0

83.4 57.1 50.0 50.0

2] 18 3

7 8 8 8

42.9 37.5 37.5 25.0

0

8 96

0

I

Nymphs

No. per infested mouse

6 17 17 ]2

34.0 7.3 7.2 2.6

]5 12 13 14

7.2 7.5 4.0 3.0

0

6 5 7

I

I

2.0 2.0 2.3 1.0

0 130

0 125

]]

I

I

0

·Data given are for the 2nd day of a 24-hr trapping period.

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Efficiellcy oj the Berlese junnel jor collecting ticks jrom nests: A disadvantage of using the Berlese funnel technique is its selectivity against desiccationsusceptible species and ins tars which may not respond quickly enough to the rapid drying of nest material. As desiccated engorged D. variabilis larvae had been observed in cotton nests, nests were handsorted after exposure in Berlese funnels to determine the efficiency of the funnels in expelling larvae. Of nests collected on the south slope in 1969 (FIG. I), funnels failed to collect 85.8% of engorged larvae, thus yielding data that would have resulted in a serious underestimation of larval numbers and an erroneous configuration of the seasonal occurrence curve. Examination of northslope nests and those collected in spring 1968 gave similar results, indicating the necessity of handsorting for engorged ticks.

]975

Jackson & DpFo]iart: Dermacentor variabilis-mouse TABLE

relationships

411

2. Gt'ometric means and compared ranked observations (tick numbers) of engorged Dermacentor variabilis larvae among PeromJ'scus leucopus nesting classesfor spring 1968and 1969, Dunbar Hill, Wisconsin. SOUTH SLOPEt

NC

PF

CHU

NORTH

CHSA CHMA CHM]

NC

PF

CHU

SLOPEt

CHSA CHMA CHM]

Spring 1968tt

x

il

N

6.4 20

]7.6

28.8

8

8

adj. H=dO.88**; x'=9.21

14.2 12

56.2

59.2

7

8

adj. H=9.20*; x'=7.38

Spring 1969

x

il

N

3.1 3.] ]8 37 adj. H=35.98";

(]8.8) 22 x'=9.2]

1.2 4.7 17 17 adj. H=I1.45";

(]5.2) ]2 x'=9.21

Spring-summer ]969 Xil N

tively). Also, when larvae and nymphs were collected directly from mice in 1966 and 1967, none was collected after 12 August and 22 August, respectively. The decline in D. variabilis infestation of mice through the summer 1967 is summarized in TABLE I.

The 2 larval peaks can be eXplained by assuming that the life history data of Smith et al. (1946) apply in Wisconsin. It is probable that most of the engorged larvae in the 1st peak originated from overwintered eggs, although some may have overwintered as unengorged larvae. According to Smith et al., the elapsed time required from the beginning of adult D. variabilis activity to the dropping of engorged larvae from mice would be approximately 52 days. Since many female D. variabilis were observed to be active in mid-April, their progeny could be present in nests as engorged larvae by 6 June, a date corresponding with the observed 2nd peak of larvae in nests. Presence of D. variabilis nymphs in nests for each slope occurred roughly between June and midAugust with exception of 5 semiengorged nymphs collected in mid-May and 1 unengorged nymph collected on 21 October 1968. Of 191 nymphs collected from 51 nests containing nymphs in spring-summer 1968 and 1969, 35.6% were unengorged, 58.6% were semiengorged, while only 5.8% were fully engorged. The low numbers of nymphs in nests were at least partially the result of sampling without replacement, i.e., removal of engorged larvae. As stated by Sonenshine & Stout (1968a), D. variabilis is not a nidicole in the concept of Audy (1958) and Camin (1963); however, the

nymphs are given advantages similar to those of nidicolous species in that concentrations of engorged larvae in mouse nests appear to contribute to nymphal success in host acquisition.

Relationships of D. variabilis larvae to P. leucopus nesting and nest location: For each slope and within each corresponding peak period of tick occurrence, significant differences were detected (X2, PO.05) were detected between logarithmic means, even though differences between the geometric means of certain comparisons appeared large. With I exception, geometric means of north slope were larger than means of south slope. If greater nest box usage implies greater host density and therefore closer host spacing (Mohr 1961), then larger means would be expected on the south slope, as, during both spring seasons, use of nesthoxes by mice was greater on the south slope (Jackson & DeFoliart 1975). This appears to agree with the results of Sonenshine & Stout (1968a) who found no relation between host spacing and mean infestation of trapped P. leucopus by D. variabilis. LITERATURE CITED

Audy, J. R. ]958. The loca]ization of disease with special reference to the zoonoses. Trans. Roy. Soc. Trap. Med. Hy~. 52: 309-28.

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1.3 2.9 (]3.1) 24.2 1.3 5.4 (19.5) 19.8 16 22 ]2 8 8 10 16 10 adj. H='26.9]"; x'= 11.34 adj. H= ]7.]8"; x'=] 1.34 tMouse nt'sting classes: NC=nonchambered, PF=platform feeding, CHU=chambered unspecified, CHSA=single adult, CHMA=multiple adults, CHMJ = multiple juveniles. ttSpring ]968, 1st peak (12.IV.-17.V.); spring 1969, 1st peak (7.IV.-]2.V.); spring-summer ]969, 2nd peak (19.V.-23.VI.); xll=geometric mean; N=no. of nests. ·Significant at P