Oviposition and eclosion periods of Ixodes didelphidis Fonseca and Aragão, 1951 (Acari: Ixodidae) under laboratory conditions

Oviposition and Eclosion Periods of Ixodes didelphidis Fonseca and Aragão, 1951 (Acari: Ixodidae) under Laboratory Conditions

Vol. 95(6): 905-908, Nov./Dec. 2000

Darci Moraes Barros-Battesti/⁺, Valeria Castilho Onofrio, Simone Michaela Simons, Virginia Lucia Nazario Bonoldi*, Natalino Hajime Yoshinari*

Laboratório de Parasitologia, Instituto Butantan, Av. Vital Brazil 1500, 05503-900 São Paulo, SP, Brasil *Laboratório de Doença de Lyme, Departamento de Clínica Médica, Faculdade de Medicina, USP, São Paulo, SP, Brasil

Oviposition and eclosion periods for Ixodes didelphidis were observed under two temperatures (25ºC and 27ºC) and 90-95% humidity. Although there was a significant increase in the eclosion period (p<0.05) and a tendency to increase the oviposition period at 25ºC, there was neither significant differences in the interval (days), until maximum peak of eclosion nor in the number of emerging larvae during the peak nor the total number of emerged larvae. These temperature values are not critical for embryological development of the species. Because at 27ºC and under high humidity the oviposition and eclosion periods are shorter, and the percentage of emerged larvae is higher, we consider this to be the ideal temperature for laboratory studies.

Key words: Ixodes didelphidis - oviposition - eclosion - laboratory conditions

The most recent studies on Brazilian species of the genus Ixodes, were carried out by Labruna et al. (1997), on the weight of eggs from six ixodid species from Brazil; Barros-Battesti (1998), on the parasitism of Ixodes didelphidis and I. loricatus on small mammals; Barros-Battesti and Knysak (1999), on the geographical distribution of Brazilian Ixodes species; Arzua and Barros-Battesti (1999), on the relationship between I. (Multi-dentatus) auritulus and hosts; and Faccini et al. (1999), on the biological characteristics of the non-parasitic phases of I. amarali. Nevertheless, the biology of these species is still unknown.

I. didelphidis Fonseca and Aragão, 1951 is a tick species endemic to Brazil, with registered occurrences in the Southern, Southeast and Central-West regions (Barros-Battesti & Knysak 1999). Both sexes are found parasiting marsupials, while immature stages feed on small wild rodents. I. didelphidis is a species closely related to I. loricatus, but they differ in the morphology of the spiracular plate. Although Morel and Perez (1978) synonymized I. didelphidis and I. loricatus Neumann, and Camicas et al. (1998) included the species in the Amerixodes subgenus, we opted to maintain them as separate species, in Ixodes subgenus, until more conclusive studies have been carried out. In addition, the difficulty in maintaining colonies in order to study each development stage is due to the lack of knowledge of physical (temperature and humidity) and biological (feeding adaptation on laboratory hosts) aspects.

In Itapevi county, State of São Paulo, where Lyme-like disease cases are recognized, I. didelphidis and I. loricatus were found naturally infected by spirochetes and seem to play an important role in their enzootic transmission cycle (Yoshinari et al. 1997, Barros-Battesti 1998, Yoshinari et al. 1999).

The aim of this study was to obtain data on the oviposition period of females and daily eclosion rate of I. didelphidis larvae under controlled conditions of temperature and humidity. This would allow an evaluation of the effects of temperature variations (25^oC and 27^oC) on oviposition and eclosion periods, total number of eggs, daily eclosion rate for each oviposition, in order to maintain colonies in laboratory.

During the first six months of 1999, females of I. didelphidis were collected from Didelphis aurita Wide-Neuwied (Didelphimorphia: Didelphidae). These were captured monthly from the woods and surrounding areas of a residential condominium in Itapevi county, SP (23^o32'45''S and 46^o56" 05''W). The area is composed of fragments of disturbed Atlantic Forest. Altitude varies from 715 m to 900 m (Ponçano et al. 1981), and the climate is type Cwb (mesothermic with dry winter and mild summer), with an annual rainfall ranging from 1300 to 1500 mm and annual mean temperature ranging from 20^oC to 22ºC.

Seven engorged females of I. didelphidis, weighing 350 mg with an average of 1.2 mm length and 0.8 mm width, were identified according to Aragão and Fonseca (1961). These females were kept in transparent vials covered with cloth and maintained in BOD (Biological Oxygen Demand) incubation chamber under high humidity (85-95%), scotophase. Three females and their eggs were exposed to a temperature of 27^oC. The remaining four females and their eggs were kept at 25^oC.

Females were examined daily. Following oviposition, all females were treated according to the technique described by Takada et al. (1994). They were then dissected in sterilized conditions, using tweezers and scalpels, in petri dishes containing 1ml of BSK medium (Barbour 1984) and 3 µl of Kanamicin antibiotic. The material was inoculated into 5 ml of BSK medium and 15 µl of Kanamicin. Cultures were maintained at 33^oC and examined weekly under dark field microscope. This procedure was necessary to exclude the possibility of colonies contaminated with spirochetes by transovarial route.

The oviposition period, total number of eggs, daily number of emerged larvae, interval (days) until maximum eclosion peak was reached, and the total eclosion period obtained in each oviposition, were recorded.

Data were analyzed by means of Mann-Whitney (U) test that used to compare independent samples. Statistical analysis was performed using the SPSS statistical program package.

All females used during the present study were not infected by spirochetes.

The total number of eggs, daily number of emerged larvae, the total eclosion period, and the maximum number of larvae obtained in the peak of eclosion, for each female, are shown on Table I.

Table I

The oviposition period (mean ± SD, range), the interval (days) until maximum eclosion peak, and the eclosion period (mean ± SD, range), registered for females exposed at temperature of 27^oC and 25^oC under high humidity, are shown on Table II.

Table II

The average percentage of eclosion was 96% (± 4.51) at 27^oC. At the temperature of 25^oC, the average percentage eclosion was 93% (± 1.07). On the first day of oviposition, the number of eggs varied between one and 20. The average number of emerged larvae until the maximum peak was 878 (± 199.10), at 27^oC, and 894.25 (± 187.04), at 25^oC. At both temperatures, it was observed that at 25^oC there was a tendency to increase the oviposition period. Consequently, there was an increase significant in the eclosion periods (U<0.001; N=7; P=0.029) under the same conditions, when they were analyzed by the Mann-Whitney statistical test (Table III).

Table III

There are evidences that in the environment, ticks remain in the host's burrow after detachment (unpublished data). It is probable that temperature and humidity are relatively constant and higher in these microhabitats. Since nothing is known about ideal conditions for colony maintenance of this Ixodes species, both temperatures were used, and the incubation chamber's humidity maintained high, taking into consideration the environmental conditions of the collection area. In a pilot test, when ticks were exposed to 27^oC and 80% humidity, the emergence percentage did not reach 30% and the eggs showed signs of dehydration.

Faccini et al. (1999) observed smaller laying and eclosion periods for an engorged I. amarali female weighing 433,5 mg and maintained at 27^oC and a relative humidity higher than 80%. The eclosion percentage registered for this species was of 34%. The low eclosion rate could be explained by the humidity of the incubation chamber and the dehydration of the eggs during manipulation, since these were weighed during the experiment. In the present study this kind of interference was avoided until the end of oviposition, when the females were removed without damaging the eggs. It is likely that this procedure favored the high eclosion rates for I. didelphidis.

The length of the oviposition period for each I. didelphidis egg was not measured, but both temperatures did not cause a significant difference in the number of eggs laid on the first day. Oliver (1989) related that, for I. ricinus, the laying of each egg can take from 3 to 12 min.

The daily eclosion rates for I. didelphidis larvae reached a peak between the fifth and seventh day (Table I). According to Oliver (1974) the daily peak for eggs produced by the majority of Ixodidae usually occurs between the third and sixth day, diminishing gradually. Though the daily oviposition rate for I. didelphidis was not registered, the daily number of emerged larvae suggests a possible relation between the daily oviposition rate and the daily eclosion rate, until the peak is reached.

Considering that high temperatures accelerate the pre-oviposition, oviposition and incubation periods of the eggs, and increases the number of eggs laid daily (Oliver 1989), it would be expected that, under different temperature conditions significant differences would occur in the oviposition and eclosion periods, in the daily eclosion rates and also in the interval (days) until peak eclosion for I. didelphidis was reached. However, despite the significant increase in eclosion period (p<0.05) at 25°C, there was no significant increase in the interval (days) until the peak of eclosion, nor in the number of larvae emerged during the peak, nor in the total number of larvae emerged in each oviposition. There was also no difference in the number of emerged larvae until the maximum peak. Despite the tendency to increase the average oviposition period, at 25ºC (Table II), the value obtained was not significant. Maybe, the value would have been significant if the sample had been larger.

Therefore, the temperature values used in this study were not critical for the embryological development of the studied species. Since at 27ºC, at high humidity, the oviposition and eclosion periods are shorter and there is a higher percentage of emerged larvae, we consider this to be the ideal temperature for laboratory studies, but we suggest further testing of humidity variation.

To Marcia CA Prata, Universidade Federal Rural do Rio de Janeiro, and Cristina A Rheims, Intituto Butantan for the critical analysis of the manuscript, Hilton F Japyassu, Intituto Butantan, for statistical assistance, Silvia HAF Cardoso and Roldon Santos do Paço for laboratory assistance.

Table I | Table II | Table III

This study was supported by the Fundação de Amparo à Pesquisa do Estado de São Paulo, project Fapesp no. 98/11666-8.

Received 28 February 2000

Accepted 5 July 2000

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Publication Dates

• Publication in this collection
  09 Nov 2000
• Date of issue
  Dec 2000

History

• Accepted
  05 July 2000
• Received
  28 Feb 2000