Biology of Microplitis mediator Haliday (Hymenoptera: Braconidae) parasitizing the wheat armyworm Mythimna (Pseudaletia) sequax Franclemont (Lepidoptera: Noctuidae)

Biologia de Microplitis mediator Haliday (Hymenoptera: Braconidae) parasitando a lagarta do trigo Mythimna (Pseudaletia) sequax Franclemont (Lepidoptera: Noctuidae)

BIOLOGICAL CONTROL

Biology of Microplitis mediator Haliday (Hymenoptera: Braconidae) parasitizing the wheat armyworm Mythimna (Pseudaletia) sequax Franclemont (Lepidoptera: Noctuidae)

Biologia de Microplitis mediator Haliday (Hymenoptera: Braconidae) parasitando a lagarta do trigo Mythimna (Pseudaletia) sequax Franclemont (Lepidoptera: Noctuidae)

Luís A. Foerster; Augusta K. Doetzer

Depto. Zoologia, Universidade Federal do Paraná, C. postal 19.020, 81531-990, Curitiba, PR

ABSTRACT

The larval endoparasitoid Microplitis mediator Haliday is described for the first time in the Neotropical Region, parasitizing the wheat armyworm Mythimna (Pseudaletia) sequax Franclemont. Females successfully parasitized the four first instars of the host, with a significant preference for the second and third, in relation to either the first or fourth instars. No parasitoids emerged from caterpillars parasitized in the fifth and sixth instars. Development of M. mediator was determined at four constant temperatures between 15°C and 30°C. The duration of the egg-larval stages varied from 32.3 days at 15°C to 8.0 days at 30°C. The pupal stage lasted 24.3 and 5.1 days at 15°C and 30°C, respectively. Development of the parasitoid from oviposition to adult emergence required the total of 251.3 degree-days (DD) above the threshold of 10.3ºC. At 20°C, longevity of females was significantly shorter when kept in oviposition activity (16.8 ± 1.81 days) in comparison to females kept without hosts (29.7 ± 1.83 days). Parasitoid females remained reproductively active until one day before death, and parasitized an average of 60.6 ± 8.40 second instar hosts during their adult life, with an average of 3.9 parasitized caterpillars/day.

Key words: Insecta, endoparasitoid, thermal requirement, biological control

RESUMO

O endoparasitóide larval Microplitis mediator Haliday é descrito pela primeira vez na Região Neotropical, parasitando a lagarta do trigo Mythimna (Pseudaletia) sequax Franclemont. Fêmeas parasitaram com sucesso hospedeiros do primeiro ao quarto instar, com preferência pelo segundo e terceiro instares. Parasitóides não emergiram de lagartas de quinto e sexto instares ofertadas para o parasitismo. O desenvolvimento de M. mediator foi determinado em quatro temperaturas constantes entre 15ºC e 30ºC. A duração dos estágios de ovo-larva variou entre 32,3 dias e 15ºC e 8,0 dias a 30ºC. A duração do estágio pupal foi entre 24,3 e 5,1 dias, a 15ºC e 30ºC respectivamente. O desenvolvimento do parasitóide da oviposição ao estágio adulto requereu o total de 251,3 graus-dia, acima da temperatura-base de 10,3ºC. A 20ºC, a longevidade das fêmeas mantidas em atividade de oviposição (16,8 ± 1,81 dias) foi significativamente menor em comparação a fêmeas mantidas na ausência de hospedeiros (29,7 ± 1,83 dias). Fêmeas do parasitóide permaneceram ativas reprodutivamente até um dia antes da morte, e parasitaram em média 60,6 ± 8,40 hospedeiros de segundo instar durante a vida, com a média de 3,9 lagartas parasitadas por dia.

Palavras-chave: Insecta, endoparasitóide, exigência térmica, controle biológico

The solitary larval endoparasitoid Microplitis mediator Haliday is a palaeartic species, distributed from Central Europe to China (Arhur & Mason 1986). In the early 1990's it was introduced in Canada for the control of the armyworm Mamestra configurata Walker, a pest of rapeseed and canola crops (Mason et al. 1998). Field surveys conducted in 1996 on wheat in Southern Paraná State, Brazil resulted in the capture of Mythimna (Pseudaletia) sequax Franclemont larvae parasitized by M. mediator. In Southern Brazil, the larval stage of M. sequax hosts more than ten species of dipterous and hymenopterous parasitoids (Gassen 1986), of which only Glyptapanteles muesebecki (Blanchard) (Hymenoptera: Braconidae) and Euplectrus ronnai (Brèthes) (Hymenoptera: Eulophidae) have been investigated (Doetzer & Foerster 1998; Yamamoto et al. 1998; Foerster et al. 1999a, b). Data on the biology of M. mediator are given by Tanaka et al. (1984) on larvae of Leucania separata (Walker) and by Arthur & Mason (1986) on M. configurata. Due to the lack of knowledge on the subject, we report results on the development of M. mediator in relation to temperature, host instar preference and adult longevity and reproduction, when the parasitoid was reared on larvae of M. sequax.

Material and Methods

A culture of M. mediator was established in laboratory from parasitized larvae of M. sequax collected on a wheat crop in Lapa, PR, Brazil. Parasitoids were maintained in the laboratory on the same host at 20 ± 1ºC, 65 ± 5% RH and a photophase of 12h. Host larvae were reared individually and fed with kicuyo grass Pennisetum clandestinum Hochts. (Foerster 1996), at similar climatic conditions as for the parasitoids.

To determine the host instar range of M. mediator, two-day old mated females of the parasitoid were exposed for 6h to 18 host larvae in a 14-cm diameter petri dish. Each dish contained three M. sequax larvae from first to sixth instar and one female parasitoid. The free-choice tests were conducted in a climatic chamber at 20°C, photophase of 12h and were repeated 10 times. After the exposure period, the females were removed and the larvae reared individually in 80 ml polythene flasks and fed with kicuyo grass. The number of parasitized caterpillars in each instar was recorded and the results were submitted to analysis of variance. Differences in the proportion of parasitized hosts in relation to the instars were compared by Tukey's test (P < 0.05).

The effect of temperature on the developmental time of M. mediator was determined from oviposition to pupation and from pupation to adult emergence at the temperatures of 15º, 20º, 25º and 30ºC ± 1ºC and photoperiod of 12h. Second instar hosts were individually exposed to mated females of the parasitoid at each temperature for 1h. After this period, the female was removed and the larvae were reared as described by Foerster (1996). The lower threshold temperature (T_o) and the thermal constant (K) in degree-days above T_o were calculated for egg+larvae, pupae, and total immature stages by the linear regression equation Y = a + bX, where "Y" is the reciprocal of the developmental time (days) and "X" is the temperature in ºC (Campbell et al. 1974).

Adult longevity and age-specific oviposition rate of M. mediator females were determined at 20ºC. Longevity was evaluated both in the presence and absence of hosts; a total of 27 newly emerged couples were maintained separately in 2 x 10 cm test tubes and fed with undiluted honey streaked on the wall of the tubes. In one of the treatments (n = 17) longevity of males and females was recorded without the exposure of hosts to the parasitoids, and in the other treatment (n = 10) 15 second instar M. sequax larvae were offered each day to the parasitoids, from adult emergence until the death of the female. After 24h of exposition, the caterpillars were removed and a new batch of hosts was introduced into the tubes. After the exposition period, the caterpillars were reared individually as described to determine the number of parasitized hosts at each day. The longevity of males and females in both treatments was compared by the 't' test for differences between means (P < 0.05).

Voucher specimens of M. mediator are deposited in the entomological collection "Padre Jesus Santiago Moure", Departamento de Zoologia, Universidade Federal do Paraná, Brazil.

Results and Discussion

Females of M. mediator successfully parasitized first to fourth instar M. sequax larvae, as observed by Tanaka et al. (1984) for the same species parasitizing L. separata. On larvae of M. configurata, successful parasitism by M. mediator occurred only in the first three instars (Arthur & Mason 1986). A significant preference for second and third instar hosts was recorded in free-choice tests in relation to the first and fourth instars (Fig. 1). Fifth and sixth instar hosts failed to produce parasitoids; however no attempts were made to determine whether females oviposited in such hosts.

Unlike parasitism by E. ronnai (Yamamoto et al. 1998), another larval parasitoid of M. sequax, caterpillars parasitized by M. mediator continued to molt, and a single parasitoid larva emerged from the host to pupate, before the host completed the larval stage.

Time of development of the egg+larval stages ranged from 8.0 to 32.3 days at 30ºC and 15ºC, respectively (Table 1). The pupal stage of M. mediator lasted 5.1 days at 30ºC and 24.3 days at 15ºC, and total development from oviposition to adult emergence ranged from 13.1 days to 56.6 days at 30ºC and 15ºC, respectively (Table 1).

The internal stages of M. mediator were more cold tolerant (T_o = 9.9ºC) than the pupal stage (T_o = 10.8ºC), and development from oviposition to adult emergence required a total of 251.3 DD above a lower threshold of 10.3ºC (Table 2). Compared to other hymenopterous parasitoids of M. sequax, the rate of development of M. mediator is faster than that of G. muesebecki, which requires 397.4 DD to reach the adult stage (Foerster et al. 1999b), and slower than E. ronnai, which completes its development in 211.4 DD (Yamamoto et al. 1998).

The longevity of female M. mediator at 20ºC was significantly shorter when in oviposition activity (mean of 16.8 days) in comparison to those not exposed to hosts (29.7 days). Males were significantly shorter lived than females in the absence of hosts, but lived longer than females in activity of oviposition (Table 3). Similar results were reported by Browning & Oatman (1985) for Microplitis brassicae Muesebeck, which attributed the shortened longevity of females exposed to hosts to the expenditure of energy in searching and oviposition and to injury due to host defensive reaction.

Females remained reproductively active until one day before death, and only after the eight day following emergence the daily oviposition rate decreased significantly in relation to the initial eight days (Fig. 2). On average, females parasitized almost four hosts/day and more than 60 caterpillars were parasitized during the lifetime of the females (Table 3). Browning & Oatman (1985) recorded for M. brassicae an average of 73.2 progeny per female at 21.1ºC during a lifetime of 19.0 days in the presence of hosts. The capacity of parasitism by M. mediator is ca. three times higher than that of G. muesebecki, a gregarious endoparasitoid of M. sequax in Brazil, which at 21ºC parasitized an average of 20.8 caterpillars during the female lifetime (Foerster et al. 1999a).

Acknowledgments

The authors are grateful to Dr. James B. Whitfield of Arkansas University for the identification of M. mediator. This research was financed by the Conselho National de Desenvolvimento Científico e Tecnológico (CNPq) and Fundação Araucária.

Literature Cited

Received 18/04/02

Accepted 30/09/02

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Publication Dates

• Publication in this collection
  03 June 2003
• Date of issue
  Jan 2003

History

• Received
  18 Apr 2002
• Accepted
  30 Sept 2002