BIOLOGY OF Microctonus sp . ( HYMENOPTERA : BRACONIDAE ) , A PARASITOID OF Cyrtomon luridus BOH . ( COLEOPTERA : CURCULIONIDAE )

Cyrtomon luridus (Boh.), a native insect of wild Solanaceae, has adapted to the medicinal plant Duboisia sp., introduced from Australia, causing damages up to 100% mortality. Microctonus sp. is the most important natural enemy of C. luridus and its potential for biological control was investigated in this work. This study was carried out in Arapongas, Paraná State, Brazil, from 1994 to 1996. Parasitism of C. luridus by Microctonus sp. was highest from February through June (maximum of 54% in May 1996), when the C. luridus adult population was decreasing. The female sex ratio of Microctonus sp. under natural conditions was 0.57 to 0.69, which was close to 1 male : 2 female. Production of males occurs parthenogetically (arrhenotoky). In the laboratory, parasitism reached 40% when two adult C. luridus were exposed per parasitoid. The number of Microctonus sp. adults that emerged per parasitized beetle ranged from 4.7 to 14.2. Larvalpupal viability was 31.7 to 64.8% and the female sex ratio ranged from 0.0 to 0.37, with prevalence of males. The egg-pupal period was 12.7 days and the pupal-adult period was 10.7 days, resulting in a mean life cycle (egg-adult) of 22.4 days for this parasitoid (25oC, 70% R.H.). This is the first report of a new species of Microctonus sp. in C. luridus.


INTRODUCTION
Species belonging to the genus Cyrtomon occur in Neotropical regions, and in Brazil, Cyrtomon luridus (Boheman) has been recorded in the States of Paraná, Santa Catarina and Rio Grande do Sul (Lanteri, 1990).This weevil develops in wild Solanaceae (wild tobacco, coerana, etc.), as well as on cotton and eucalyptus (Sérgio A. Vanin 1 ).C. luridus is a univoltine insect; its larvae feed on roots and adults consume leaves of plants.It has Sci.Agric.(Piracicaba, Braz.), v.61, n.5, p.538-541, Sept./Oct.2004 adapted to and seriously damages a medicinal plant, Duboisia sp.(Solanaceae) (hybrids between D. myoporoides and D. leichhardtii) (Ohlendorf, 2002), causing up to 100% mortality.Introduced from Australia, this plant is rich in scopolamine, an alkaloid widely used for human and animal health.

MATERIAL AND METHODS
Adults of C. luridus were collected in the field from Duboisia sp., from February 1994 to June 1996, in Arapongas, PR, Brazil (23º21'21'' S;51º29'27'' W).Individual insects were placed in rearing containers (200-mL plastic cups), covered with plastic (PVC) film, along with Duboisia sp.leaves as a food source.The containers were maintained in the laboratory (25 ± 2 o C.; R.H. 70 ± 10%; 12h photo phase), to determine the degree of parasitism.The female sex ratio [s.r.= females / (males + females)] of Microctonus sp. that emerged from the adult weevils was calculated.
Parasitoids and adults of C. luridus were maintained in the same Duboisia sp.containers described above.They were exposed to adult Microctonus sp. to determine the parasitism rates.In this case, the ratio of adult C. luridus per Microctonus sp.female ranged from 1 to 3, with continuous host exposure until death of the parasitoid occurred.The number of replications ranged from 20 to 184, depending on the number of parasitoids available.
Larva-pupal viability, number of parasitoids that emerged per parasitized beetle, female sex ratio in the laboratory and the duration of the life cycle (egg to adult) in Microctonus sp., were obtained by examining progeny daily.The superparasitism or polyembriony were not the object of this study.The type of reproduction of Microctonus sp. was determined by examining the sex ratio of progeny of previously mated (during 24h) or unmated females with 30 replications, and data presented as average ± standard deviation.

RESULTS AND DISCUSSION
The rate of parasitism of C. luridus by Microctonus sp.under field conditions ranged from 0 to 54%, from February 1994 to June 1996.Parasitism rates were higher from February to June, when adult populations of C. luridus on Duboisia sp. were low, compared to October to January (Figure 1).The highest parasitism rate occurred in May 1996 (54%), in areas with taller plants (about 2.0 m) and fully developed foliage, which apparently is a preferred habitat of Microctonus sp.The best time to introduce the parasitoid probably depends on the development stage of Duboisia, which was not investigated in this study.However, at the final phase of emergence of adult C. luridus, the parasitism rate was not high enough to avoid economic damage to Duboisia sp.If this parasitoid is to be mass-reared in the laboratory, it probably should be released in the field in September, when adults begin to emerge, in order to maximize the degree of control.In isolated areas containing trap plants left as refuges for the weevils to be controlled with insecticides, there was no parasitism.Therefore, parasitoid releases should be avoided in areas subjected to chemical control practices.
The parasitism pattern observed in Brazil is typical of Microctonus sp.In New Zealand, Goldson et al. (1990) found 85-100% infestation by Microctonus aethiopoides (Loan) in beetles remaining from an annual generation of Sitona discoideus (Gyllenhal), with minimal parasitism when new beetles emerged.The lack of oviposition by the parasitoid during the winter (overwintering diapause) resulted in minimal parasitism in August and September.In addition, Goldson et al. (1998) observed in New Zealand three generations per year of Microctonus hyperodae (Loan), which were produced in Listronotus bonariensis (Kuschel) after diapause in September.In this case, the maximum parasitism rate reached 90% in March, decreasing rapidly in April, with the appearance of new adults from neighboring fields free of parasitoids.This conclusion supports the suggestion to release Microctonus sp. in September to control C. luridus.However, more studies are necessary.In contrast, Copley & Grant (1998) observed poor colonization by M. aethiopoides introduced for the control of Hypera postica (Gyllenhal) in alfalfa fields in Tennessee, USA.Barratt et al. (1997) obtained only a maximum parasitism rate of 6.0% in Sitona lepidus (Gyllenhal) by M. aethiopoides or M. hyperodae in laboratory cage tests, indicating that these species would be ineffective for the biological control of this pest in New Zealand pastures.
The sex ratio of Microctonus sp.under natural conditions was from 0.57 ± 0.11 to 0.69 ± 0.10, which is close to 1 male : 2 female, very close to 0.60-0.65 found in the nature in M. aethiops (Nees) (= aethiopoides Loan) by Fusco & Hower (1974).In the laboratory, the sex ratio ranged from 0.16 ± 0.13 to 0.37 ± 0.13 (Table 1), with a predominance of male parasitoids.Therefore, rearing conditions were probably inadequate for normal mating or the continuous oviposition produced more males, as demonstrated by Fusco & Hower (1974).Beetles parasitized by unfertilized females of Microctonus sp.produced only males.Therefore, reproduction in Microctonus sp. can be sexual or by arrhenotokous parthenogenesis, a common occurrence in hymenoptera.
The maximum parasitism rate in the laboratory was 40% in May/95 (Table 2), when two adults C. luridus were placed with one female Microctonus sp. in each rearing chamber.However, parasitism rates of 30.7, 19.5 and 20.7% were obtained in July/95 with the same 2:1 ratio.In general, the percentage of parasitism for this host/parasitoid ratio was higher than that obtained with 1:1 (33% and 32%, respectively) or 3:1 (19%), both of them resulting in a low level of parasitism.In all cases, C. luridus attempted to avoid attack by the parasitoid by moving the last pair of legs.Premature mortality of the infested beetles also was observed.Premature mortality is a typical result from superparasitism, which is common in arrhenotokous parasitoids, but it was not the object of this study.However, premature mortality also can be caused by bacterial transmission from the invasion of pathogens in the wounds produced by Microctonus oviposition (Jackson & McNeill, 1998).
Overall, parasitism of C. luridus by Microctonus sp. in this study was lower than results found by other authors, who investigated different species and host/ parasitoid densities.In the New Zealand, the mean parasitism rate of L. bonariensis by Microctonus hyperodae was 58.7% (Goldson et al., 1993), with a ratio of 40 beetles : 1 female parasitoid.Fusco & Hower (1974) found a maximum parasitism of 51.6% when Hypera postica was parasitized by M. aethiops with 50:1 ratio, with a low percentage of premature mortality.
The mean number of adult Microctonus sp. that emerged from each parasitized C. luridus ranged from 4.7 ± 1.9 to 14.2 ± 8.0, corresponding to a larva-pupa viability from 31.7 ± 9.9 % to 64.8 ± 20.4 % (Table 1).The larvae of Microctonus sp.change into pupae protected by cocoons, after they leave the beetles.The mean duration of the full cycle (egg-adult) was 22.4 ± 1.1 days, including 11 to 14 days (mean of 12.7 ± 1.1 days) for the eggpupa period, and 9 to 13 days (mean 10.7 ± 0.95 days) for the pupa-adult period.These values were very close to those found by Goldson et al. (1993), who reported a life cycle of 22.4 days for M. hyperodae raised on L. bonariensis, although variations in fecundity can be different according to habitats, exposure to the host and genetically distinct populations (Phillips et al., 1996;Winder et al., 1997).
Difficulties with mass rearing of Microctonus sp. for release in the field are the main limiting factor for using this parasitoid for augmentative biological control of C. luridus.An artificial diet for C. luridus would facilitate the rearing of this parasitoid.Also, a successful biological control program with Microctonus sp. may depend on the timing of release (at the time of the first appearance of C. luridus adults) (Figure 1).

Figure 1 -
Figure 1 -The parasitism rates of Microctonus sp. in the Cyrtomon luridus population on Duboisia sp.under field conditions.
1 n = number of observations.