Flight activity and invasion of houses by Triatoma sordida and T. guasayana were studied in the Province of Santiago del Estero, Argentina. Spontaneous findings of both species in houses were recorded from 1982 to 1989. Light trap collections were performed in 1982, 1983 and 1984, at the woods surrounding the settlements of Amamá (43 houses) and Trinidad (19 houses). Most of the 101 triatomines collected, were unfed and negative for Trypanosoma cruzi. T. guasayana predominated over T. sordida, and both appeared on the lighted screens between 19-31 min (mean 24) after dusk and the catch time was 30-45 min. Although entomological evaluation of 41 houses at Amamá performed in September 1985, just before insecticidal spraying, showed that Triatoma infestans predominated, adults of T. guasayana were collected in sleeping places, in 7 houses (17%). Most triatomines invading houses from then up to 1990 were flying T. guasayana (20/27) and females outnumbered males. Three non-infected T. guasayana females were fed on man and two T. guasayana males positive for "T. cruzi like" trypanosomes were unfed. Therefore, visiting hungry adults could transmit T. cruzi to people and introduce wild parasites to the domestic cycle. T. guasayana stands as the main potential substitute of T. infestans in the studied area, and it might play there the same role as T. sordida in Brazil.

Anastrepha fraterculus is an important pest of commercial fruits in South America. The variability observed for morphological and behavioural traits as well as genetic markers suggests that A. fraterculus represents a complex of synmorphic species rather than a single biological species. We studied the correlation between geographical distribution and genetic variation in natural populations from Argentina and south Brazil. Fragments of the mitochondrial gene COII were sequenced in 28 individuals. The matrix of aligned sequences was phylogenetically analysed by parsimony, yielding a cladogram of haplotypes. Based on Templeton's nested method, no clade showed any geographic pattern for the gene COII, indicating lack of significant association between haplotypic variability and geographic distribution. The analysis of nucleotide substitution distances by Neighbour-Joining algorithm showed that geographically distant populations exhibit low genetic distances. The corresponding trees clustered the populations without showing any geographic pattern. This result suggests that the populations studied are not reproductively isolated.

Trypanosoma cruzi infection in sylvatic mammals of the quebracho woods of the eastern part of Santiago del Estero province, Argentina, was studied from October 1984 to December 1987. 301 mammals of 20 different species were caught. T. cruzi, characterized biologically and biochemically, was isolated by xenodiagnosis from 23 of 72 (32%) Didelphis albiventris opposums, 2/36 (5.5%) Conepatus chinga skunks, and one ferret (Galictis cuja). 53 opossum refuges were located and triatomine bugs were found in 2 of them: one male Triatoma infestans, infected with T. cruzi, and 5 uninfected nymphs of T. sordida, had all fed on opossum blood. Electrophoretic zymogram patterns of the T. cruzi populations isolated from opossums and skunks were similar to isoenzyme profiles already described for populations isolated from infected humans in Argentina. The small number of triatomines found in the opossum refuges seems inadequate to account for the prevalence of T. cruzi infection recorded for these mammals, so other possible contaminative routes of infection should be investigated.

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Herbivore host specialization includes changes in behavior, driven by locally induced adaptations to specific plants. These adaptations often result in sexual isolation that can be gauged through detection of reduced gene flow between host associated populations. Hypothetically, reduced gene flow can be mediated both by differential response to specific plant kairomones and by the influence of larval diet on some adult traits such as pheromone composition. These hypotheses could serve as a model to explain rapid radiation of phytophagous tephritid fruit flies, a group that includes several complexes of cryptic species. The South American Fruit Fly Anastrepha fraterculus (Wiedemann) is a complex of at least seven cryptic species among which pheromone mediated sexual isolation resulted in rapid differentiation. Cryptic species also exhibit differences in host affiliation. In search of a model explaining rapid radiation in this group, we studied host plant chemical composition and genetic structure of three host associated sympatric populations of A. fraterculus. Chemical composition among host plant fruit varied widely both for nutrient and potentially toxic secondary metabolite content. Adaptation to plant chemistry appears to have produced population differentiation. We found host mediated differentiation to be stronger between populations exploiting sympatric synchronic hosts differing in chemical composition, than between populations that exploit hosts that fruit in succession. Gene flow among such host associated populations was extremely low. We propose as a working hypothesis for future research, that for those differences to persist over time, isolating mechanisms such as male produced sex pheromones and female preferences resulting from adaptation to different larval diets should evolve.

SUMMARY -Parallel polymorphisms for B chromosome and supernumerary segments on M,, s. and S,. pairs have been detected in two Argentinian populations of Dichroplus elongatus, Raco (RA) and Tafi Viejo (TV) (Tucuman Province). All these polymorphisms seem to be temporally stable. In all samples of TV, it was observed a tendency towards an excess of the B + SS, class. SS, produces a total restriction of chiasma formation in proximal position in the involved pair in heterozygous and homozygous males. In TV a consistent dose effect of B chromosome on interstitial chiasma (X;) frequency was detected in males. The X; frequency vs. number of supernumeraries plot fits to a linear regression. In both populations, the individuals with B's showed a significant increase in macro-and microspermatid production. This effect is more marked at RA. The cytophotometric analysis of DNA content revealed that in both types of males, B and non B carriers, most macrospermatids are diploid suggesting a probably origin by cytokinesis blocking. However, spermatids with 3 to 5C contents in B carriers were also detected showing that other mechanism (i.e. cell fusion) may also account to explain the effects of B's on abnormal sperm formation.

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