Masahiro Takagi

Effect of landscape structure on anopheline mosquito density and diversity in northern Thailand: Implications for malaria transmission and control

The influence of landscape structure on anopheline mosquito density and diversity was studied in a comparison of agricultural and forested landscapes in northern Thailand. Agriculture locations had significantly higher landscape diversity, more patches, smaller mean patch sizes, and more complex patch shapes than forest locations. Mosquito collections were undertaken during both dry and wet seasons from October 1997 to December 1999. The density of two forest-associated species, Anopheles maculatus s.s. and Anopheles minimus s.l., both primary malaria vectors in Thailand, was significantly higher in forest locations in at least one season. The density of two paddy field-associated species, Anopheles aconitus and Anopheles hyrcanus group did not differ between locations. Anopheles aconitus is a secondary malaria vector and An. hyrcanus group is not considered as a vector in Thailand. The density of An. minimus s.l. was positively related to forest mean patch size, various water and paddy field landscape metrics and negatively related to landscape diversity. Anopheles hyrcanus group was also positively related to water metrics. Anopheline species diversity was negatively related to landscape diversity. Forest fragmentation resulting from human economic activities often increases landscape heterogeneity, which may result in a reduction in anopheline species diversity, as was the case in this study. There are indications that the effect of fruit orchards on anopheline diversity might be different in the dry season compared to the wet season. Fruit orchard landscape metrics affected species diversity negatively in the dry season and positively in the wet season. One reason for this could be that pesticides are typically applied in fruit orchards during the dry season. The conversion of forests to fruit orchards is a major land-use change in northern Thailand. These results show the complexity of vector status in northern Thailand and that vector and agriculture pest control are intricately interrelated. It is therefore important to include both the public health and agricultural sectors in controlling malaria vectors in the country. Our results also indicate that if landscape management should be used for malaria control in northern Thailand large-scale reduction and fragmentation of forest cover would be needed. Such drastic actions do not agree well with current global objectives concerning forest and biodiversity conservation

Characteristics of Anopheles minimus (Diptera: Culicidae) Larval Habitats in Northern Thailand

Abstract The effects of breeding habitat characteristics on the larval density of Anopheles minimus Theobald were studied in a perennial stream in the foothills of Chiang Mai province, northern Thailand. Data on 41 different variables related to plant cover and stream physical attributes in 200 sections, each 10 m long, were collected along with larval data during the dry and wet seasons of 1998 and 1999. Larval density was significantly higher in the dry season than in the wet season. In the dry season, An. minimus density was negatively correlated with water current velocity, height of aquatic large-leaved plants, and height of riparian small-leaved plants; and positively correlated with the cover of riparian ferns. The dry season prediction model, which explained 51% of the variation of An. minimus, was as follows: ymin = 0.1980–0.1733*water velocity – 0.0317*height of aquatic large-leaved plants – 0.0249*height of riparian small-leaved plants + 0.0192*cover of riparian ferns – 0.0170*height of stream banks. The influence of vegetation characteristics on larval density may not be as large as previously assumed. We conclude that factors other than those measured here may account for a large part of the variation in larval density.

Comparative Study on Nocturnal Behavior of Aedes aegypti and Aedes albopictus

Abstract Nocturnal behavior of nonblood-fed females of Aedes aegypti (L.) and Aedes albopictus (Skuse) was studied using an automatic recording device equipped with a photoelectric sensor. Carbon dioxide, heating, and the contrast of the black and white colors were used as attractive cues for mosquitoes. The nocturnal host-seeking activity positively correlated with the increasing light intensity in both species. Ae. aegypti was found to be more sensitive to light than Ae. albopictus. The threshold of light intensity for the activation of the nocturnal host-seeking activity was <0.1 lx (≈0.01 foot candle) in Ae. aegypti and >10 lx (≈1 foot candle) in Ae. albopictus. Complete darkness during the daytime deactivated the host-seeking activity of both species, irrespective of their increasing flight activity controlled by their intrinsic circadian rhythms. This finding suggested that visual cues are indispensable for host-seeking behavior. The eye parameter value, the product of the ommatidial diameter, and the interommatidial angle were significantly larger in Ae. aegypti than those in Ae. albopictus, indicating that the eye of Ae. aegypti is more adapted to a darker environment.

Crossing experiments of Anopheles minimus species C and putative species E

ABSTRACT In the Anopheles minimus complex, 2 sibling species (A and C) are generally accepted. Recently, a 3rd species, provisionally designated An. minimus species E, has been described from the Ryukyu Archipelago, Japan, based on crossing experiments (A and E), DNA analysis, mitotic karyotypes, and some morphological characteristics. The present study reports the results of crossing experiments between species C and putative species E. Hybridization between the progeny of An. minimus species C from Thailand and putative species E from Japan revealed postzygotic genetic incompatibility. Although F1 hybrid progeny were obtained from both directions of crosses, the hybrid males from C female × E male crosses were completely sterile, with atrophied testes and accessory glands. In addition, the external terminalia of all of these males never completely rotated and the males failed to copulate by artificial mating. In E female × C male crosses, the hybrid males showed partially sterile testes in which most spermatozoa were abnormal (enlarged head) and inactive, and they had very little success in inseminating females. The salivary gland polytene chromosomes of F1 hybrid larvae from species C female × species E male showed a fixed heterozygous inversion on the 3L arm. Those hybrids F1 from species E female × species C male showed partial asynapsis on identified arms (2R and 3L) and a fixed heterozygous inversion on the 3R arm. When the hybrid females from both directions of crosses were F1 backcrossed with either C or E males, they produced male progeny with abnormal spermatozoa. Study of mating behavior in a 30 × 30 × 30-cm cage showed that the C males failed to mate with either C or E females, indicating that species C males cannot breed in confined spaces (lack stenogamy). Putative species E males had little success in inseminating species C females. This study provides strong evidence of genetic incompatibility between An. minimus species C and putative species E, supporting previous data that species E is a distinct species in the An. minimus complex.

Malaria vector densities in transmission and non-transmission areas during 23 years and land use in Chiang Mai province, Northern Thailand

Accumulated anopheline mosquito collection protocols from Chiang Mai province, northern Thailand were compiled and analyzed with the objective to describe and compare the variation in density of malaria vectors between two areas: transmission area (TA) and non-transmission area (NTA), and between two periods: 1977 to 1989 and 1990 to 1999. We hypothesized that the reasons for differences in transmission between areas were caused by higher or increasing densities of malaria vectors in TA and lower or decreasing densities in NTA. We further hypothesized that low vector densities in NTA could be caused by changes in land use, specifically increased landscape diversity and forest fragmentation. Our results indicate that the vector situation in Chiang Mai province is very complex. There were no significant differences in density of the main malaria vectors, Anopheles minimus sensu latu and A. dirus s.l. between areas (TA and NTA). However, the change in density of A. minimus s.l. from the first to the second period was significantly different; TA having an increasing trend and NTA a decreasing trend. Furthermore, the reduction of malaria vectors in the NTA, especially A. minimus s.l. may have been a result of increased landscape diversity and forest fragmentation. There were minor landscape changes in the TA and large landscape changes in the NTA. It was concluded that changes in malaria mosquito densities and land use are important for explaining differences in malaria transmission between areas, but that other factors such as human movement and behavior, and socioeconomic conditions probably also play a role in the transmission of malaria in Chiang Mai province. Die gesammelten Aufzeichnungen zu Anophelesmucken-Erfassungen wurden mit dem Ziel zusammengestellt, die Variation der Dichte der Malariavektoren zwischen zwei Gebieten, einem Ubertragungsgebiet (TA) und einem Nicht-Ubertragungsgebiet (NTA), sowie zwischen den Zeitraumen 1977 bis 1989 und 1990 bis 1999 zu beschreiben und zu vergleichen. Wir stellten die Hypothese auf, dass die Unterschiede in der Ubertragung zwischen den Gebieten durch die hohere oder zunehmende Dichte der Malariavektoren in den TA und niedrigere oder abnehmende Dichte in den NTA verursacht wurden. Wir vermuteten zudem, dass die niedrige Vektorendichte in den NTA durch Veranderungen der Landnutzung, insbesondere durch eine zunehmende Landschaftsdiversitat

Sensitivity of the larvivorous copepod species, Mesocyclops pehpeiensis and Megacyclops viridis, to the insect growth regulator, pyriproxyfen

ABSTRACT The effects of the insect growth regulator pyriproxyfen were evaluated on the mortality, fecundity, longevity, and predation capability of 2 species of copepods, Mesocyclops pehpeiensis Hu and Megacyclops viridis (Jurine), under laboratory conditions. Pyriproxyfen showed no significant effects on either the development or reproduction of M. pehpeiensis at 0.1 ppm, which is a 10-fold greater concentration than the reported effective dosage for controlling mosquito larvae (0.01 ppm). In contrast, the development of M. viridis was impaired by pyriproxyfen at 0.1 ppm. An 80% reduction in nauplius survivorship was observed in the experimental (treated) group compared with the control group. Although the application of pyriproxyfen caused high mortality in the nauplius stage of this species, the pyriproxyfen-treated group developed faster, killed more mosquito larvae, yielded more eggs per oviposition event, and survived longer than the control group. These results indicate that pyriproxyfen caused mortality in the earlier stages of this sensitive species but that the surviving individuals were those that were selected for significantly faster development, better predation ability, and greater longevity during their reproductive stage. Therefore, under natural conditions, pyriproxyfen would cause modifications in the characteristics of a copepod population rather than its complete loss. Our results suggest that the combined application of copepods and pyriproxyfen to control Aedes populations is feasible. However, repeated application of pyriproxyfen may cause changes in copepod populations and communities in the affected ecosystem.