Effects of the ant Formica fusca on the transmission of microsporidia infecting gypsy moth larvae.

Transmission plays an integral part in the intimate relationship between a host insect and its pathogen that can be altered by abiotic or biotic factors. The latter include other pathogens, parasitoids, or predators. Ants are important species in food webs that act on various levels in a community structure. Their social behavior allows them to prey on and transport larger prey, or they can dismember the prey where it was found. Thereby they can also influence the horizontal transmission of a pathogen in its host's population. We tested the hypothesis that an ant species like Formica fusca L. (Hymenoptera: Formicidae) can affect the horizontal transmission of two microsporidian pathogens, Nosema lymantriae Weiser (Microsporidia: Nosematidae) and Vairimorpha disparis (Timofejeva) (Microsporidia: Burenellidae), infecting the gypsy moth, Lymantria dispar L. (Lepidoptera: Erebidae: Lymantriinae). Observational studies showed that uninfected and infected L. dispar larvae are potential prey items for F. fusca. Laboratory choice experiments led to the conclusion that F. fusca did not prefer L. dispar larvae infected with N. lymantriae and avoided L. dispar larvae infected with V. disparis over uninfected larvae when given the choice. Experiments carried out on small potted oak, Quercus petraea (Mattuschka) Liebl. (Fagaceae), saplings showed that predation of F. fusca on infected larvae did not significantly change the transmission of either microsporidian species to L. dispar test larvae. Microscopic examination indicated that F. fusca workers never became infected with N. lymantriae or V. disparis after feeding on infected prey.

Influence of the forest caterpillar hunter Calosoma sycophanta on the transmission of microsporidia in larvae of the gypsy moth Lymantria dispar.

The behaviour of predators can be an important factor in the transmission success of an insect pathogen. We studied how Calosoma sycophanta influences the interaction between its prey [Lymantria dispar (L.) (Lepidoptera, Lymantriidae)] and two microsporidian pathogens [Nosema lymantriae (Microsporidia, Nosematidae) and Vairimorpha disparis (Microsporidia, Burellenidae)] infecting the prey.Using laboratory experiments, C. sycophanta was allowed to forage on infected and uninfected L. dispar larvae and to disseminate microsporidian spores when preying or afterwards with faeces.The beetle disseminated spores of N. lymantriae and V. disparis when preying upon infected larvae, as well as after feeding on such prey. Between 45% and 69% of test larvae became infected when C. sycophanta was allowed to disseminate spores of either microsporidium.Laboratory choice experiments showed that C. sycophanta did not discriminate between Nosema-infected and uninfected gypsy moth larvae. Calosoma sycophanta preferred Vairimorpha-infected over uninfected gypsy moth larvae and significantly influenced transmission.When C. sycophanta was allowed to forage during the latent period on infected and uninfected larvae reared together on caged, potted oak saplings, the percentage of V. disparis infection among test larvae increased by more than 70%. The transmission of N. lymantriae was not affected significantly in these experiments.Beetles never became infected with either microsporidian species after feeding on infected prey.We conclude that the transmission of N. lymantriae is not affected. Because no V. disparis spores are released from living larvae, feeding on infected larvae might enhance transmission by reducing the time to death and therefore the latent period.

Vertical transmission and overwintering of microsporidia in the gypsy moth, Lymantria dispar.

Vertical transmission and the overwintering success of three different microsporidia infecting Lymantria dispar (Lepidoptera: Lymantriidae) larvae were investigated. Endoreticulatus schubergi, a midgut pathogen, was transmitted to offspring via female and male via the egg chorion (transovum transmission). Between 8% and 29% of the emerging larvae became infected. No spores of E. schubergi were found in surface-washed eggs. Nosema lymantriae, a microsporidium that causes systemic infections, was transovarially transmitted. Between 35% and 72% of the progeny were infected. Vairimorpha disparis, a fat body pathogen, was not vertically transmitted. The infectivity of spores that overwintered in cadavers of infected L. dispar varied by species, placement in the environment, and weather conditions. Spores of E. schubergi were still infective after an eight month exposure period of cadavers on the ground. Spores of N. lymantriae and V. disparis remained highly infective only when cadavers overwintered under a more or less continuous snow cover for four months.

Horizontal and vertical transmission of a Nosema sp. (Microsporidia) from Lymantria dispar (L.) (Lepidoptera: Lymantriidae).

The gypsy moth, Lymantria dispar L. (Lepidoptera, Lymantriidae), a serious defoliator of deciduous trees, is an economically important pest when population densities are high. Outbreaking populations are, however, subject to some moderating influences in the form of entomopathogens, including several species of microsporidia. In this study, we conducted laboratory experiments to investigate the transmission of an unusual Nosema sp. isolated from L. dispar in Schweinfurt, Germany; this isolate infects only the silk glands and, to a lesser extent, Malpighian tubules of the larval host. The latent period ended between 8 and 15 days after oral inoculation and spores were continuously released in the feces of infected larvae until pupation. Exclusion of feces from the rearing cages resulted in a 58% decrease in horizontal transmission. The silk of only 2 of 25 infected larvae contained microsporidian spores. When larvae were exposed to silk that was artificially contaminated with Nosema sp., 5% became infected. No evidence was found for venereal or transovum (including transovarial) transmission of this parasite.

Studies on the impact of two Nosema isolates from Bulgaria on the gypsy moth (Lymantria dispar L.).

We investigated host-parasite interactions of two Nosema-type microsporidian isolates recovered from populations of Lymantria dispar L. in northwestern Bulgaria, one near Veslec and one near Levishte. Bioassay studies produced information on development, stage specific mortality, pupation, and adult eclosion of infected individuals. Horizontal transmission of the two isolates was investigated in a second set of experiments. At dosages ranging from 2 x 10(2) to 5 x 10(4) spores/microl, the infection rates varied between 77 and 100% for the isolate from Veslec and between 92 and 99% for the Levishte isolate. The Veslec isolate caused a slightly higher mortality rate and the median time to death was shorter compared to the isolate from Levishte. The total mortality for both isolates varied between 79 and 99%, independent of spore dosages. A lower relative growth rate was recorded for male and female L. dispar larvae infected with either isolate during the third larval instar and a higher relative growth rate during the fourth instar compared to the control groups. Pupal weight did not differ significantly among females, but male infected pupae were heavier than the controls. Nosema sp. [Veslec] was as efficiently transmitted as Nosema sp. [Levishte]; 42% of the susceptible larvae became infected with the Veslec isolate when uninfected larvae were exposed to infected larvae; 43% of larvae became infected with the Nosema sp. [Levishte]. The latency period varied between 7 and 8 days for both isolates.