François Lorenzetti

Variation in the Susceptibility of the Forest Tent Caterpillar (Lepidoptera: Lasiocampidae) to Bacillus thuringiensis variety kurstaki HD-1: Effect of the Host Plant

Abstract Host-mediated effect on the efficacy of Bacillus thuringiensis Berliner against larvae of the forest tent caterpillar, Malacosoma disstria Hübner, was investigated under controlled conditions. Host plants used in this study were quaking aspen, Populus tremuloides Michx., a preferred host, and sugar maple, Acer saccharum Marsh., a secondary host. Larvae were reared in the laboratory on leaves of these hosts, and upon reaching the third, fourth, and fifth instar, they were fed leaves treated with one of a range of concentrations of B. thuringiensis variety kurstaki HD-1 suspensions. Larvae were tested on the host on which they were feeding before the 4-d bioassays. The estimated LC50s were 100-fold greater on quaking aspen than on sugar maple. Also, there was a decrease in efficacy over the whole ranges of concentrations with larval age on both hosts. LC50s varied approximately two-fold between third and fifth instar. These results indicate that host-mediated effects on B. thuringiensis efficacy warrant more interest. In particular, they strongly indicate that the host plant modifies the interaction between B. thuringiensis and a target insect, and offer the opportunity to investigate the mechanism(s) that may be involved in the enhancement of B. thuringiensis toxicity.

Perception of aspen and sun/shade sugar maple leaf soluble extracts by larvae of Malacosoma disstria

We investigated the behavioral feeding preference and the chemoreception of leaf polar extracts from trembling aspen, Populus tremuloides, and from sun and shade sugar maple, Acer saccharum, by larvae of the polyphagous forest tent caterpillar, Malacosoma disstria, a defoliator of deciduous forests in the Northern Hemisphere. Three polar extracts were obtained from each tree species: a total extract, a water fraction, and a methanol fraction. M. disstria larvae were allowed ad libitum access to an artificial diet from eclosion to the fifth instar. Two-choice cafeteria tests were performed comparing the mean (±SE) surface area eaten of the total extracts, and the following order of preference was obtained: aspen > sun maple < shade maple. Tests with the other fractions showed that M. disstria larvae preferred the total aspen extract to its water fraction, and the latter to its methanol fraction. The response to sun maple was similar to aspen. However, for the shade maple experiment, there was no difference between the total extract and its water fraction. Electrophysiological recordings for aspen showed that the sugar-sensitive cell elicited more spikes to the water fraction, followed by the total extract, and finally the methanol fraction. Spike activity to stimulations of sun and shade maple extracts revealed a similar trend, where methanol fraction > water fraction > total extract. Our findings are discussed in light of previously known information about this insects performance on these host plants.

Evidence for spatial niche partitioning in predaceous aphidophaga: Use of plant colour as a cue

A field experiment involving aphid-free control and nutrient-stressed plants of 5 maize (Zea mays L.) genotypes was conducted to determine if predaceous aphidophaga use plant cues, such as colour, to select plants on which to forage. Nutrient stress resulted in plants lighter in colour (Yellow) than control plants in all the maize genotypes. Coccinellids were significantly more abundant on yellow plants than on greener control plants whereas chrysopids were significantly more numerous on controls in 3 out of 5 maize genotypes. These two groups of predators may use plant colour to partition habitat spatially and exploit their aphid prey while minimizing intraguild interactions.RésuméUne expérience de terrain avec des plants de maïs (Zea mays L.) de 5 génotypes différents et non infestés par des pucerons a été menée afin de déterminer si les prédateurs aphidiphages utilisaient un critère particulier pour choisir un plant. Plus précisément, l’intérêt était de savoir si les prédateurs étaient capables de discriminer entre des plants nutritionnellement stressés et des plants normalement fertilisés, étant donné que des carences nutritionnelles conduisent à des plants en apparence plus pâles (jaune). Des différences significatives ont effectivement été trouvées dans les choix qu’ont faits les prédateurs. Les coccinelles étaient plus abondantes sur les plants stressés que sur les plants non stressés alors que les chrysopes étaient plus nombreux sur les plants non stressés d’au moins 3 des 5 génotypes utilisés. Il est suggéré que les deux groupes de prédateurs se servent de la couleur des plants afin d’exploiter différentiellement dans l’espace les colonies de pucerons présentes dans un habitat donné. De cette façon, ces deux groupes de prédateurs minimisent les effets négatifs des interactions intraguildes.

Effects of a Forest Tent Caterpillar Outbreak on the Dynamics of Mixedwood Boreal Forests of Eastern Canada

Abstract: In boreal mixedwood stands dominated by trembling aspen (Populus tremuloides), forest tent caterpillar (Malacosoma disstria, FTC) outbreaks are recurrent events whose effects on stand dynamics are poorly documented. To describe and characterize the effects of FTC outbreaks, we assessed canopy opening, gap size, and understory tree recruitment in 12 stands dominated by trembling aspen that had experienced different levels of defoliation (in terms of severity and duration) during the last outbreak in northwestern Quebec (1999–2002). The study showed a significant increase in canopy opening and gap size with defoliation intensity. Furthermore, the proportion of large gaps and aspen mortality increased with defoliation intensity. Balsam fir (Abies balsamea) regeneration benefited from changes in canopy structure caused by the FTC, while aspen did not. Forest succession in mixedwood stands that had been defoliated for 1 y was not profoundly affected, while multiple years of defoliation likely caused more rapid canopy transition from aspen to fir. By creating a variety of gaps, FTC outbreaks modify stand structure in ways that differ from succession to coniferous dominance controlled by single-stem exclusion.

PRODUCTION OF GLANDS IN LEAVES OF POROPHYLLUM SPP. (ASTERACEAE) : ECOLOGICAL AND GENETIC DETERMINANTS, AND IMPLICATIONS FOR INSECT HERBIVORES

1 The mean number of translucent glands containing potent insecticidal allelochemicals in leaves of Porophyllum gracile was observed to vary up to nearly twofold under field conditions in the area of Tucson, Arizona (USA). 2 The production of glands appeared to be a beneficial trait to reduce herbivory in the field since individuals of P. gracile bearing 2.5 glands per leaf were submitted to an approximately fivefold higher herbivory pressure than those having 4.8 glands per leaf. 3 To investigate if the production of glands in leaves of P. gracile is under genetic control, seeds were collected from individuals for which this trait varied under field conditions. There was no correlation for the mean number of glands per leaf between individuals of P. gracile sampled in the field and their progeny seedlings grown under controlled conditions. This suggested that the production of glands in leaves of P. gracile is not strongly hereditary. 4 The resource allocated to the formation of foliar glands, which was determined as the ratio of total volume of glands per unit area of leaf, in seedlings of P. ruderale and P. gracile was enhanced by up to one order of magnitude by both nitrogen fertilization, 15 vs. 0 mm of NOin the watering solution, and high light regime, 380 vs. 50 ,mole photon m-2 s-5. These results suggest that the production of glands in leaves of Porophyllum spp. depends greatly on resource availability. 5 The natural plasticity in the production of glands in leaves of P. ruderale was in turn employed in controlled laboratory experiments to confirm the potential benefit of the glands to repress insect herbivory as observed in the field. It was demonstrated that adults of the red-legged grasshopper are repelled by the volatiles emitted from the glands and that they feed less on leaves bearing more glands.