Population-specific responses to light influence herbivory in the understory shrubLindera benzoin

Data on Herbivore Performance and Plant Herbivore Damage Identify the Same Plant Traits as the Key Drivers of Plant-Herbivore Interaction

Data on plant herbivore damage as well as on herbivore performance have been previously used to identify key plant traits driving plant-herbivore interactions. The extent to which the two approaches lead to similar conclusions remains to be explored. We determined the effect of a free-living leaf-chewing generalist caterpillar, Spodoptera littoralis (Lepidoptera: Noctuidae), on leaf damage of 24 closely related plant species from the Carduoideae subfamily and the effect of these plant species on caterpillar growth. We used a wide range of physical defense leaf traits and leaf nutrient contents as the plant traits. Herbivore performance and leaf damage were affected by similar plant traits. Traits related to higher caterpillar mortality (higher leaf dissection, number, length and toughness of spines and lower trichome density) also led to higher leaf damage. This fits with the fact that each caterpillar was feeding on a single plant and, thus, had to consume more biomass of the less suitable plants to obtain the same amount of nutrients. The key plant traits driving plant-herbivore interactions identified based on data on herbivore performance largely corresponded to the traits identified as important based on data on leaf damage. This suggests that both types of data may be used to identify the key plant traits determining plant-herbivore interactions. It is, however, important to carefully distinguish whether the data on leaf damage were obtained in the field or in a controlled feeding experiment, as the patterns expected in the two environments may go in opposite directions.

Phenoseasonal subcanopy light dynamics and the effects of light on the physiological ecology of a common understory shrub, Lindera benzoin

The purpose of this work was to quantify the variation of subcanopy spatiotemporal light dynamics over the course of a year and to link it to the physiological ecology of the understory shrub, Lindera benzoin L. Blume (northern spicebush). Covering all seven phenoseasons of a deciduous forest, this work utilized a line quantum sensor to measure the variation in subcanopy light levels under all sky conditions at different times of the day. A total of 4,592 individual subcanopy measurements of photosynthetic photon flux density (PPFD, μmol m^-2 s^-1) were taken as 15-second spatially-integrated one-meter linear averages to better understand the dynamism of light exposure to L. benzoin. Both open (n = 2, one continuous and one instantaneous) and subcanopy location (n = 25) measurements of PPFD were taken on each sampling date in and near the forested plot (Maryland, USA). In addition, we explored the effect of four photointensity-photoperiod combinations on the growth of L. benzoin under controlled conditions to compare to field conditions. On average, understory PPFD was less than 2% of open PPFD during the leafed months and an average of 38.8% of open PPFD during leafless winter months, indicating that: (1) often overlooked woody surfaces intercept large amounts of light; and (2) spicebush within the plot receive limited light even in early spring before canopy leaf-out. Statistical results suggested phenoseason accounted for nearly three-quarters of the variation in incident radiation between the three plant canopy heights. Spicebush under controlled conditions exhibited the highest fitness levels at an intensity of 164.5 μmol m^-2 s^-1 for 12-hour duration. Similarly, spicebush growth in the field occurred at subcanopy locations receiving higher incidence of PPFD (i.e., >128 μmol m^-2 s^-1). Results suggest that the ecological niche for these plants is very specific in terms of light intensity.

Light conditions affect the performance of Yponomeuta evonymellus on its native host Prunus padus and the alien Prunus serotina

The bird cherry ermine moth, Yponomeuta evonymellus L., is considered an obligatory monophagous insect pest that feeds only on native European Prunus padus L. In recent years, however, increased larval feeding on alien P. serotina Ehrh. has been observed. In both species, general defoliation is extensive for shade grown trees, whereas it is high in P. padus, but very low in P. serotina, when trees are grown in full light conditions. The aim of the present study was to identify how the plant host species and light conditions affect the performance of Y. evonymellus. The influence of host species and light condition on their growth and development, characterized by the parameters of pupation, adult eclosion, body mass, potential fecundity, and wing size, was measured in a 2 × 2 experimental design (two light treatments, two hosts). In comparison with high light (HL) conditions, a greater percentage of pupation and a longer period and less dynamic adult emerge was observed under low light (LL) conditions. The effect of host species on these parameters was not significant. In contrast, mass, fecundity and all of the studied wing parameters were higher in larvae that grazed on P. padus than on P. serotina. Similarly the same parameters were also higher on shrubs in HL as compared with those grown under LL conditions. In general, light conditions, rather than plant species, were more often and to a greater extent, responsible for differences in the observed parameters of insect development and potential fecundity.

Sex Ratio and Body Mass of Adult Herbivorous Beetles Depend on Time of Occurrence and Light Conditions

Body mass and sex ratio (F/M) of folivorous insects are easily measured parameters that are commonly used to assess the effect of food quality, living conditions, and preferences on the selection of favourable sites for offspring. A study was conducted on the polyphagous beetle, Gonioctenaquinquepunctata (a pest of the native Prunus padus and alien P. serotina) and on the monophagous beetle, Alticabrevicollis coryletorum (a pest of Corylus avellana). Both species have a similar life cycle with emergence of current-year adults in summer, and reproduction of 1-year-old insects in spring. A. brevicollis coryletorum feeds primarily on sunlit shrubs, while G. quinquepunctata prefers shaded leaves. The present study assessed the effect of time of occurrence(insect age) on body mass in both sexes and on the sex ratio F/M, taking into account the influence of light conditions associated with their favoured food source (sunlit vs. shaded leaves). We hypothesized that a change in body mass in current-year insects would be determined by the amount of consumed food, while the sex ratio would be stable, when in 1-year-old insects females would die shortly after oviposition, while males would be active for a prolonged time. Results confirmed the hypothesis that changes in mass of current-year beetles was determined by the amount of food intake. We also found that in spring, unfertilized females coexist with fertilized ones and that the latter females live for some time after oviposition; resulting in fluctuations of the mean mass for females. In both species, 1-year-old beetles were heavier than current-year. The preference of A. brevicollis coryletorum for sunlit leaves results in a higher body weight than in G. quinquepunctata in both seasons. The data are consistent and indicate seasonal fluctuations in body mass and changes in the sex ratio in 1-year-old beetles, due to the entrance into their reproductive period.