Dispar Caterpillars Research Articles

Response of cavity nesting birds to Lymantria dispar (Lepidoptera) and aerial spraying – An experimental approach

Eruptive population dynamics of forest insect species regularly attract the interest of ecologists but also often evoke debates among stakeholders concerning impacts that outbreaks or pest control activities have on forest communities including insectivorous birds. Lymantria dispar is the most serious native defoliator in mixed oak-broadleaf forests in Central Europe. To investigate the treatment effects of L. dispar density, aerial application of the insecticide Mimic (tebufenozide) and nest box accessibility for L. dispar caterpillars on nest box success of cavity nesting birds, we deployed 352 nest boxes in a full factorial experimental design with 11 replicates. L. dispar density was predicted by egg mass counts made in the year before the study. The accessibility of nest boxes for L. dispar caterpillars was manipulated in all study sites by placing nest boxes on trunks for easy access to caterpillars and on branches for more difficult access. Successful nest boxes (binary metric that combines failed breeding attempt and empty nest box) and nest productivity (number of fledged chicks in successful boxes) were monitored in the peak year of a L. dispar outbreak, during which half of the plots were treated with Mimic, as well as in the following year when L. dispar populations had widely collapsed, and no further treatments were applied. L. dispar and non-target caterpillar biomass was sampled by canopy fogging to quantify prey abundance. Caterpillar biomass was significantly reduced by Mimic in the treatment year and remained low in the post-treatment year. Despite the reduction of caterpillars, our zero-inflation model found no effect of any tested variables on nest box success of the early brood. However, in the late broods the number of successful nest boxes was reduced in Mimic treated plots in the first year by 42%, while unaffected by initial L. dispar density. The number of successful nest boxes in the second year was lower in areas with initially high L. dispar densities and there was a marginal carry-over effect of insecticide treatment. L. dispar caterpillars entered nest boxes on the trunk in higher numbers than boxes on the branch, but nest box position had no effect on nest box success of birds.We conclude that despite its strong negative impact on caterpillar prey, aerial application of Mimic targeting L. dispar caused an increase in the number of failed boxes in late broods in sprayed plots, particularly in the year of application and to some extent in the year after. We presume this delayed effect of Mimic only on late broods to be caused by Mimic-induced caterpillar shortages during first broods leading to an increased parental effort and a subsequent reduced number of second broods. The generally limited impact of Mimic might be explained by the overall extensive availability of alternate prey in oak forests in May during first broods and the ability of tits to increase their parental effort in times of caterpillar shortage. In unsprayed sites, L. dispar densities can create legacies of impact in following years, but caterpillars do not directly affect birds nest box success despite entering cavities.

Aboveground phytochemical responses to belowground herbivory in poplar trees and the consequence for leaf herbivore preference

Belowground (BG) herbivory can influence aboveground (AG) herbivore performance and food preference via changes in plant chemistry. Most evidence for this phenomenon derives from studies in herbaceous plants but studies in woody plants are scarce. Here we investigated whether and how BG herbivory on black poplar (Populus nigra) trees by Melolontha melolontha larvae influences the feeding preference of Lymantria dispar (gypsy moth) caterpillars. In a food choice assay, caterpillars preferred to feed on leaves from trees that had experienced attack by BG herbivores. Therefore, we investigated the effect of BG herbivory on the phytochemical composition of P. nigra trees alone and in combination with AG feeding by L. dispar caterpillars. BG herbivory did not increase systemic AG tree defences like volatile organic compounds, protease inhibitors and salicinoids. Jasmonates and salicylic acid were also not induced by BG herbivory in leaves but abscisic acid concentrations drastically increased together with proline and few other amino acids. Leaf coating experiments with amino acids suggest that proline might be responsible for the caterpillar feeding preference via presumptive phagostimulatory properties. This study shows that BG herbivory in poplar can modify the feeding preference of AG herbivores via phytochemical changes as a consequence of root-to-shoot signaling.

Parameters of oxidative stress, cholinesterase activity, Cd bioaccumulation in the brain and midgut of Lymantria dispar (Lepidoptera: Lymantriidae) caterpillars from unpolluted and polluted forests

Cadmium (Cd) can display a variety of different effects on living organisms. The objectives of the present study were to investigate Cd bioaccumulation and differences in parameters of oxidative stress (activities of the enzymes: superoxide dismutase, catalase, glutathione reductase, ascorbate peroxidase, glutathione S-transferase, and amounts of non-enzymatic free sulfhydryl groups and total glutathione) and cholinesterase activity in larval brain and midgut tissues of the polyphagous forest insect Lymantria dispar collected from unpolluted and polluted oak forests. Fourth instar L. dispar caterpillars from the unpolluted forest had higher body mass but accumulated more Cd in comparison to caterpillars from the polluted forest. In both populations the midgut was more sensitive than the brain to the prooxidative effects of Cd. Enzyme activities and amounts of non-enzymatic parameters tended to be greater in midgut tissues than in the brain, except for cholinesterase activity. Parameters of oxidative stress had higher values in caterpillar tissues from the polluted than from the unpolluted oak forest. The observed differences between the two natural populations point to the importance of knowing the history of population exposure to environmental pollution when monitoring forest ecosystems.

Acetylcholinesterase and heat shock protein 70 response in larval brain tissue of Lymantria dispar L. (Lepidoptera, Limantriidae) upon chronic exposure to benzo(a)pyrene.

The effects of different concentrations of benzo(a)pyrene (BaP) (2, 10, 20, 100, 200, and 2000ng/g dry food weight) on the activity of acetylcholinesterase (AChE) and concentration of heat-shock protein 70 (hsp70) in fifth instar Lymantria dispar brain tissue were investigated. Inhibition of AChE is a general effect biomarker at the individual level. We observed the most efficient inhibition of this enzyme at medium and high BaP concentrations (20, 100, 200, and 2000ng/g dry food weight). Western blot analysis revealed the presence of two hsp 70 isoforms, with changed amounts depending on the BaP concentration. A fluctuating response in hsp70 concentration to ingested BaP was observed: with decreased levels in the groups fed 2 and 100ng BaP/g dry food weight, but increased concentrations in the groups given 10 and 2000ng/g dry food weight. Even though we detected changes in AChE activity and hsp70 concentration in L. dispar brain tissue upon exposure to different BaP concentrations, we cannot characterize them as sensitive and reliable biomarkers for this xenobiotic in L. dispar caterpillars.

Leaf rust infection reduces herbivore-induced volatile emission in black poplar and attracts a generalist herbivore.

Plants release complex volatile blends after separate attack by herbivores and pathogens, which play many roles in interactions with other organisms. Large perennials are often attacked by multiple enemies, but the effect of combined attacks on volatile emission is rarely studied, particularly in trees. We infested Populus nigra trees with a pathogen, the rust fungus Melampsora larici-populina, and Lymantria dispar caterpillars alone and in combination. We investigated poplar volatile emission and its regulation, as well as the behavior of the caterpillars towards volatiles from rust-infected and uninfected trees. Both the rust fungus and the caterpillars alone induced volatile emission from poplar trees. However, the herbivore-induced volatile emission was significantly reduced when trees were under combined attack by the herbivore and the fungus. Herbivory induced terpene synthase transcripts as well as jasmonate concentrations, but these increases were suppressed when the tree was additionally infected with rust. Caterpillars preferred volatiles from rust-infected over uninfected trees. Our results suggest a defense hormone crosstalk upon combined herbivore-pathogen attack in poplar trees which results in lowered emission of herbivore-induced volatiles. This influences the preference of herbivores, and might have other far-reaching consequences for the insect and pathogen communities in natural poplar forests.

Tannic acid modulates the activity of bombyxin- and PTTH-producing neurons in Lymantria dispar L. (Insecta: Lepidoptera) caterpillars

Lymantria dispar caterpillars were exposed for 3 days to tannic acid, added to the artificial diet in the following concentrations: 1, 2.5 and 5%. Changes in the morphometric parameters of medial A2 bombyxin immunopositive and dorsolateral L2` prothoracicotropic (PTTH) immunopositive neurosecretory neurons (nsn; sizes of six nsn, the size of their nuclei, and quantification of cytoplasm density) were analysed upon caterpillars’ exposure to this allelochemical. Protein band density, obtained by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS PAGE) electrophoresis, from regions of bombyxin and PTTH molecular mass (Mr) were estimated. A2 nsn decreased in size, while the size of their nuclei did not change significantly. Relative cytoplasm density and relative band density from the bombyxin Mr region (Mr 4–5 kD) were decreased upon exposure to 1% and 2.5% tannic acid, while the addition of 5% tannic acid to artificial diet increased all of these parameters. Results suggested that lower concentrations of tannic acid stimulate the release of neurosecretory material from A2 nsn, most probably bombyxin-like protein. High tannic acid concentration (5%) probably allocates resources towards induction of other stress protective components instead of bombyxin-mediated metabolic stress response mechanisms. In L2` nsn, the size of nsn, nuclei and the relative cytoplasm density were increased, and relative band density from the region of big PTTH form (Mr 11–15 kD) was higher in all treatments in comparison to control. Results indicate that PTTH is synthesised and stored in the cytoplasm of L2` nsn.

The RING for gypsy moth control: Topical application of fragment of its nuclear polyhedrosis virus anti-apoptosis gene as insecticide

Numerous studies suggest a cellular origin for the Lymantria dispar multicapsid nuclear polyhedrosis virus (LdMNPV) anti-apoptosis genes IAPs, thus opening a possibility to use the fragments of these genes for modulation of host metabolism. We report here the strong insecticidal and metabolic effect of single-stranded antisense DNA fragment from RING (really interesting new gene) domain of gypsy moth LdMNPV IAP-3 gene: specifically, on reduction of biomass (by 35%) and survival of L. dispar caterpillars. The treatment with this DNA fragment leads to a significantly higher mortality rates of female insects (1.7 fold) accompanied with the signs of apoptosis. Additionally, we show increased expression of host IAP-1, caspase-4 and gelsolin genes in eggs laid by survived females treated with RING DNA fragment accompanied with calcium and magnesium imbalance, indicating that the strong stress reactions and metabolic effects are not confined to treated insects but likely led to apoptosis in eggs too. The proposed new approach for insect pest management, which can be considered as advancement of “microbial pesticides”, is based on the application of the specific virus DNA, exploiting the knowledge about virus–pest interactions and putting it to the benefit of mankind.

Warming and drought do not influence the palatability of Quercus pubescens Willd. leaves of four European provenances

In the context of global warming, the impact of extreme drought events on trees and biotic interactions with herbivore insects is widely unknown. A faster range expansion of insects in a changing climate could lead to mass propagations of pests in forests. Therefore, the aim was to investigate the influence of climatic alterations on leaf palatability. We exposed juvenile Quercus pubescens Willd. individuals of four European provenances (Bulgaria, Germany, Hungary, and Italy) to warming and drought. In addition, we conducted a palatability experiment with the pre-exposed Q. pubescens leaves and the caterpillars of the generalist forest pest Lymantria dispar L. (gypsy moth). Consumed leaf dry material, density of trichomes, and specific leaf area were examined. Surprisingly, neither warming nor drought affected the leaf palatability, but palatability was related to the density of trichomes. The Bulgarian provenance of Q. pubescens, which had the lowest density of trichomes, was most palatable. These findings suggest that global warming and drought might not lead to more frequent infestations of the four tested European Q. pubescens provenances by L. dispar caterpillars in the future.

Searching for synergism: effects of combinations of phenolic compounds and other toxins on oxidative stress in Lymantria dispar caterpillars

Synergistic combinations of plant allelochemicals are commonly believed to increase their effectiveness as defenses against insect herbivores. For example, temperate deciduous trees produce large amounts of phenolic compounds (primarily tannins), but many of these trees also produce smaller amounts of other potentially toxic compounds. This study tested the hypothesis that mixtures of phenolics and other types of toxins produce a greater effect on oxidative stress (a measure of toxicity) than do phenolics alone. Oxidative stress was measured in Lymantria dispar (gypsy moth) larvae as a shift in the redox balance of glutathione (GSH) towards a higher percentage of its oxidized form (glutathione disulfide; GSSG). We began by showing that larvae that ingested ellagitannins and chlorogenic acid (phenolics) contained greatly elevated levels of reactive oxygen species in their midgut contents, but this was not sufficient to shift the redox balance of GSH in their midgut tissues. Therefore, the phenolic compounds were tested in pairwise combinations with juglone (a quinone), rutin, kaempferol, or quercitrin (flavonoids), quinine, berberine, gramine, or glaucine (alkaloids), and soy or Quillaja saponins. When each of these allelochemical combinations was treated on hybrid poplar (Populus tremula × P. alba) leaves and consumed by L. dispar larvae for a 2-day period, none significantly shifted the redox balance of GSH. GSH levels were induced by the combination of phenolics with an alkaloid and a saponin, suggesting that a “cocktail” of allelochemicals might produce a more effective plant defense. GSH levels were also found to increase with larval age, both within and between instars. Increased levels of GSH may help explain the greater resistance of late-instar than early-instar larvae to plant defenses. Overall, the results of this study do not support the hypothesis that the combined effects of phenolic compounds and other toxins in sugar maple (Acer saccharum) leaves produce a synergistic increase in oxidative stress in L. dispar larvae. Further work is needed to test the general hypothesis that the allelochemical diversity in plants functions, in part, to produce synergistic toxicological effects in insect herbivores.

Rapid induced resistance of silver birch affects both innate immunity and performance of gypsy moths: the role of plant chemical defenses

In this study we tested the effects of rapid induced resistance of the silver birch, Betula pendula, on the performance and immune defense of the gypsy moth, Lymantria dispar. We also measured the effects of defoliation on the concentrations of plant secondary metabolites, particularly on phenolics and terpenoids. It was found that severe natural defoliation (by moth larvae) of silver birch led to an increase in lipophilic flavonoids on the leaf surface. The concentration of some simple phenolics and monoterpenes (linalool and geraniol) also increased, while that of several glycosides of quercetin decreased. The female pupal weights and survival rates of moths decreased, and larval development time increased, when the insects fed on defoliated trees. However, the feeding of caterpillars with the leaves of defoliated trees led to an increase in lysozyme-like activity in their hemolymph, with an increase in their ability to encapsulate potential parasites. Our data show that the silver birch deploys a rapid chemical defense against gypsy moth larvae. We suggest that lipophilic flavonoids are important compounds in the direct silver birch defense against L. dispar caterpillars. The increased strength of immune defense of insects exposed to trees that had deployed a rapid induced resistance may be an adaptation of the herbivores to resist the rising density of parasites when host population density is high.

Activity of gypsy moth dorsolateral neurosecretory neurons under increased rearing density

Lymantria dispar caterpillars were reared under two different rearing densities for the first three days of the 4th larval instar: 5 larvae that were kept in a Petri dish (V = 80 ml) belonged to the intense stress (D1 group); 5 larvae that were kept in a plastic cup (V = 300ml) belonged to the group exposed to less intense stress (D2 group). In the control group, single larvae were reared in a Petri dish. Morphometric changes in L1, L2 and L2? dorsolateral neurosecretory neurons (nsn) were analyzed. After keeping 5 larvae in a Petri dish, the size of L2 neurosecretory neurons (nsn) significantly increased. Rearing 5 larvae in a plastic cup significantly increased the size of L1 nsn nuclei and the number of L2?nsn. A decrease in relative band densities in the region of molecular masses (11-15 kD) that correspond to prothoracicotropic hormones in the gypsy moth was observed in the electrophoretic profiles that were obtained after both treatments in comparison to the control group.

Hydrolyzable tannins as “quantitative defenses”: Limited impact against Lymantria dispar caterpillars on hybrid poplar

The high levels of tannins in many tree leaves are believed to cause decreased insect performance, but few controlled studies have been done. This study tested the hypothesis that higher foliar tannin levels produce higher concentrations of semiquinone radicals (from tannin oxidation) in caterpillar midguts, and that elevated levels of radicals are associated with increased oxidative stress in midgut tissues and decreased larval performance. The tannin-free leaves of hybrid poplar ( Populus tremula × P. alba) were treated with hydrolyzable tannins, producing concentrations of 0%, 7.5% or 15% dry weight, and fed to Lymantria dispar caterpillars. As expected, larvae that ingested control leaves contained no measurable semiquinone radicals in the midgut, those that ingested 7.5% hydrolyzable tannin contained low levels of semiquinone radicals, and those that ingested 15% tannin contained greatly increased levels of semiquinone radicals. Ingested hydrolyzable tannins were also partially hydrolyzed in the midgut. However, increased levels of semiquinone radicals in the midgut were not associated with oxidative stress in midgut tissues. Instead, it appears that tannin consumption was associated with increased metabolic costs, as measured by the decreased efficiency of conversion of digested matter to body mass (ECD). Decreased ECD, in turn, decreased the overall efficiency of conversion of ingested matter to body mass (ECI). Contrary to our hypothesis, L. dispar larvae were able to maintain similar growth rates across all tannin treatment levels, in part, because of compensatory feeding. We conclude that hydrolyzable tannins act as “quantitative defenses” in the sense that high levels appear to be necessary to increase levels of semiquinone radicals in the midguts of caterpillars. However, these putative resistance factors are not sufficient to decrease the performance of tannin-tolerant caterpillars such as L. dispar.