The role of nodding stems in the goldenrod-gall-fly interaction: A test of the "ducking" hypothesis

Herbivores are among the most pervasive selective forces acting on plants, and the number of plant chemicals that presumably evolved for defense against herbivory is immense. In contrast, biologists are only beginning to appreciate the important roles that architectural traits can play in antiherbivore defense. One putative architectural-resistance trait is the nodding stem apex of some goldenrods (Solidago; Asteraceae). Individuals of S. altissima genets that undergo temporary nodding in the late spring (i.e., "candy-cane" ramets) have been shown to be more resistant than individuals of erect-stemmed genets to certain apex-attacking herbivores. We tested the hypothesis that the greater resistance of candy-cane ramets is accomplished by the ramets' "ducking" from the herbivores. In a greenhouse experiment, nodding candy-cane ramets were significantly more resistant to oviposition by the gall-inducing fly Eurosta solidaginis than were ramets of the same genets that had been experimentally straightened. The straightened candy-cane stems were just as susceptible to ovipositions as were ramets of erect-stemmed genets. Thus, ducking indeed appears to confer a resistance advantage to candy-cane genets of S. altissima.     

Associational resistance for mule’s ears with sagebrush neighbors

Many examples of associational resistance have been reported, in which a plant’s neighbors reduce the rate of damage by herbivores that it experiences. Despite 30 years of interest and hundreds of examples of associational resistance, we still know very little about how plants avoid their herbivores. This lack of mechanistic understanding prevents us from predicting when or where associational resistance will be important or might affect species’ distributions. I demonstrate here that the plant neighborhoods that surrounded focal mule’s ears (Wyethia mollis) individuals affected the damage they received. In particular, distance between a focal mule’s ears individual and its nearest sagebrush neighbor (Artemisia tridentata) was a good predictor of how much leaf area the mule’s ears would lose to herbivores over 2 years. Mule’s ears close to sagebrush suffered less loss than those with more distant nearest sagebrush neighbors. Mule’s ears with near sagebrush neighbors suffered half the leaf loss as mule’s ears with sagebrush experimentally removed. This associational resistance was probably not caused by sagebrush attracting or increasing populations of predators of generalist herbivores. Sagebrush is known to emit chemicals that are feeding deterrents to generalist grasshoppers and these deterrents were probably involved here. Volatile chemicals emitted by damaged sagebrush have been found to induce resistance in neighboring plants of several species. However, I found no evidence for such eavesdropping here as mule’s ears gained associational resistance from sagebrush neighbors whether or not those sagebrush neighbors had been experimentally damaged. Understanding the mechanisms responsible for associational resistance is critical to predicting where and when it will be important.     

Strong but opposing effects of associational resistance and susceptibility on defense phenotype in an African savanna plant

The susceptibility of plants to herbivores can be strongly influenced by the identity, morphology and palatability of neighboring plants. While the defensive traits of neighbors often determine the mechanism and strength of associational resistance and susceptibility, the effect of neighbors on plant defense phenotype remains poorly understood. We used field surveys and a prickle‐removal experiment in a semi‐arid Kenyan savanna to evaluate the efficacy of physical defenses against large mammalian herbivores in a common understory plant, Solanum campylacanthum. We then quantified the respective effects of spinescent Acacia trees and short‐statured grasses on browsing damage and prickle density in S. campylacanthum. We paired measurements of prickle density beneath and outside tree canopies with long‐term herbivore‐exclusion experiments to evaluate whether associational resistance reduced defense investment by decreasing browsing damage. Likewise, we compared defense phenotype within and outside pre‐existing and experimentally created clearings to determine whether grass neighbors increased defense investment via associational susceptibility. Removing prickles increased the frequency of browsing by ~25%, and surveys of herbivory damage on defended leaves suggested that herbivores tended to avoid prickles. As predicted, associational resistance and susceptibility had opposing effects on plant phenotype: individuals growing beneath Acacia canopies (or, analogously, within large‐herbivore exclosures) had a significantly lower proportion of their leaves browsed and produced ~ 70–80% fewer prickles than those outside refuges, whereas plants in grass‐dominated clearings were more heavily browsed and produced nearly twice as many prickles as plants outside clearings. Our results demonstrate that associational resistance and susceptibility have strong, but opposing, effects on plant defense phenotype, and that variable herbivore damage is a major source of intraspecific variation in defense phenotype in this system.     

The life history and host specificity of theEchium borer,Phytoecia coerulescens [Col.: Cerambycidae]

The cerambycid stem borerPhytoecia coerulescens (Scopoli) is a possible biological control agent for the weedEchium plantagineum L. in Australia. The adult beetle oviposits at the base of stems ofEchium spp. and a few otherBoraginaceae. The larva damages the flower stem by boring both upwards and downwards and eventually, at the end of the season, girdling the stem at its base. It overwinters in the rootstock of its host plant. It does not attack plants of agricultural importance in the field and host restriction to a small group of boraginaceous plants was confirmed in the laboratory, withEchium spp. as principal hosts. It is therefore considered thatP. coerulescens is a safe agent to introduce into Australia for the control ofE. plantagineum.     

Plant neighborhood influences colonization of Brassicaceae by specialist and generalist aphids

A plant's own characteristics, but also those of its neighbors, might have an impact on its probability of being colonized by herbivorous insects. A plant might be less colonized and experience associational resistance when it grows near repellent neighbors. In contrast, it might be more colonized and experience associational susceptibility near attractive neighbors. To date, mechanisms that drive associational defense are not really understood. In order to gain insights into the occurrence of associational resistance versus associational susceptibility under field conditions, we conducted an experiment to determine the influence of neighboring plants on the colonization of a focal plant by aphids. The focal plant was always Brassica oleracea. The neighbors were B. oleracea (control), B. napus, B. nigra, or Solanum lycopersicum, which represent contrasting levels of physical and chemical defenses. The focal plant, B. oleracea, was more colonized by the specialist aphid Brevicoryne brassicae, and experienced associational susceptibility when it was surrounded by B. nigra or B. napus. In contrast, B. oleracea was less colonized by the generalist aphid Myzus persicae, and experienced associational resistance when it was surrounded by S. lycopersicum, B. nigra or B. napus. Neighboring plants had no significant impact on host plant choice by the generalist aphid Macrosiphum euphorbiae. In conclusion, attraction or repulsion of the specialist aphid B. brassicae and the generalist aphid M. persicae by B. nigra, B. napus, and S. lycopersicum resulted in associational susceptibility or associational resistance for B. oleracea.     

Effects of spatial distribution on plant associational defense against herbivory

Several studies have shown that consumption of a focal plant by herbivores depends not only on its own defense traits but also on the characteristics of the neighboring plants. A number of studies have reported on plant associational defense in relation to neighboring plant palatability but the effect of the spatial distribution of the focal plant within patches of different neighboring plants has received less attention. We conducted a manipulative experiment to determine whether and how spatial distribution of focal plants affects the associational defense between plant species. In our experimental setup sheep encountered two patches varying in spatial distribution of the focal plant within patches (dispersed or clumped) and patch quality, good patch and bad patch, where the focal plant, Lathyrus quinquenervius, was neighbored to high- (Chloris virgata) or low-palatable (Kalimeris integrifolia) species, respectively. Results showed that, when focal plants were dispersed within both patches, the risk of attack was significantly lower for focal plants in the patches with low- than high-palatable neighbors, indicating associational defense. Alternatively, when focal plants were clumped within both patches, they were consumed in bad-patch as much as in good-patch plots, which indicates the absence of associational defense. However, if the focal plants have different spatial distributions in the two patches (dispersed in good-patch and clumped in bad-patch or vice versa), sheep foraging success for focal plants was greatly reduced in dispersed spatial pattern irrespective of the palatability of neighboring plants. Therefore, we concluded that spatial distribution is as important as traits of neighboring plants in predicting vulnerability of the focal plant to grazing by generalist herbivores. The outcome of plant associational defense for different types of neighborhood strongly depends on the magnitude of herbivore foraging selectivity between and within patches, which further depended on the contrasts between plant species or between patches.     

Seasonality of herbivory and communication between individuals of sagebrush

Seasonal changes in herbivore numbers and in plant defenses are well known to influence plant–herbivore interactions. Some plant defenses are induced in response to herbivore attack or cues correlated with risk of attack although seasonal variation in these defenses is relatively poorly known. We previously reported that sagebrush becomes more resistant to its herbivores when neighboring plants have been experimentally clipped with scissors. In this study we asked whether herbivory to leaves of sagebrush varied seasonally and whether there was seasonal variation in natural levels of damage when neighbors were clipped. We found that sagebrush accumulated most chewing damage early in the season, soon after the spring flush of new leaves. This damage was caused by generalist grasshoppers, deer, specialist caterpillars, beetles, gall makers, and other less common herbivores. Sagebrush showed no evidence of preferentially abscising leaves that had been experimentally clipped. Experimental clipping by Trirhabda pilosa beetle larvae caused neighbors to accumulate less herbivore damage later that season, similar to results in which clipping was done with scissors. Induced resistance caused by experimentally clipping a neighbor was affected by season; plants with neighbors clipped in May accumulated less damage throughout the season relative to plants with unclipped neighbors or neighbors clipped later in the summer. We found a correlation between seasonal herbivore pressure, damage accumulated by plants, and induced responses to experimentally clipping neighbors. The causal mechanisms responsible for this correlation are unknown although a strong seasonal effect was clear.     

Associational resistance mediates interacting effects of herbivores and competitors on fireweed performance

Neighboring plants may have dual roles in relation to focal plants, being both competitors, and mediators of associational resistance (AR), or associational susceptibility (AS) to herbivores. Even though these two roles are played out simultaneously in nature, they have traditionally been studied separately. We conducted a two-by-two factorial experiment by manipulating the occurrence of neighbors (meadowsweet, Filipendula ulmaria) and shared mammal herbivores (moose, Alces alces) to investigate how the two simultaneously affect the performance of focal fireweed (Chamerion angustifolium) plants.We found that interactions between mammal herbivory and neighbors had significant effects on fireweed performance. These effects were explained by two counteracting mechanisms. First, when mammal herbivores were present, fireweed experienced AR such that plants growing outside of meadowsweet patches had an almost tripled risk of being browsed compared to plants growing among meadowsweet, resulting in reduced fireweed reproduction and growth. Second, when mammals were excluded, the association with meadowsweet had a direct negative effect on fireweed growth rates – mainly due to competition for light. Additionally, for the first time, we show that association with neighbors affected mammal-induced branching in fireweed.This study demonstrates that AR mediated by mammal herbivores can flip plant–plant interactions from competitive (−) to facilitative (+), and affect plant architecture. As AR and competition are probably simultaneously co-occurring forces, interactions between the two are likely common and thus of fundamental importance for populations and communities.     

Biomechanical properties of the emergent aquatic macrophyte Sparganium erectum: Implications for fine sediment retention in low energy rivers

This paper is concerned with the biomechanical properties of the emergent aquatic macrophyte, Sparganium erectum. We present observations of adjustments in the physical characteristics and biomechanical properties of S. erectum during the growing season (April-November) from the River Blackwater, UK. When a pulling device is attached to plant stems to measure their resistance to uprooting, individual plants show remarkable strength in their above- and below-ground biomass (median stem strength when stems break away from the underground biomass, 78 N, median rhizome strength, 39 N) and high resistance to uprooting (median uprooting resistance when entire plants uproot, 114 N). This provides the potential for the species to protect and reinforce the generally soft, silty sediments that it often retains and within which its rhizomes and roots develop in lower energy river environments. There is a propensity for plant stems to break before the plant is uprooted at the beginning and end of the growth season, but for the stems to have sufficient strength in mid season for plant uprooting to dominate. This ensures that rhizome and root systems remain relatively undisturbed at times when the silty sediments in which they grow are poorly protected by above-ground biomass. In contrast, rhizome strength remains comparatively invariant through the growing season, supporting the plant's potential to have a protective/reinforcing effect on fine sediments through the winter when above ground biomass is absent.     

Observational evidence of herbivore‐specific associational effects between neighboring conspecifics in natural,dimorphic populations of Datura wrightii

Associational effects—in which the vulnerability of a plant to herbivores is influenced by its neighbors—have been widely implicated in mediating plant–herbivore interactions. Studies of associational effects typically focus on interspecific interactions or pest–crop dynamics. However, associational effects may also be important for species with intraspecific variation in defensive traits. In this study, we observed hundreds of Datura wrightii—which exhibits dimorphism in its trichome phenotype—from over 30 dimorphic populations across California. Our aim was to determine whether a relationship existed between the trichome phenotype of neighboring conspecifics and the likelihood of being damaged by four species of herbivorous insects. We visited plants at three timepoints to assess how these effects vary both within and between growing seasons. We hypothesized that the pattern of associational effects would provide rare morphs (i.e., focal plants that are a different morph than their neighbors) with an advantage in the form of reduced herbivory, thereby contributing to the negative frequency‐dependent selection previously documented in this system. We found the best predictor of herbivory/herbivore presence on focal plants was the phenotype of the focal plant. However, we also found some important neighborhood effects. The total number of plants near a focal individual predicted the likelihood and/or magnitude of herbivory by Tupiochoris notatus, Lema daturaphila, and Manduca sexta. We also found that velvety focal plants with primarily sticky neighbors are more susceptible to infestation by Tupiochoris notatus and Lema daturaphila. This does not align with the hypothesis that associational effects at the near‐neighbor scale contribute to a rare‐morph advantage in this system. Overall, the results of our study show that the number and trichome‐morph composition of neighboring conspecifics impact interactions between D. wrightii and insect herbivores.     

Orientation of flight for physically disturbed spotted lanternflies,Lycorma delicatula, (Hemiptera,Fulgoridae)

The spotted lanternfly, Lycorma delicatula, (Hemiptera, Fulgoridae) is an invasive pest to Korea and the United States, originating from China or Southeast Asia. Immature L. delicatula feed on a wide range of plants, but the adults are more host-selective, often preferring the tree of heaven, Ailanthus altissima. We performed field studies to evaluate adult movement in relation to A. altissima after disturbance. The Ailanthus trees were in a mixed suburban forested situated at the southern border of an open grassy field. Female adult lanternflies were manually disturbed from feeding on the trunks of large A. altissima trees. In the first experiment, insects were disturbed directly from the tree using a ballpoint pen to simulate a predatory attack. These insects usually flew initially southward away from the tree line toward a sunlit field, but turned northward back toward the tree line. In the second experiment, to simulate an initially successful predatory attack, they were manually taken from trees, and allowed to escape. The females immediately opened their wings in an apparent aposematic display. They then either immediately flew toward the sunlit open field, or remained with their wings splayed open for a prolonged period.     

水体溶氧影响陆生植物喜旱莲子草(Alternanthera philoxeroides)和牛鞭草(Hemarthria altissima)对完全水淹的耐受力

溶氧是水环境中一个重要的环境因子,为了探讨水中的溶氧含量水平是否会对陆生植物的耐淹能力造成影响,研究了陆生植物喜旱莲子草(Alternanthera philoxeroides)和牛鞭草(Hemarthria altissima)在遭受不同溶氧含量水体完全淹没后的生长表现、存活情况和非结构碳水化合物的变化。实验结果表明:(1)水体中的溶氧含量显著影响了处于完全水淹环境中的喜旱莲子草和牛鞭草的存活。受高溶氧水体完全水淹的喜旱莲子草和牛鞭草主茎的完好程度和存活叶的数量均显著高于遭受低溶氧水体完全水淹的喜旱莲子草和牛鞭草,喜旱莲子草和牛鞭草在高溶氧水体完全水淹后的生物量比低溶氧水体完全水淹后要高;(2)水体中的溶氧含量显著影响了处于完全水淹环境中的喜旱莲子草和牛鞭草的生长,受高溶氧水体完全水淹的喜旱莲子草主茎伸长生长和不定根生长显著强于受低溶氧水体完全水淹的喜旱莲子草,在不定根的生长上牛鞭草也具有同样的表现。(3)高溶氧水环境有利于减小被完全淹没的喜旱莲子草和牛鞭草的碳水化合物消耗,两种植物在受高溶氧完全水淹后体内具有的非结构性碳水化合物含量均比受低溶氧完全水淹后高。(4)喜旱莲子草比牛鞭草能更好地耐受完全水淹,当处于低溶氧完全水淹时表现得更为明显,本研究表明入侵物种喜旱莲子草比本地物种牛鞭草具有更强的环境适应能力和水淹耐受能力。     

Reproductive strategy of the sugi bark borer,Semanotus japonicus (Coleoptera: Cerambycidae) on Japanese cedar,Cryptomeria japonica

The inner bark of Japanese cedar,Cryptomeria japonica D. Don., is the main food of the sugi bark borer,Semanotus japonicus Lacordaire, but may also be involved in resistance to attack by the borer. I used newly hatched larvae to inoculate cedar logs that had been cut 2-weeks to 12-months earlier, and undamaged living cedar trees. On living trees, all larvae were killed by resin flow from the traumatic resin canals between the outer- and inner bark or between the inner bark and cambium. In logs that were cut more than 4 months prior to inoculation, larvae were unable to complete development. In logs cut 2 weeks prior to inoculation, almost all larvae were able to reach the adult stage but they were smaller in size than adults from living trees damaged by this borer. These results suggest that poor nutrient conditions in the inner bark of logs affected development and survival of the borer. Because of its nutritional advantage, living cedars provide a better environment for the sugi bark borer. However, all larvae are killed by resin flow, suggesting that this insect is “in between” being primary and secondary with respect to living trees, or a “weak” primary insect. The sugi bark borer seems to develop early in the season so that early instar larvae encounter reduced amounts of resin flow and so that late-instar larvae feed mostly in summer when nutrient levels in the inner bark are at their highest.     

森林生态系统中植食性昆虫与寄主的互作机制、假说与证据

围绕"多样性稳定性"假说、"联合抗性假说"、"生长势假说"、"胁迫假说"、以及下调、上调和推拉等机制与假说提出的背景与实验验证的证据,力图辨析其概念以及它们之间的相互关系。作者认为,多样性-稳定性机制关注森林生态系统的功能,是基于群落甚至景观层次。多样性条件下的联合抗性机制和联合易感性应属于稳定性中的抵抗力范畴。联合抗性机制的主要基础是基于资源集中假说和天敌假说,这些观点在种群层次上更易理解;上调力和下调力机制是以食物网底部的资源与顶端的天敌来探讨这种互作关系。因此,资源集中与上调力有着对应关系,而天敌假说只是下调力机制中的一个层面而已。植物生长势假说和植物胁迫假说力图从植物个体或种的群体的生长状态出发解析植食性动物的对寄主的选择趋势。上述有关植食性昆虫与寄主互作的机制、假说与证据是基于不同的层面提出的,因而在解析研究目标时,由于基本面的差异有可能会得出不同的结论。以近年来的研究进展和研究成果为依据有针对性地阐述这些理论对森林有害生物生态调控技术的指导作用,其中,联合抗性和联合易感性理论对指导森林有害生物生态控制具有更直接的指导作用。进一步提出了相应的亟待解决的科学问题。     

The possible origin of thick stem in Chinese wild soybean (Glycine soja)

Thicker, erect stem and enlarged seeds are characteristic of the domestication of cultivated soybeans (Glycine max) from its progenitor, wild soybean (G. soja). Wild soybeans have different stem thicknesses but the thick stem as defined here appears in a small number of small-seeded wild soybeans (≤2.0 g/100-seeds) in China. However, little attention has been paid to this phenomenon in considering the origin of thick stem in wild soybean. Here, we addressed this question through the study of a mixed field of wild, semi-wild and cultivated soybeans. Thick-stemmed samples had lower sensitivity to light period, higher mean genetic diversity (H _e = 0.090, H = 0.535) and higher mean multilocus outcrossing rate (t _m = 9.77 %), while thin-stemmed plants were the opposite (H _e = 0.029, H = 0.416) and lower mean outcrossing rate (t _m = 5.88 %). F statistics calculations indicated that there was genetic differentiation between the thin and thick stems. UPGM cluster analysis showed that not only were thick-stemmed wild soybeans genetically different from thin-stemmed ones, but they were also genetically closer to semi-wild soybean, to varying degrees completely dependent on seed size. These data strongly implied that the plants with thick stems had more complicated genetic backgrounds than the thin-stemmed ones, and that they were related to cultivated soybeans. This study suggests that if plants have distinctly thick stems (an average 2.5-fold thicker than other thin-stemmed plants) or stems similar to semi-wild plants and/or near to local soybeans in a natural wild population adjacent to farmlands, such plants could be cultivar-introgressive offspring.     

DISPERSION OF SEEDLINGS OF THE PRAIRIE COMPASS PLANT,SILPHIUM LACINIATUM (ASTERACEAE)

Although the dispersal of seeds around individual plants (the seed shadow) has frequently been characterized, the dispersion of seedlings around plants (the seedling shadow) has rarely been examined. We mapped 101 and 149 seedlings of the prairie compass plant (Silphium laciniatum) that appeared in our study area in 1987 and 1990 following mass flowering in 1986 and 1989. We also mapped the locations of flowering stems which appeared in 1986 and 1989 and recorded the number of flowerheads at each stem location. The frequency distributions of distance between a seedling and the nearest flowering stem were identical in the 2 years, with a median distance of 1.0 m. The large size and lack of wind-dispersal structures of compass plant seeds (achenes) are responsible for their limited dispersal. From estimates of the total seed production in the study area in 1986 and 1989, we calculated that about 1% of seeds became seedlings in each year. Flowering stem locations with a higher number of flowerheads had a significantly higher density of seedlings around them. This indicates that recruitment to compass plant populations is not a “lottery”; individual plants that produce more seeds produce, on average, more seedlings.     

Tests of associational defence provided by hairy plants for glabrous plants of Arabidopsis halleri subsp. gemmifera against insect herbivores

1. Trichome‐producing (hairy) and trichomeless (glabrous) plants of Arabidopsis halleri subsp. gemmifera were investigated to test whether plant resistance to herbivory depends on the plants' phenotypes and/or the phenotypes of neighbouring plants (associational effects). 2. A common garden experiment was conducted in which the relative frequency of hairy and glabrous plants was manipulated. Two species of leaf‐chewing insects (larvae of a white butterfly and a cabbage sawfly) were found less often on hairy plants than on glabrous plants. By contrast, the numbers of aphids and flea beetles did not differ significantly between hairy and glabrous plants. For none of these insects did abundance depend on the frequency of the two plant morphs. 3. A field survey was conducted in two natural populations of A. halleri. In the first population, a species of white butterfly was the dominant herbivore, and hairy plants incurred less leaf damage than glabrous plants across 2 years. By contrast, in the other population, where flea beetles were dominant, there were no consistent differences in leaf damage between the two types of plants. In neither of the two populations was any evidence found of associational effects. 4. This study did not provide any conclusive evidence of associational effects of anti‐herbivore resistance, but it was discovered that trichomes can confer resistance to certain herbivores. Given the results of previous work by the authors on associational effects against a flightless leaf beetle, such associational effects of the trichome dimorphism of A. halleri were herbivore‐specific.     

Skewed adult sex ratios observed early in the North American invasion of Lycorma delicatula (Hemiptera: Fulgoridae)

The spotted lanternfly, Lycorma delicatula, (Hemiptera Fulgoridae) is an invasive pest in Korea and the United States, but originating from mainland Asia. Both tree of heaven, Ailanthus altissima, and grapevine (Vitis) are known to be preferred hosts of adults. However, much is unknown about its adult behavior with respect to mating or dispersal patterns. In 2015, just after it was discovered in the United States, we performed an observational study to elucidate movement by adults in relation to host plants. We noted weekly changes in their presence and sex ratio on host plants at four sites in a quarantine zone in Berks County, PA. Just after adult emergence, the trunks of large (>15 cm diameter at breast height) A. altissima hosted nearly exclusively females, while smaller A. altissima, Vitis, Salix nigra, and other less commonly inhabited tree species, had high proportions of males. By October 1st, greater proportions of males were observed on the trunks of the larger A. altissima trees. Pairings of males and females were observed most frequently on the smaller A. altissima and Vitis. At the very end of the season, a large mixed-sex population was found on Salix nigra, which was the only deciduous tree species in the area still bearing green leaves.