Parasitoid-plant mutualism: parasitoid attack of herbivore increases plant reproduction

We tested whether a plant's life time seed production is increased by parasitization of herbivores in a tritrophic system, Arabidopsis thaliana (Brassicaceae) plants, Pieris rapae (Lepidoptera: Pieridae) caterpillars and the solitary endoparasitoid Cotesia rubecula (Hymenoptera: Braconidae). We established seed production for intact A. thaliana plants, plants that were mechanically damaged, plants fed upon by parasitized caterpillars and plants fed upon by unparasitized caterpillars. In the first experiment, with ecotype Landsberg (erecta mutant), herbivory by unparasitized P. rapae caterpillars resulted in a strongly reduced seed production compared to undamaged plants. In contrast, damage by P. rapae caterpillars that had been parasitized by C. rubecula did not result in a significant reduction in seed production. For the second experiment with the ecotype Columbia, the results were identical. Plants damaged by unparasitized caterpillars only produced seeds on regrown shoots. Seed production of plants that had been mechanically damaged was statistically similar to that of undamaged plants. Production of the first ripe siliques by plants fed upon by unparasitized caterpillars was delayed by 18–22 days for Landsberg and 9–10 days for Columbia. We conclude that parasitization of P. rapae by C. rubecula potentially confers a considerable fitness benefit for A. thaliana plants when compared to plants exposed to feeding damage by unparasitized P. rapae larvae. Plants that attract parasitoids and parasitoids that respond to herbivore-induced plant volatiles will both experience selective advantage, justifying the use of the term mutualism for this parasitoid-plant interaction. This type of mutualism is undoubtedly very common in nature.     

Neopolyploidy in Spartina pectinata Link: 1. Morphological analysis of tetraploid and hexaploid plants in a mixed natural population

Prairie cordgrass has been reported as a multi-polyploidy species having three cytotypes: tetra- (2n?=?4x?=?40), hexa- (2n?=?6x?=?60), and octoploid (2n?=?8x?=?80). A mixed-ploidy population comprising tetraploids and hexaploids was recently found at a single location in Illinois. However, adaptation and morphological differences between tetra- and hexaploids occurring in natural conditions as well as the contact zones of these cytotypes have yet to be determined. In this study, the cytotypes of 147 individuals of prairie cordgrass collected across the contact zone (4x?+?6x) were determined by flow cytometry using somatic G1 nuclei, and the results were confirmed by chromosome counts. Nineteen morphological characteristics were compared between the cytotypes. Tetra- and hexaploid plants have 2C genome sizes of 1.57 and 2.36?pg with chromosome counts of 40 and 60, respectively. This increase in polyploidy resulted in a greater variability of morphological expression in Illinois prairie cordgrass. Substantial differences in the flowering time, stomatal size, and plant morphological characteristics were observed between tetra- and hexaploids. The results indicate that the increasing of ploidy level in prairie cordgrass resulted in increased plant size in ploidy mixtures. The recent event of ploidy mixtures in prairie cordgrass natural populations offers unique opportunities for studying the formation and establishment of neopolyploidy.     

Impacts of granivorous and frugivorous arthropods on pre-dispersal seed production of western juniper (<Emphasis Type="Italic">Juniperus occidentalis</Emphasis>)

Arthropods impact seed production in various juniper species, but effects of pre-dispersal seed predation are generally unknown for arthropods that feed on western juniper (Juniperus occidentalis). From 2009 to 2013, we quantified impacts of three arthropod granivores and a frugivorous insect on western juniper seed production at two N. California field sites—Madeline and Shinn Peak. Insect larvae were genetically identified using the COI barcode region. Seed damage by all arthropod taxa varied both spatially and temporally. Juniper berry mites (Trisetacus quadricetus) had the greatest effect on reducing seed production at Madeline, and granivorous moths (Periploca spp. and Argyresthia spp.) had the greatest effect at Shinn Peak. Three findings supported the predator satiation hypothesis, which suggests that unpredictable masting events overwhelm responses of seed predators. First, estimated berry production had significant negative effects on the proportion of seeds damaged across sites by a chalcidoid wasp (Eurytoma juniperina) and by granivorous moths at Shinn Peak. Second, seed damage by granivorous moths was significantly reduced in a mast year of juniper berry production. Third, number of seeds per berry positively affected seed damage across sites by granivorous moths in all years except the mast year as well as damage by a frugivorous tortricid moth (Henricus infernalis) at Madeline. Distance to neighboring trees had positive, site-specific effects on damage by granivorous moths and Henricus, perhaps due to limited movement of parasitoids that attack these insects. Our results suggest that arthropod damage can significantly impact seed production of western juniper.     

Effect of pine foliage damage on the incidence of larval diapause in the pine caterpillar Dendrolimus punctatus (Lepidoptera: Lasiocampidae)

The pine caterpillar Dendrolimus punctatus (Walker) with a larval facultative diapause is one of the most destructive insect pests of the pine tree Pinus massoniana in China. The larvae feeding on pine trees with different damage levels were studied to determine the induction of diapause under both laboratory and field conditions. Developmental duration of larvae before the third instar was the longest when fed with 75%–90% damaged needles, followed by 25%–40% damaged needles and intact pine needles, whereas mortalities did not differ among different treatments under the conditions of 25° and critical photoperiod 13.5:10.5 L:D. At 25°, no diapause was induced under 15:9 L:D, whereas 100% diapause occurred under 12:12 L:D regardless of the levels of needle damage. Incidences of larvae entering diapause when they were fed with intact, 25%–40% and 75%–90% damaged pine needles were 51.7%, 70.8% and 81% under 13.5:10.5 L:D, respectively. Similar results were obtained in the field experiment. Incidence of diapause was significantly different among the pine needle damage levels of pine trees when the photoperiod was close to the critical day length, indicating that the effect of host plants on diapause induction was dependent on the range of photoperiod. The content of amino acid and sugar decreased and tannin increased in pine needles after feeding by the pine caterpillars, suggesting that changed levels of nutrients in damaged needles or a particular substance emitted by damaged pine trees was perhaps involved in the diapause induction of the pine caterpillar.     

Wheat stem sawfly‐infested plants benefit from parasitism of the herbivorous larvae

1 Parasitoids Bracon cephi (Gahan) and Bracon lissogaster Muesebeck and their herbivorous host the wheat stem sawfly Cephus cinctus Norton, a pest of wheat Triticum aestivum, were investigated for yield in T. aestivum grown in the field. 2 Wheat stem sawfly‐infested stems had a higher yield potential than uninfested stems. However, final reproductive output was not significantly different between ears on infested stems that supported complete larval development compared with ears on uninfested stems. 3 Stems containing parasitized larvae and stems containing larvae that died before completing their development had a higher mean number of seeds and seed weight, when accounting for number of fertile spikelets of each ear, than either infested with live larvae and uninfested stems. 4 The results obtained suggest that larval feeding prevented infested stems from attaining their yield potential, and that the negative impact of the pest on wheat yield was reduced when late instar sawfly larvae were parasitized. Even though some feeding occurs before parasitism, this early damage has a comparatively low impact on yield. 5 This is the first study to show a yield benefit and enhanced plant fitness due to the wheat stem sawfly parasitoids B. cephi and B. lissogaster. This results from the maintenance of increased seed number and seed weight in the higher yielding stems that are preferentially infested by this pest.     

Herbivore and Fungal Pathogen Exclusion Affects the Seed Production of Four Common Grassland Species

Insect herbivores and fungal pathogens can independently affect plant fitness, and may have interactive effects. However, few studies have experimentally quantified the joint effects of insects and fungal pathogens on seed production in non-agricultural populations. We examined the factorial effects of insect herbivore exclusion (via insecticide) and fungal pathogen exclusion (via fungicide) on the population-level seed production of four common graminoid species (Andropogon gerardii, Schizachyrium scoparium, Poa pratensis, and Carex siccata) over two growing seasons in Minnesota, USA. We detected no interactive effects of herbivores and pathogens on seed production. However, the seed production of all four species was affected by either insecticide or fungicide in at least one year of the study. Insecticide consistently doubled the seed production of the historically most common species in the North American tallgrass prairie, A. gerardii (big bluestem). This is the first report of insect removal increasing seed production in this species. Insecticide increased A. gerardii number of seeds per seed head in one year, and mass per seed in both years, suggesting that consumption of flowers and seed embryos contributed to the effect on seed production. One of the primary insect species consuming A. gerardii flowers and seed embryos was likely the Cecidomyiid midge, Contarinia wattsi. Effects on all other plant species varied among years. Herbivores and pathogens likely reduce the dispersal and colonization ability of plants when they reduce seed output. Therefore, impacts on seed production of competitive dominant species may help to explain their relatively poor colonization abilities. Reduced seed output by dominant graminoids may thereby promote coexistence with subdominant species through competition-colonization tradeoffs.     

Interactive effects of soil fertility and herbivory on Brassica nigra

Soil nutrient availability may affect both the amount of damage that plants receive from herbivores and the ability of plants to recover from herbivory, but these two factors are rarely considered together. In the experiment reported here, I examined how soil fertility influenced both the degree of defoliation and compensation for herbivory for Brassica nigra plants damaged by Pieris rapae caterpillars. Realistic levels of defoliation were obtained by placing caterpillars on potted host plants early in the life cycle and allowing them to feed until just before pupation on the designated plant. Percent defoliation was more than twice as great at low soil fertility compared to high (48.2% and 21.0%, respectively), even though plants grown at high soil fertility lost a greater absolute amount of leaf area (38.2 cm² and 22.1 cm², respectively). At both low and high soil fertility, total seed number and mean mass per seed of damaged plants were equivalent to those of undamaged plants. Thus soil fertility did not influence plant compensation in terms of maternal fitness. However, the pathways used to achieve compensation in seed production were different at low and high soil fertility. At low soil fertility, relative leaf growth rates (area added per inital area per day) of damaged plants were drastically reduced over the second week of caterpillar feeding. Damaged plants recovered the leaf area lost to herbivory in the two weeks following insect removal by increasing leaf relative growth rates above the levels seen for undamaged plants, but the replacement of leaf tissue lost to herbivory came at the expense of stem biomass. At high soil fertility, relative leaf growth rates of damaged plants were similar to those of undamaged plants both over the second week of caterpillar feeding and following caterpillar removal, and stem biomass was not affected by herbivory. These results suggest that higher levels of soil nutrients increased the ability of plants to stay ahead of their herbivores as they were being eaten. Because damaged plants at high soil fertility were able to maintain leaf growth rates to a greater extent than damaged plants at low soil fertility, they did not fall as far behind undamaged plants over the period of insect feeding and did not have as much catching up to do after feeding ended to compensate for herbivory.     

Symptoms, Distribution and Abundance of the Stem-Boring Caterpillar, Blastobasis repartella (Dietz), in Switchgrass

A potential pest of switchgrass, Panicum virgatum L., was first detected in South Dakota in 2004, where death of partially emerged leaves was noted in a small proportion of tillers. Similar “dead heart” symptoms were observed in switchgrass in Illinois during 2008 and adults of a stem-boring caterpillar were collected and identified as Blastobasis repartella (Dietz). In 2009, a survey of the central United States was used to estimate the distribution and abundance of this insect. In eight northern states, B. repartella was consistently found in both cultivated plots and natural stands of switchgrass. In four southern states, B. repartella was not detected. However, because symptoms are conspicuous for a short period of time, failure to collect stem-borers on one survey date for each southern location does not necessarily define the limit of distribution for B. repartella. Sampling in four northern states showed the proportion of tillers damaged by B. repartella ranged from 1.0–7.2%. Unlike some caterpillars that feed on native grasses, it appears that the egg-laying behavior of adult moths may preclude the use of prescribed burns as an effective method to suppress this stem-boring caterpillar. As a potential pest of switchgrass planted for biomass production, near-term research needs include refining the geographic distribution of B. repartella, quantifying potential losses of switchgrass biomass, and determining whether switchgrass may be bred for resistance this and other stem-boring insects.     

Taxonomic and Functional Structure of Phytophagous Insect Communities Associated with Grain Amaranth

Amaranthus are worldwide attacked mainly by leaf chewers and sucker insects. Stem borers and leaf miners follow in importance, while minor herbivores are leaf rollers, folders, and rasping-sucking insects. The herbivorous community observed on Amaranthus spp. in Argentina was consistent with the information reported worldwide both in guild composition and order proportion. Amaranth plants had a higher number of phytophagous species in their native rather than in its introduced range. Occurrence of insect guilds differed in space and time. The highest density of leaf chewers was observed shortly after the emergence of plants, while higher density of borer and sucker insects coincided with reproductive stages of the crop. The sucking guild was observed mainly at panicles, while the insects within the leaf chewer group were registered in both leaves (92.6%, n?=?746 adults) and inflorescences (7.4%). The borer guild was also recorded in stems and inflorescences; however, the density of larvae in stems was about four times as high as the density observed in panicles (n?=?137 larvae).     

水稻穗期大螟危害习性初步观察

于2009年常规中稻的蜡熟前期、中期和黄熟期、枯熟期,分别取样观察大螟Sesamia inferens Walker的钻蛀危害习性。结果发现:单株螟害的蛀孔数一般为2～3个,多的达5个,而且表现白穗株>枯穗株>虫伤株。螟害株的蛀孔部位随着稻穗趋向成熟和大螟虫龄的增大,逐渐由稻株的中上部节间向中下部节间转移。其中,白穗株与枯穗株有70%以上的蛀孔分别分布于稻株由上向下的第1～3与3～5节间上;在水稻的这4个生长时期中,虫伤株有90%左右的蛀孔分别分布于稻株由上向下的第1～3、1～4和2～5、3～5节间上,全株虫量有85%以上依次分布于第1～3、2～4和3～5、3～5节间上,单株虫量4个时期最高分别为18、6、3、2头,同一节间内的4个时期最高虫量分别为12、5、1、1头;单个节间有多头螟虫时,80%以上为1～3龄幼虫。随着稻穗趋向成熟,白穗株和枯穗株上的虫量均逐渐下降,而虫伤株上的虫量则相对稳定或有上升的趋势;至枯熟期后,虫伤株的有虫株率和有虫孔率均稳定在30%以上。     

Above- and Belowground Prairie Cordgrass Response to Applied Nitrogen on Marginal Land

Prairie cordgrass (Spartina pectinata, Link.) has been evaluated for its biomass potential because of its high yield, relatively low nutrient demand, and diverse geographical adaptation. Our objectives were to determine (1) biomass production potential of prairie cordgrass in South Dakota and Kansas under varying nitrogen levels, (2) the effect of N on prairie cordgrass yield components (tillers m^?2 and tiller mass), and (3) the effect of N on yield and N concentration of belowground biomass. Older stands of Red River prairie cordgrass (RR-PCG) in South Dakota and Atkins prairie cordgrass (AT-PCG) in Kansas were fertilized with 0, 56, 112 and 168 kg N ha^?1 from 2008 to 2011 in South Dakota and in 2009 and 2010 in Kansas. Experimental design at all locations was a 4?×?4 Latin square. Prairie cordgrass was harvested around a killing frost in October and early November. Biomass production ranged from 5.50 to 13.69 Mg ha^?1 in South Dakota and 5.33 to 12.51 Mg ha^?1 in Kansas. Prairie cordgrass yield did not increase significantly with N application at any location or year. Across years, tiller density ranged from 536 to 934 tillers m^?2 for RR-PCG in South Dakota and from 234 to 315 tillers m^?2 for AT-PCG in Kansas. Neither tiller density or tiller mass was affected by N rate at any location in any year. Belowground biomass production to a depth of 25 cm was equal to or greater than aboveground biomass. However, it was not affected by N rate in all locations by any year. Understanding prairie cordgrass nitrogen-use dynamics to improve biomass and nutrient management will be essential for future investigations. Findings of this study are important to support the notion that prairie cordgrass biomass production in two different environments can be achieved with minimal N inputs.     

Parasitoid load affects plant fitness in a tritrophic system

Plants attacked by herbivorous insects emit volatile compounds that attract predators or parasitoids of the herbivores. Plant fitness increases when these herbivorous insects are parasitized by solitary parasitoids, but whether gregarious koinobiont parasitoids also confer a benefit to plant fitness has been disputed. We investigated the relationship between parasitoid load of the gregarious Cotesia glomerata (L.) (Hymenoptera: Braconidae), food consumption by larvae of their host Pieris brassicae L. (Lepidoptera: Pieridae), and seed production in a host plant, Brassica nigra L. (Brassicaceae), in a greenhouse experiment. Plants damaged by caterpillars containing single parasitoid broods produced a similar amount of seeds as undamaged control plants and produced significantly more seeds than plants with unparasitized caterpillars feeding on them. Increasing the parasitoid load to levels likely resulting from superparasitization, feeding by parasitized caterpillars was significantly negatively correlated with plant seed production. Higher parasitoid brood sizes were negatively correlated with pupal weight of Cotesia glomerata , revealing scramble competition leading to a fitness trade-off for the parasitoid. Our results suggest that in this tritrophic system plant fitness is higher when the gregarious parasitoid deposits a single brood into its herbivorous host. A prediction following from these results is that plants benefit from recruiting parasitoids when superparasitization is prevented. This is supported by our previous results on down-regulation of synomone production when Brassica oleracea was fed on by parasitized caterpillars of P. brassicae . We conclude that variable parasitoid loads in gregarious koinobiont parasitoids largely explain existing controversies about the putative benefit of recruiting these parasitoids for plant reproduction.     

Effect of Heliothis subflexa herbivory on fruit abscission by Physalis species: the roles of mechanical damage and chemical factors

1. Insect oral secretions are important for the induction of a number of plant responses, but the relative role of mechanical damage in the induction of these responses is often not well understood. Damage from the frugivore Heliothis subflexa, a specialist on Physalis species, causes herbivore‐induced fruit abscission. In this field study, we examined the separate and combined effects of mechanical damage and H. subflexa oral secretions on Physalis fruit abscission. 2. To determine the relative role of mechanical and chemical factors, the following treatments were administered to fruit: (1) three levels of mechanical damage, (2) natural herbivore damage by control larvae and by larvae surgically treated to inhibit saliva secretion, and (3) injection of H.subflexa oral secretions and a water control. Abscission of mechanically damaged fruit with and without the addition of oral secretions was also compared. 3. Mechanical damage was sufficient to cause fruit abscission, and the addition of oral secretions to mechanically damaged fruit did not cause an increase in fruit abscission. Normal caterpillars and those treated to inhibit saliva secretion caused similar abscission rates. 4. Though most studies examining the effects of insect oral secretions on induced plant responses find these chemical stimuli to be important or essential, the results of the present study showed that oral secretions are not necessary for fruit abscission. Future work is needed to determine the relative importance of mechanical damage in herbivore‐induced plant responses in other systems.     

The effect of the CpTi gene in strawberry against attack by vine weevil (Otiorhynchus sulcatus F. Coleoptera: Curculionidae)

The degree of protection against insect feeding conferred upon transgenic strawberry lines expressing the Cowpea trypsin inhibitor was evaluated under glasshouse conditions. Insect bioassays were carried out using vine weevil (Otiorhynchus sulcatus) in two experiments and in both experiments there was a highly significant reduction in damage by weevil larvae on the transgenic lines.     

Insect Herbivory of Ovules and Seeds in Native and Restored Prairies

Insect herbivory of ovules and seeds is known to have a negative impact on individual plant reproductive success by reducing overall seed set. Although this seed set reduction has been well documented in plant populations found in native habitats, little work has been done on populations found in restored habitats. In a 4-year study (2001–2004), I investigated the herbivory of the Gelechiid moth ( Coleotechnites eryngiella ) of Eryngium yuccifolium populations in native and restored prairies. Data were collected from 20 E. yuccifolium populations (10 native prairies/10 restored prairies) in Illinois. Percent herbivory and percent seed set were determined for each population. From 2001 to 2004, percent herbivory in prairie restorations ranged from 0 to 93 and percent seed set ranged from 2 to 82. In native prairies, percent herbivory ranged from 0 to 98 and percent seed set ranged from 0 to 71. No differences were found between native and restored prairies for percent herbivory or percent seed set. However, significant differences were found among years for percent seed set. This study shows that the antagonistic effects of insect herbivory can reach similar levels in restored and native prairies.     

辽东栎叶片昆虫取食形状多样性及其变化模式

取食辽东栎(Quercus liaotungensis)叶片的植食性昆虫有丰富的多样性和重要的生态功能,昆虫取食叶片留下的形状变化多,易观察,可以作为昆虫物种多样性监测的手段之一。研究北京西部东灵山地区辽东栎叶片被植食性昆虫取食状况,发现昆虫对辽东栎叶片的危害非常普遍,叶片被食频率约为90％,被食面积约5％,取食状可分为9类,其中以缘食和孔食为主,共记录20余种植性昆虫,其中蛾类幼虫和甲虫是主要取食种类,植食性昆虫集中出现在叶片发育的早期阶段（5－6月份）,7月份后,大多数幼虫发育成熟,取食叶片的面积减少。各取食状所涉及昆虫种类的组成不同,面积呈不同的季节变化模式,其中缘食状和孔食状之间有显著的正相关关系;辽东栎叶片在展叶初的叶面积最低,在7月底均达到最大值,然后呈下降趋势,单位面积重量也在展叶初最低,在6月底接近最大值,不同坡向的辽东栎林和叶片面积,单位面积重量及叶片被食面积均有显著差异,偏阳坡（东南坡） 高于偏阴坡（西北坡）。以上结果表明,辽东栎叶片维持了昆虫物种多样性,昆虫的生长发育与辽东栎的物候规律相互协调,昆虫种类的取食呈季节和空间变化。     

Nesaecrepida infuscata: a biological control agent of the invasive plant Mimosa pigra

Mimosa pigra L. is a serious weed of wetlands of Australia, Asia and Africa. A suite of established biocontrol agents have been introduced in Australia and some Asian countries, but better control is needed. Nesaecrepida infuscata (Schaeffer) (Coleoptera: Chrysomelidae) is a common insect on M. pigra in tropical America. The larvae develop on the roots while the adults feed on the leaves. As both roots and leaves of M. pigra are relatively undamaged in the introduced range, this species has potential to limit the growth, survival and seed production. Furthermore, it is abundant in the dry season and so inflicts damage when most other agents are not active. In host specificity tests, larvae did not develop on any of the 65 test plant species other than M. pigra. Adult feeding on test plant species other than M. pigra was minimal. Based on these results, this insect has been released in Australia.