Competition and co-existence of Zoophthora radicans and Pandora blunckii, two co-occurring fungal pathogens of the diamondback moth, Plutella xylostella

The entomopathogenic fungi Zoophthora radicans and Pandora blunckii co-occur in field populations of Plutella xylostella and, therefore, are likely to interact during the infection process. We have investigated the possible outcomes of these interactions in the laboratory. Using four isolates, two of each fungal species, inter-specific interaction experiments were done in Petri dishes and on intact plants. In Petri dish experiments, larvae were inoculated directly using sporulating mats of mycelium, both species had the same opportunity to infect and only the relative concentration of conidia of each pathogen species applied was manipulated. In the intact plant experiments, larvae were placed onto fungus-contaminated plants, inoculation was passive and the probability of infection by either or both species of fungi depended on larval activity and proximity to inoculum. In the Petri dish experiment, the species with the largest concentration of conidia out-competed the other regardless of virulence, and results were similar in the intact plant experiment. The ecological implications for competition or co-existence of these two pathogens in the field are discussed.     

The influence of density-dependent factors on larval development in native and invasive populations of <Emphasis Type="Italic">Harmonia axyridis</Emphasis> (Pall.) (Coleoptera,Coccinellidae)

Effects of the number of larvae per Petri dish (1, 5, and 10) on the preimaginal development of individuals of the native (Irkutsk, southern Siberia) and invasive (Sochi, the Northern Caucasus) populations of the multicolored Asian ladybird Harmonia axyridis were investigated in the laboratory. The experiments were conducted under short (12 h) and long (18 h) day conditions; the larvae were fed on the green peach aphid Myzus persicae or on the eggs of the grain moth Sitotroga cerealella. An increase in the number of larvae developed in one Petri dish resulted in a significant decrease in the rate of development in individuals from both populations which fed on aphids. Survival decreased with an increase in the number of larvae developed in one Petri dish fed on both prey species, but only in larvae from the invasive population of H. axyridis. The weight of emerging adults decreased with the number of larvae per dish in individuals from both study populations, but only when fed on aphids. These data suggest that the influence of density-dependent factors on the development of H. axyridis depends significantly on larval prey species. In addition, larvae from the invasive population have somewhat more aggressive interactions with competitors, this possibly having been one of the prerequisites for invasion.     

Preference for plants damaged by conspecific larvae in ovipositing cabbage root flies: influence of stimuli from leaf surface and roots

In laboratory dual-choice assays females of the cabbage root fly, Delia radicum, prefer for oviposition plants with roots damaged by conspecific larvae to undamaged controls. Cauliflower and kale plants were inoculated with root fly eggs (25 per plant) and the hatching larvae were allowed to feed on the roots for various periods of time (1–17 days). After 4 (cauliflower) or 5 (kale) days of larval feeding the oviposition preference was most pronounced and flies laid between 64% and 68% of their eggs near plants with damaged roots. Later, with increasing damage but fewer surviving, and thus actively feeding, larvae, the magnitude of the preference declined. The preference for plants already damaged by conspecific larvae may contribute to the previously observed aggregated distribution of D. radicum eggs in Brassica crop fields.Further experiments revealed that the sensory cues inducing this oviposition preference originate from the complex consisting of the damaged roots, the surrounding substrate (soil) and associated microbes, rather than from the aerial plant parts. In choice assays using the root-substrate complex of damaged and control plants (aerial parts removed), the observed preference for damaged roots was similar to that found for the entire plant but was more pronounced. The damaged roots alone, compared to control roots, received up to 72% (cauliflower) and 75% (kale) of the eggs. By contrast, surrogate leaves sprayed with methanolic leaf surface extracts from the most preferred plants which had been damaged were not discriminated from surrogate leaved sprayed with extracts of the respective control plants. Analysis of glucosinolate levels in methanolic leaf surface extracts revealed that root damage resulted in enhanced concentrations of indole-glucosinolates on the leaf surface in kale but not in cauliflower. Although indole-glucosinolates are oviposition stimulants for the cabbage root fly, the induced changes were apparently too small to influence oviposition behaviour.     

Influence of Insect Larvae and Plant Roots on the Host-finding Behaviour of Heterorhabditis megidis

We studied the host-finding and dispersion behaviour of Heterorhabditis megidis (strain NLHE 87.3) in the presence of Galleria mellonella or Otiorhynchus sulcatus larvae and strawberry roots. In large Petri dishes (19 cm diameter) filled with moist sand (8% w/w), and incubated at 15°C over 24 h, infective juveniles (IJs) responded positively to the presence of G. mellonella , to roots of a single strawberry plant and to O. sulcatus larvae in direct contact with roots of a single strawberry plant. A neutral or negative response was observed when IJs were presented with only O. sulcatus larvae or a combination of several strawberry plants with O. sulcatus larvae, either in contact or not in contact with the roots. IJs responded strongly to the combination of plant roots and feeding larvae indicating that the tritrophic interaction formed by IJs - O. sulcatus larvae - strawberry plants may be an infochemical-mediated interaction.     

Fungus gnats vector Fusarium oxysporum f.sp. radicislycopersici1

Fungus gnat adults transported Fusarium oxysporum f.sp. radicis-lycopersici from Petri dish culture and infected host plants to the roots and hypocotyls of healthy tomato and bean plants. The source of the fungus did not affect the ability of fungus gnats to transport the fungus to healthy hosts. The presence of fungus gnat larvae in media in which young tomato plants were grown did not increase the incidence of plant infection by the pathogen. Fungus gnat adults appear to aid in the dissemination of F. oxysporum f.sp. radicis-lycopersici.     

Effect of various exogenous and endogenous factors on microspore embryogenesis in Indian mustard (<Emphasis Type="Italic">Brassica juncea</Emphasis> (L.) Czern and Coss)

Summary The influence of donor plant growth environment, microspore development stage, culture media and incubation conditions on microspore embryogenesis was studied in three Indian B. juncea varieties. The donor plants were grown under varying environments: field conditions, controlled conditions, or a combination of the two. The correlation analysis between the bud size and microspore development stage revealed that the bud size is an accurate marker for donor plants grown under controlled conditions, however, the same does not hold true for the field-grown plants. The buds containing late uninucleate microspores collected from plants grown under normal field conditions up to bolting stage and then transferred to controlled environment were observed to be most responsive with genotypic variability ranging from 10 to 35 embryos per Petri dish, irrespective of the other factors. NLN medium containing 13% sucrose was found to be most suitable for induction of embryogenesis The fortification of this medium with activated charcoal, polyvinylpyrrolidone, colchicine, or growth regulators (6-benzylaminopurine and 1-naphthaleneacetic acid) was observed to be antagonistic for microspore embryogenesis, while silver nitrate (10 μM) had a significant synergistic effect. A post-culture high-temperature incubation of microspores at 32.5±1°C for 10–15 d was found most suitable for high-frequency production of microspore embryos. The highest frequency of microspore embryogenesis (78 embryos per Petri dish) was observed from the late uninucleate microspores (contained in bud sizes 3.1–3.5 nm irrespective of genotype) cultured on NLN medium containing 13% sucrose and silver nitrate (10 μM), and incubated at 32.5°C for 10–15 d.     

Behavioral Response to Larval Tracks and the Influence of Tracks on Intraguild Scavenging by Coccinellid Larvae

A paired design was used to determine that Harmonia axyridis 4th instars were not influenced by the presence of conspecific larval tracks, but well-fed H. axyridis 4th instars spent less time on plants that contained tracks left by Coleomegilla maculata 4th instars. To determine if the presence of larval tracks influences intraguild scavenging by H. axyridis 4th instars, dead 4th instars were placed in Petri dishes that contained or did not contain larval tracks. The presence of larval tracks did not influence the feeding frequency or the amount of time before feeding. However, larvae dragged their pygopod on dish surfaces more frequently if the dish contained larval tracks. In addition, starved H. axyridis larvae were more likely to feed on the prey and dragged their pygopod less frequently than well-fed larvae.     

Effect of arbuscular mycorrhizal fungi (Glomus intraradices) on the oviposition of rice water weevil (Lissorhoptrus oryzophilus)

Root-feeding insects are important drivers in ecosystems, and links between aboveground oviposition preference and belowground larval performance have been suggested. The root-colonizing arbuscular mycorrhizal fungi (AMF) play a central role in plant nutrition and are known to change host quality for root-feeding insects. However, it is not known if and how AMF affect the aboveground oviposition of insects whose offspring feed on roots. According to the preference–performance hypothesis, insect herbivores oviposit on plants that will maximize offspring performance. In a greenhouse experiment with rice (Oryza sativa), we investigated the effects of AMF (Glomus intraradices) on aboveground oviposition of rice water weevil (Lissorhoptrus oryzophilus), the larvae of which feed belowground on the roots. Oviposition (i.e., the numbers of eggs laid by weevil females in leaf sheaths) was enhanced when the plants were colonized by AMF. However, the leaf area consumed by adult weevils was not affected. Although AMF reduced plant biomass, it increased nitrogen (N) and phosphorus concentrations in leaves and N in roots. The results suggest that rice water weevil females are able to discriminate plants for oviposition depending on their mycorrhizal status. The discrimination is probably related to AMF-mediated changes in plant quality, i.e., the females choose to oviposit more on plants with higher nutrient concentrations to potentially optimize offspring performance. AMF-mediated change in plant host choice for chewing insect oviposition is a novel aspect of below- and aboveground interactions.     

Environmentally Controlled Sex Expression in Meloidodera floridensis

Larvae of Meloidodera floridensis develop as females after feeding on pine roots, but become males under conditions of starvation. Seventy to 80% of the larvae kept in tap water at 23 C for 4 months underwent one or two molts, developing as males, and more than 50% became adult males. Ninety-six percent of the larvae that entered pine roots became females and only 4% developed as males. There is evidence that the latter did not feed on the roots. In comparison with tap water, solutions of cholesterol, testosterone propionate and β-estradiol did not significantly affect the percentage of larvae that developed into males. Larvae kept in soil without a host plant did not develop into males. Most of them exhausted their energy supply and died without undergoing any development. We conclude that sex expression in M. floridensis is to a large extent controlled by environmental factors. Under natural conditions of feeding on a host plant, larvae develop as females according to their genetic constitution (thelytokous organism). Under conditions of starvation, however, sexual differentiation proceeds toward the male direction, probably as a result of alteration of the hormonal balance of the larvae and the subsequent activation of different sites of genetic function.     

The Influence of Diet and Population Density on Maturation of Females from Native and Invasive Populations of the Multicolored Asian Ladybird <Emphasis Type="Italic">Harmonia axyridis</Emphasis> (Pallas) (Coleoptera,Coccinellidae)

Population density can influence insect reproduction both directly and indirectly causing quantitative and qualitative changes in the feeding regime. We investigated the impact of diet and population density on reproductive maturation of females from native (Irkutsk) and invasive (Sochi) populations of the multicolored Asian ladybird Harmonia axyridis. During the study the beetles fed either on the green peach aphid, Myzus persicae, or on less suitable factitious food (eggs of the grain moth Sitotroga cerealella). Population density was determined by the number of females and males placed in a standard Petri dish. The experiments showed that feeding on the grain moth eggs (other conditions being equal) delayed maturation and increased the tendency to enter reproductive diapause in females from the native but not from the invasive population of H. axyridis. In addition, the preoviposition period increased with the number of females but decreased with the number of males in a dish, although these effects were observed only in individuals from the invasive population and their strength depended on the food of the beetles. Earlier we have demonstrated that the impact of the density-dependent factors on the larvae of the invasive population of H. axyridis was stronger than that on the larvae of the native population because the larvae of the invasive population have somewhat more aggressive interactions with competitors. In the present study, similar differences were revealed between females of these populations.     

THE BIOLOGY AND CONTROL OF OTIORRHYNCHUS CLAVIPES BONSD. (RHYN. COLEOP.), A PEST OF STRAWBERRIES

Otiorrkynchus clavipes Bonsd. is a serious pest of strawberries around Cheddar, Somerset. The weevils nibble the foliage, the larvae eat the roots of the plants, and burrow into the rootstock.
The beetles appear in two waves, pupating in the autumn and emerging en masse in the spring, or pupating in the late spring and summer and emerging in succession between mid-June and the end of August. Both sexes are found and can be distinguished by the posterior thoracic sterna, which are concave in males and convex in females. Mating is alternated with egg-laying and the beetles lay between 100 and 300 eggs per female. On emergence the beetles eat voraciously, and roughly one egg is laid for every 10 sq.mm. of leaf consumed. Both parthenogenetic and fertile eggs are laid and hatch in between 17 and 24 days.
The larvae burrow rapidly when hatched and can only penetrate soil spaces which will admit their head capsules. The distribution of head capsule widths in mixed populations of larvae of all ages shows there are five instars. While feeding, the larvae are clustered within the root range of the plants and find their way to their food from plant to plant, guided by the exudates from the strawberry roots. As the larvae grow older they descend in the soil and turn to white soft pupae in smooth earthen cells 6–8 in. below the plants.
Adult damage to foliage is unimportant but larval damage to roots is serious and has been largely responsible for restricting the effective life of the strawberry beds to at most 2 years. Although weevil attack can be avoided by growing the crop in a 1-year rotation, control is difficult owing to the intensive cropping, which creates the most suitable conditions for the spread of the infestations. The adults are killed with 10% DDT dust; and the larvae by DDT and BHC preparations poured on to the soil, at a pint per plant, or by DD injection. The limitations and possibilities of various methods of control are discussed.     

The effect of plant identity and the level of plant decay on molecular gut content analysis in a herbivorous soil insect

Plant roots represent an important food source for soil‐dwelling animals, but tracking herbivore food choices below‐ground is difficult. Here, we present an optimized PCR assay for the detection of plant DNA in the guts of invertebrates, using general plant primers targeting the trnT‐F chloroplast DNA region. Based on this assay, we assessed the influence of plant identity on the detectability of ingested plant DNA in Agriotes click beetle larvae. Six different plant species were fed to the insects, comprising a grass, a legume and four nonlegume forbs. Moreover, we examined whether it is possible to amplify DNA of decaying plants and if DNA of decayed plant food is detectable in the guts of the larvae. DNA of the ingested roots could be detected in the guts of the larvae for up to 72‐h post‐feeding, the maximum digestion time tested. When fed with living plants, DNA detection rates differed significantly between the plant species. This may be ascribed to differences in the amount of plant tissue consumed, root palatability, root morphology and/or secondary plant components. These findings indicate that plant identity can affect post‐feeding DNA detection success, which needs to be considered for the interpretation of molecularly derived feeding rates on plants. Amplification of plant DNA from decaying plants was possible as long as any tissue could be retrieved from the soil. The consumption of decaying plant tissue could also be verified by our assay, but the insects seemed to prefer fresh roots over decaying plant material.     

Effect of Cry3Bb1-expressing transgenic corn on plant-to-plant movement by western corn rootworm larvae (Coleoptera: Chrysomelidae)

Dispersal of larvae of the western corn rootworm, Diabrotica virgifera virgifera LeConte, in specific combinations of transgenic corn expressing the Cry3Bb1 protein and nontransgenic, isoline corn was evaluated in a 2-yr field study. In total, 1,500 viable western corn rootworm eggs were infested in each subplot. Each year, plant damage and larval recovery were evaluated among four pedigree combinations (straight transgenic; straight nontransgenic corn; nontransgenic corn with a transgenic central, infested plant; and transgenic corn with a nontransgenic central, infested plant) on six sample dates between egg hatch and pupation. For each subplot, the infested plant, three successive plants down the row (P1, P2, and P3), the closest plant in the adjacent row of the plot, and a control plant were sampled. The number of western corn rootworm larvae recovered from transgenic rootworm-resistant plants adjacent to infested nontransgenic plants was low and not statistically significant in either 2001 or 2002. In 2001, significantly fewer larvae were recovered from transgenic rootworm-resistant plants than from nontransgenic plants when both were adjacent to infested, nontransgenic plants. In 2002, significantly more neonate western corn rootworm larvae were recovered from nontransgenic plants adjacent to infested, transgenic rootworm-resistant plants than nontransgenic plants adjacent to infested, nontransgenic plants on the second sample date. Together, these data imply that both neonate and later instar western corn rootworm larvae prefer nontransgenic roots to transgenic rootworm-resistant roots when a choice is possible. However, when damage to the infested, nontransgenic plant was high, western corn rootworm larvae apparently moved to neighboring transgenic rootworm-resistant plants and caused statistically significant, although only marginally economic, damage on the last sample date in 2001. Implications of these data toward resistance management plan are discussed.     

Sex‐biased oviposition by a nursery pollinator on a gynodioecious host plant: Implications for breeding system evolution and evolution of mutualism

Dioecy, a breeding system where individual plants are exclusively male or female, has evolved repeatedly. Extensive theory describes when dioecy should arise from hermaphroditism, frequently through gynodioecy, where females and hermaphrodites coexist, and when gynodioecy should be stable. Both pollinators and herbivores often prefer the pollen‐bearing sex, with sex‐specific fitness effects that can affect breeding system evolution. Nursery pollination, where adult insects pollinate flowers but their larvae feed on plant reproductive tissues, is a model for understanding mutualism evolution but could also yield insights into plant breeding system evolution. We studied a recently established nursery pollination interaction between native Hadena ectypa moths and introduced gynodioecious Silene vulgaris plants in North America to assess whether oviposition was biased toward females or hermaphrodites, which traits were associated with oviposition, and the effect of oviposition on host plant fitness. Oviposition was hermaphrodite‐biased and associated with deeper flowers and more stems. Sexual dimorphism in flower depth, a trait also associated with oviposition on the native host plant (Silene stellata), explained the hermaphrodite bias. Egg‐receiving plants experienced more fruit predation than plants that received no eggs, but relatively few fruits were lost, and egg receipt did not significantly alter total fruit production at the plant level. Oviposition did not enhance pollination; egg‐receiving flowers usually failed to expand and produce seeds. Together, our results suggest that H. ectypa oviposition does not exert a large fitness cost on host plants, sex‐biased interactions can emerge from preferences developed on a hermaphroditic host species, and new nursery pollination interactions can arise as negative or neutral rather than as mutualistic for the plant.     

Fine‐tuning of defences and counter‐defences in a specialised plant–herbivore system

1. The plant–herbivore arms race has been postulated to be a major driver for generating biological and biochemical diversity on Earth. Herbivore feeding is reduced by the production of chemical and physical barriers, but increases plant resistance against subsequent attack. Accordingly, specialisation is predicted to be an outcome of herbivores being able to circumvent plant‐induced defences. 2. Using a specialised plant–herbivore system, in which adult chrysomelid beetles (Chrysochus auratus) feed on leaves and larvae feed on roots of dogbane (Apocynum spp.), this study investigated whether root latex and cardenolides are effective against the soil‐dwelling larvae, and whether such defences could be circumvented by the herbivore. 3. Across two Apocynum species, C. auratus larvae were not affected by latex production or cardenolide amounts and diversity. By contrast, cardenolide apolarity was detrimental to larval growth. Yet larval feeding decreased average root cardenolide apolarity in A. cannabinum and larvae performed better on those plants. Finally, above‐ground induction rendered the plants more toxic by increasing root cardenolide apolarity and maintaining it, even during subsequent larval herbivory. 4. Therefore, the intimate relationship and interaction between Chrysochus and Apocynum are maintained by a delicate balance of herbivore manipulation and plant chemical induction.     

Preference for outbred host plants and positive effects of inbreeding on egg survival in a specialist herbivore

Inbreeding can profoundly affect the interactions of plants with herbivores as well as with the natural enemies of the herbivores. We studied how plant inbreeding affects herbivore oviposition preference, and whether inbreeding of both plants and herbivores alters the probability of predation or parasitism of herbivore eggs. In a laboratory preference test with the specialist herbivore moth Abrostola asclepiadis and inbred and outbred Vincetoxicum hirundinaria plants, we discovered that herbivores preferred to oviposit on outbred plants. A field experiment with inbred and outbred plants that bore inbred or outbred herbivore eggs revealed that the eggs of the outbred herbivores were more likely to be lost by predation, parasitism or plant hypersensitive responses than inbred eggs. This difference did not lead to differences in the realized fecundity as the number of hatched larvae did not differ between inbred and outbred herbivores. Thus, the strength of inbreeding depression in herbivores decreases when their natural enemies are involved. Plant inbreeding did not alter the attraction of natural enemies of the eggs. We conclude that inbreeding can significantly alter the interactions of plants and herbivores at different life-history stages, and that some of these alterations are mediated by the natural enemies of the herbivores.     

A simple method for reducing moisture condensation on Petri dish lids

Condensation on the lids of Petri dishes, used to culture plant tissues, can often obscure the view of the contents of the dish and interfere with data collection. Under the high humidity conditions that exist in the culture container, a small temperature drop causes water to condense on the inside lid and sides of the container. Mild condensation causes “fogging” while continual or repeated rounds of condensation result in the formation of water droplets. To control condensation in the standard plant tissue culture Petri dish, a simple method was developed whereby the lid of the culture dish was modified, to buffer the lid from temperature fluctuations. Polymer discs, which were the same diameter as the Petri dish lid, were either placed on the top of the lids of existing dishes or surface-sterilized and used in place of the lid. Polymer discs of varying thicknesses and type, and possessing different thermal conductivities, were evaluated for their abilities to reduce the rate of condensation formation. Petri dishes with modified lids were placed under reduced temperature conditions. Condensation, forming on the lids of the dishes was quantified over time using image analysis. Gray value determinations indicated that the thicker polymer discs with the lowest thermal conductivities provided the best protection against condensation. Placement of polymer discs on the top of Petri dishes is a relatively simple method that can be used to buffer the lid from small temperature changes and minimize condensation problems.     

No evidence of modulation of indirect plant resistance of Brassica rapa plants by volatiles from soil-borne fungi

1. Upon herbivory, plants emit specific herbivore-induced plant volatiles (HIPVs) that can attract natural enemies of the herbivore thus serving as indirect plant resistance. Not only insect herbivores, but microorganisms may also affect HIPV emission before or after plant colonisation, which in turn can affect behaviour of natural enemies of the herbivore. Yet, it remains elusive whether volatiles from microorganisms influence HIPV emission and indirect plant resistance.
2. In this study, we investigated whether exposure of Brassica rapa roots to volatiles from soil-borne fungi influence HIPV emission and the recruitment of natural enemies of Pieris brassicae larvae.
3. Using a two-compartment pot system, we performed greenhouse and common-garden experiments, and we profiled plant HIPV emission.
4. We found that exposure of plant roots to fungal volatiles did not affect the number of P. brassicae larvae recollected from the plants, suggesting a neutral effect of the fungal volatiles on natural predation. Likewise, in a greenhouse, similar numbers of larvae were parasitised by Cotesia glomerata wasps on control plants as on fungal volatile-exposed plants. Additionally, chemical analysis of HIPV profiles revealed no qualitative and quantitative differences between control plants and fungal volatile-exposed plants that were both infested with P. brassicae larvae.
5. Together, our data indicate that root exposure to fungal volatiles did not affect indirect plant resistance to an insect herbivore. These findings provide new insight into the influence of indirect plant resistance by fungal volatiles that are discussed together with the effects of fungal volatiles on direct plant resistance.