Honeydew analysis for detecting phloem transport of plant natural products : Implications for host-plant resistance to sap-sucking insects

Pyrrolizidine alkaloids and beehive products: A review

Pyrrolizidine alkaloids (PA) are secondary metabolites of plants, which are mostly found in the genus Senecio, Echium, Crotalaria, and Eupatorium. The presence of 1,2-unsaturated PA in foods is a concern to food regulators around the world because these compounds have been associated to acute and chronic toxicity, mainly in the liver. The intake foods with PA/PANO usually occur through accidental ingestion of plants and their derivatives, besides to products of vegetal-animal origin, such as honey. PA/PANO are transferred to honey by their presence in nectar, honeydew, and pollen, which are collected from the flora by bees. In addition to honey, other beekeeping products, such as pollen, royal jelly, propolis, and beeswax, are also vulnerable to PA contamination. In this context, this review provides information about chemical characteristics, regulation, and toxicity, as well as summarizes and critically discusses scientific publications that evaluated PA in honeys, pollens, royal jelly, and propolis.

Phytochemistry-mediated disruption of ant-aphid interactions by root-feeding nematodes

Plants link interactions between aboveground and belowground organisms. Herbivore-induced changes in plant chemistry are hypothesized to impact entire food webs by changing the strength of trophic cascades. Yet, few studies have explored how belowground herbivores affect the behaviors of generalist predators, nor how such changes may act through diverse changes to the plant metabolome. Using a factorial experiment, we tested whether herbivory by root-knot nematodes (Meloidogyne incognita) affected the aboveground interaction among milkweed plants (Asclepias fascicularis or Asclepias speciosa), oleander aphids (Aphis nerii), and aphid-tending ants (Linepithema humile). We quantified the behaviors of aphid-tending ants, and we measured the effects of herbivore treatments on aphid densities and on phytochemistry. Unexpectedly, ants tended aphids primarily on the leaves of uninfected plants, whereas ants tended aphids primarily at the base of the stem of nematode-infected plants. In nematode-infected plants, aphids excreted more sugar per capita in their ant-attracting honeydew. Additionally, although plant chemistry was species-specific, nematode infection generally decreased the richness of plant secondary metabolites while acting as a protein sink in the roots. Path analysis indicated that the ants' behavioral change was driven in part by indirect effects of nematodes acting through changes in plant chemistry. We conclude that belowground herbivores can affect the behaviors of aboveground generalist ant predators by multiple paths, including changes in phytochemistry, which may affect the attractiveness of aphid honeydew rewards.

Excretion of non-metabolized insecticides in honeydew of striped pine scale

Honeydew production is a characteristic of soft scales and other hemipteran insects. Honeydew has the capacity to alter the ecology of predators and parasitoids because it is used as a food resource and can contain insecticidal proteins from hemipteran host plants. We examined honeydew excreted by the striped pine scale (Hemiptera: Coccidae), Toumeyella pini (King), after feeding on pine trees treated with systemic insecticides to determine whether they could eliminate insecticidal compounds in honeydew. Imidacloprid and spirotetramat were applied at labeled rates to soil or foliage. Water sensitive paper was used to measure honeydew production and liquid chromatography coupled to mass spectrometry (LC-MS) to analyze excreted insecticide concentrations. Foliar and soil applications of imidacloprid caused a 25-fold reduction honeydew produced by scales six days after treatment (DAT). In contrast, spirotetramat treatments did not affect honeydew production. Parent compounds of both insecticides were detected in honeydew. However, on imidacloprid treated plants, these compounds were detected at similar concentrations in honeydew collected at 4 DAT from soil and foliar treatments. Imidacloprid was only detected from soil treatments at 8 DAT. Similarly, the spirotetramat parent compound was found 4 DAT after soil and foliar treatments, but only at 8 DAT in foliar treatments. At this time the concentration of spirotetramat in honeydew was six-fold higher than at 4 DAT. We conclude that striped pine scales excrete insecticides in honeydew even when the toxicant greatly reduces honeydew production. Honeydew excretion is thus a mechanism of bioaccumulation and has the potential to harm honeydew-feeding organisms.

Glycosides of the Neurotoxin Tutin in Toxic Honeys Are from Coriaria arborea Phloem Sap, Not Insect Metabolism

Some honeys contain the neurotoxin tutin (1) plus hyenanchin (2), 2-(β-d-glucopyranosyl)tutin (3), and 2-[6'-(α-d-glucopyranosyl)-β-d-glucopyranosyl]tutin (4). These honeys are made by bees collecting honeydew from passionvine hoppers feeding on the sap of tutu plants ( Coriaria spp.). We report a LC-MS study showing that all these picrotoxanes are of plant, not insect, origin. Hyenanchin was barely detectable and the diglucoside was not detectable in C. arborea leaves, but tutu phloem sap contained all four compounds at concentrations up to the highest found in honeydew. It is proposed that the diglucoside may function as a transport form of tutin, analogous to sucrose transport in phloem.

Host-Tree Selection by the Invasive Argentine Ant (Hymenoptera: Formicidae) in Relation to Honeydew-Producing Insects

The Argentine ant, Linepithema humile (Mayr; Hymenoptera: Formicidae), is one of the world's most hazardous invasive species, and thus its eradication from Japan is important. Physical and chemical controls can be expensive and cause strong adverse effects on local terrestrial ecosystems regardless of their high efficacy. Here, presence/absence of host-tree selection by Argentine ants was investigated to understand the ant-honeydew-producing insects interactions in order to develop new cultural controls compatible with biodiversity conservation. Abundance of Argentine ants and their tree utilization ratio was measured among dominant roadside trees (Cinnamomum camphora, Myrica rubra, Nerium indicum, Rhaphiolepis indica var. umbellata, Juniperus chinensis var. kaizuka) in two areas around Kobe, Japan. Almost all ants collected were Argentine ants suggesting that native ants would have been competitively excluded. Tree utilization of Argentine ants clearly differed among host trees. Abundance of both Argentine ants and honeydew-producing insects and tree utilization rate of the ants were significantly lower in especially C. camphora and J. chinensis. Few Argentine ants were observed trailing on C. camphora, J. Chinensis, and N. indicum, most probably due to low abundance of honeydew-producing insects on these trees with the toxic and repellent chemical components. On the other hand, high abundance of both Argentine ants and homopterans were found in M. rubra and especially R. indica. We suggest that reductions of R. indica and M. rubra would lead to a decrease in abundance of honeydew-producing insects, and thus effectively control populations of Argentine ants. At the same time, planting of C. camphora, J. Chinensis, and N. indicum may also play a role in restraint efficacy against invasion of the invasive ants.

Effects of intraspecific and intra-individual differences in plant quality on preference and performance of monophagous aphid species

Plant chemistry is one of the main drivers of herbivore distribution. Monophagous herbivore species are highly specialized, but even within their only host species the chemistry varies. The herbivore's choice is initially mainly guided by volatile plant compounds. Once on the plant, particularly for aphids the phloem quality affects their performance. However, little is known about the intraspecific and intra-individual variation in phloem sap and their influences on monophagous aphids. To determine potential mechanisms involved in aphid colonization, we tested the effects of intraspecific chemical variation in Tanacetum vulgare, which produces different chemotypes, on the preference of two monophagous aphid species. Moreover, we measured the performance of the aphids on different plant parts (stem close to the inflorescence, young and old leaves) of these chemotypes and analyzed their phloem sap composition. Both species preferred the β-thujone (THU) over the trans-carvyl acetate (CAR) chemotype in dual-choice assays. Survival of Macrosiphoniella tanacetaria was neither affected by intraspecific nor intra-individual variation, whereas the reproduction was highest on stems. In contrast, Uroleucon tanaceti survived and reproduced best on old leaves of the preferred chemotype. The sugar, organic acid and amino acid composition pronouncedly differed between phloem exudates of different plant parts, but less between chemotypes. Unexpectedly, high concentrations of amino acids did not necessarily enhance aphid performance. These different performance optima may cause niche differentiation and, therefore, enable co-existence. In conclusion, the tremendous variation in plant chemistry even within one species can affect the distribution of highly specialized aphids at various scales aphid species-specifically.

Mechanisms and evolution of plant resistance to aphids

Aphids are important herbivores of both wild and cultivated plants. Plants rely on unique mechanisms of recognition, signalling and defence to cope with the specialized mode of phloem feeding by aphids. Aspects of the molecular mechanisms underlying aphid-plant interactions are beginning to be understood. Recent advances include the identification of aphid salivary proteins involved in host plant manipulation, and plant receptors involved in aphid recognition. However, a complete picture of aphid-plant interactions requires consideration of the ecological outcome of these mechanisms in nature, and the evolutionary processes that shaped them. Here we identify general patterns of resistance, with a special focus on recognition, phytohormonal signalling, secondary metabolites and induction of plant resistance. We discuss how host specialization can enable aphids to co-opt both the phytohormonal responses and defensive compounds of plants for their own benefit at a local scale. In response, systemically induced resistance in plants is common and often involves targeted responses to specific aphid species or even genotypes. As co-evolutionary adaptation between plants and aphids is ongoing, the stealthy nature of aphid feeding makes both the mechanisms and outcomes of these interactions highly distinct from those of other herbivore-plant interactions.

Plant-derived differences in the composition of aphid honeydew and their effects on colonies of aphid-tending ants

In plant-ant-hemipteran interactions, ants visit plants to consume the honeydew produced by phloem-feeding hemipterans. If genetically based differences in plant phloem chemistry change the chemical composition of hemipteran honeydew, then the plant's genetic constitution could have indirect effects on ants via the hemipterans. If such effects change ant behavior, they could feed back to affect the plant itself. We compared the chemical composition of honeydews produced by Aphis nerii aphid clones on two milkweed congeners, Asclepias curassavica and Asclepias incarnata, and we measured the responses of experimental Linepithema humile ant colonies to these honeydews. The compositions of secondary metabolites, sugars, and amino acids differed significantly in the honeydews from the two plant species. Ant colonies feeding on honeydew derived from A. incarnata recruited in higher numbers to artificial diet, maintained higher queen and worker dry weight, and sustained marginally more workers than ants feeding on honeydew derived from A. curassavica. Ants feeding on honeydew from A. incarnata were also more exploratory in behavioral assays than ants feeding from A. curassavica. Despite performing better when feeding on the A. incarnata honeydew, ant workers marginally preferred honeydew from A. curassavica to honeydew from A. incarnata when given a choice. Our results demonstrate that plant congeners can exert strong indirect effects on ant colonies by means of plant-species-specific differences in aphid honeydew chemistry. Moreover, these effects changed ant behavior and thus could feed back to affect plant performance in the field.

Phloem sap collection from lettuce (Lactuca sativa L.): Chemical comparison among collection methods

The chemical composition of phloem sap from lettuce, collected by three different methods, was compared. Phloem sap from stylectomy samples contained sucrose and 14 amino acids. Honeydew and EDTA chelation samples showed considerable breakdown of sucrose into fructose and glucose, several additional amino acids, and large differences in relative concentrations of amino acids, when compared to stylectomy samples. Honeydew contained considerable amounts of other oligosaccharides, and few proteins in low amounts, while EDTA showed many proteins. HPLC chromatograms showed numerous unidentified secondary plant compounds in honeydew and EDTA samples. Comparison of phloem sap samples from near-isogenic susceptible and resistant lines showed no relation of phloem sap composition with monogenic resistance to the aphidNasonovia ribisnigri.

Within-plant variation in concentrations of amino acids, sugar, and sinigrin in phloem sap of black mustard,Brassica nigra (L.) Koch (Cruciferae)

Although within-plant variation in the nutrient and allelochemical composition of phloem sap has been invoked to explain patterns of host use by phloem-feeding insects, little is known about within-plant variation in phloem chemistry. Here I describe a new technique in which I use the green peach aphid,Myzus persicae Sulz., to investigate within-plant variation in the concentrations of chemicals in the phloem sap of black mustard,Brassica nigra (L.) Koch (Cruciferae). Relationships between the concentrations of chemicals in aphid diets and honeydew were established using honeydew from aphids fed on artificial diets with known concentrations of amino acids, sucrose, and sinigrin. These relationships were applied to honeydew from aphids fed on different aged leaves of black mustard to estimate the concentrations of the chemicals in phloem sap. Sinigrin concentration was estimated to be high (>10 mM) in phloem sap in young leaves, calling into question the prevailing opinion that phloem sap contains only low concentrations of allelochemicals. High concentrations may function as defenses against sap-feeding herbivores. Within-plant variation in phloem sap composition was high: (1) young leaves had high concentrations of nutrients (216 mM amino acids, 26% sugar) and sinigrin (>10 mM); (2) mature and presenescent leaves had lower concentrations of nutrients (77-83 mM amino acids, 19-20% sugar) and low concentrations of sinigrin (1-2 mM); and (3) senescing leaves had high concentrations of nutrients (199 mM amino acids, 25% sugar) and low concentrations of sinigrin (3 mM).

Secondary metabolites from the phloem of Piper solmsianum (Piperaceae) in the honeydew of Edessa meditabunda

INTRODUCTION

The phytochemistry of species of the genus Piper has been studied extensively, including Piper solmsianum. However, no studies have addressed the phytochemistry of the sap content of Piper species.

OBJECTIVE

To evaluate the transferring of secondary compounds from the saps of P. solmsianum to the honeydew of Edessa meditabunda.

METHODOLOGY

The honeydew of E. meditabunda and saps of P. solmsianum were analysed by GC-MS, (1) H-NMR and LC-MS.

RESULTS

The lignan (-)-grandisin and the phenylpropanoid (E)-isoelemicin were detected in both saps of P. solmsianum and honeydew of E. meditabunda.

CONCLUSION

Analysis of honeydew secreted by the sap-sucking insect E. meditabunda indicated that (-)-grandisin and (E)-isoelemicin are absorbed from the phloem of Piper solmsianum.

Phloem alkaloid tolerance allows feeding on resistant Lupinus angustifolius by the aphid Myzus persicae

We examined the hypothesis that the polyphagous green peach aphid (Myzus persicae) shows clone-specific adaptation to the narrow-leafed lupin (Lupinus angustifolius) containing toxic quinolizidine alkaloids. We compared the performance of a lupin-feeding clone of M. persicae from Western Australia to that of nine clones of the same species collected from eastern Australian locations, where narrow-leafed lupins rarely occur. Mean relative growth rate (MRGR) and colonization ability varied among the M. persicae clones on one aphid-susceptible and two aphid-resistant lupin varieties. The performance of the lupin-feeding clone was better than that of all other clones on the resistant narrow-leafed lupin varieties "Tanjil" and "Kalya", indicating that successful lupin feeding is not a characteristic of the species. Gas chromatography-mass spectrometry analyses (GC-MS) of phloem from the different lupin varieties detected differences in the quantities of two alkaloid compounds identified as 13-OH-lupanine and lupanine. The lupin-feeding M. persicae clone also showed better performance on artificial diet amended with lupanine. The results suggest that the M. persicae clone collected from Western Australia is adapted to feed successfully on narrow-leafed lupin, and that this adaptation may involve improved tolerance of lupanine in its diet.

Effects of 1,4-benzoxazin-3-one derivatives from maize on survival and fecundity of Metopolophium dirhodum (Walker) on artificial diet

Effects of 2-beta-D-glucopyranosyloxy-4-hydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA-Glc) and DIMBOA-Glc N-O-methylated (HDMBOA-Glc), two compounds present in high concentration in maize, were tested on the aphid Metopolophium dirhodum reared on artificial diet. HDMBOA-Glc and DIMBOA-Glc decrease survival of adults with an LD50 of I mM and 5.6 mM, respectively, after 72 hr of feeding. These compounds also decrease the fecundity of the aphids at concentrations of 2 mM and 1 mM, respectively. At concentrations of 2 mM HDMBOA-Glc and 8 mM DIMBOA-Glc in the diet, the average lifetime fecundity of 10 females is near zero. Offspring mortality on diet with 2 mM DIMBOA-Glc is significantly higher than with the control diet. In contrast, HDMBOA-Glc has no effect on the survival of offspring. The possibility that these compounds protect Poaceae against aphids is discussed.

[Presence of metacyclical promastigotes of Leishmania pifanoi in the hypopharynx of Lutzomyia youngi and their sugar consumption]

The presence of metacyclical promastigotes of Leishmania pifanoi in the hypopharyngeal duct of Lutzomyia youngi is reported. The insects were experimentally infected by engorgement on the tarsal lesions of hamsters. The metacyclics, whose morphology is illustrated, appeared in the hypopharynx 5 to 9 days after engorgement; they were more frequently found in the insects fed on unrefined commercial sugar. They role of amino derivates of glucose and galactose, as well as of amino acids in the development and migration of metacyclics, is investigated.

The evolution of soldiers in aphids

1. Defensive individuals, termed soldiers, have recently been discovered in aphids, Soldiers are typically early instar larvae, and in many species the soldiers are reproductively sterile and morphologically and behaviourally specialized. 2. Since aphids reproduce parthenogenetically, we might expect soldier production to be more widespread in aphids than it is. We suggest that a more useful way to think about these problems is to attempt to understand how a clone (rather than an individual) should invest in defence and reproduction. 3. Known soldiers are currently restricted to two families of aphids, the Pemphigidae and Hormaphididae, although they are distributed widely among genera within these families. We discuss the use of a phylogenetic perspective to aid comparative studies of soldier production and we demonstrate this approach using current estimates of phylogenetic affinities among aphids. We show that the distribution of soldier production requires a minimum of six to nine evolutionary origins plus at least one loss. 4. At least four main types of soldiers exist and we present and discuss this diversity of soldiers. 5. Most soldier-producing species produce soldiers within plant galls and we discuss the importance of galls for the evolution of soldiers. 6. We summarize the evidence on the interactions between soldiers and predators and between soldier-producing aphids and ants. 7. We present an optimality model for soldier investment strategies to help guide investigations of the ecological factors selecting for soldiers. 8. The proximate mechanisms of soldier production are currently very poorly understood and we suggest several avenues for further research.