P. Scutareanu

Volatiles from Psylla-infested pear trees and their possible involvement in attraction of anthocorid predators

Previous work showed that anthocorid predators aggregate around gauze cages containing Psylla-infested trees in a pear orchard. Because anthocorids responded to odor from Psylla-infested leaves in a laboratory test, it was hypothesized that these aggregative responses in the field were triggered by olfaction of compounds associated with Psylla injury. We present chemical analyses of volatiles from damaged and undamaged plants and studies on behavioral responses of anthocorid predators to compounds released by damaged plants. Leaf headspace volatiles from clean and Psylla-infested pear trees were collected on Tenax and identified by GC-MS after thermodesorption. Twelve volatiles were found exclusively in headspace samples from Psylla-infested leaves. Six were present in significantly higher quantities in samples from infested leaves: the monoterpene, (E,E)-α-farnesene, the phenolic, methyl salicylate, and the green leaf compounds, (Z)-3-hexen-1-yl acetate, (Z)-3-hexen-1-ol, 1-hexyl-acetate, and 1-penten-3-ol. These compounds are known to be produced by plants, and damage by pear psyllids seems to trigger their emission. Blend composition varied and was partly correlated with tree or leaf age and degree of Psylla infestation. To study whether compounds associated with leaf injury elicit olfactory responses in anthocorid predators, apple-extracted (E,E)-α-farnesene, synthetic methyl salicylate, and (Z)-3-hexen-1-yl acetate were offered in a Y-tube olfactometer to field-collected adult Anthocoris spp. Significant positive responses were found to both the monoterpene and the phenolic, but not to the green leaf volatile. The results lend support to the hypothesis that predator attraction to herbivore-infested pear trees is mediated by herbivory-induced plant volatiles.

Do anthocorid predators respond to synomones from Psylla-infested pear trees under field conditions?

Because Y‐tube olfactometer experiments in the laboratory showed a response of anthocorid bugs to odour from Psylla‐infested leaves, it was of interest to assess its relevance under field circumstances. This was done by measuring the density of predatory bugs on pear trees adjacent to Psylla‐infested or control trees that were covered with fine mesh gauze‐screens. In this way odours from these caged trees could spread through the screen, while contact with the Psylla prey in the cage was prevented. The density of anthocorid predators around cages with heavily infested trees was significantly higher than around uncaged control trees and around cages containing uninfested or little infested trees. Covering a cage with Psylla‐infested trees by an airtight plastic sheet led to an immediate drop in the density of anthocorid predators, whereas removal of the sheet led to predator aggregation again. The results of these field experiments strongly support the hypothesis that anthocorid predators respond to volatile chemicals emanating from Psylla‐infested pear trees.

Constitutive and herbivore-induced volatiles in pear, alder and hawthorn trees

Summary. Qualitative and quantitative differences among pear cultivars were found in constitutive and Cacopsylla-induced volatiles, depending on experimental treatment of the trees (i.e., uninfested and partly or completely infested by psyllids). Blend differences were also found between pear cultivars and wild-type pear, alder and hawthorn–the latter trees are frequently present in pear orchard hedgerows. ¶Interesting differences were found in the presence of methyl salicylate and (E,E)-α-farnesene, two compounds previously found to mediate attraction of predatory bugs towards psyllid-infested pear trees. Methyl salicylate is expressed constitutively and is induced systemically by infestation in the whole plant of all four cultivars. (E,E)-α-farnesene on the other hand showed also systemic induction in Bartlett, NY10355 and Beurré Hardy, but in partially infested Conference trees it was induced locally, only in herbivore-damaged leaves. No methyl salicylate or (E,E)-α-farnesene were identified in honeydew. In field collected headspace samples of alder leaves infested by aphids and leaf beetles we found methyl salicylate but no (E,E)-α-farnesene, whereas in uninfested hawthorn neither were identified. Insight in the variability of damage-related pear volatiles will have important implications for integrated pest management in the field.

Interactions between arthropod predators and plants: A conspiracy against herbivorous arthropods?

Plants provide protection (domatia), alternative food (nectar, exudates, pollen) and chemical lures which benefit predatory arthropods and thereby protect plants against herbivorous arthropods. Experimental evidence for these indirect effects is reviewed and hypotheses are provided to explain why plants invest in attracting, feeding and protecting predatory arthropods despite the fact that (1) other organisms not beneficial to the plant may utilize these facilities too, and (2) also competing neighbour-plants may profit from these investments. It is argued that although the plant may benefit under certain conditions, plant-bodyguard interactions do not have a single anticipated outcome. Instead, these interactions may have a range of positive, neutral and negative outcomes, the exact value of which depends on position in space and moment in time. The benefits to the plant investing in indirect defence depend on the availability and responsiveness of predatory arthropods, the abundance and responses of cheaters, the degree to which neighbouring plants profit from these investments, and the extent to which other plants in the environment invest or harbour profitable prey. Possibilities for indirect plant defence to arise from coevolution are discussed.

Cross‐correlation analysis of fluctuations in local populations of pear psyllids and anthocorid bugs

1. To test whether predatory anthocorids migrate into pear orchards when populations of pear psyllids are building up, a cross‐correlation analysis was carried out on their population numbers. Predator and prey population sizes were assessed weekly in 3 consecutive years (1991–93) by sampling pear leaves for eggs and nymphs of psyllids and pear tree branches for adult psyllids, as well as adults and nymphs of predatory anthocorids. The time‐series consisted of numbers (per leaf or branch) averaged over preselected pear trees in an orchard and, in addition, over other trees selected along the hedgerows flanking the orchard.

Leaf volatiles and polyphenols in pear trees infested by Psylla pyricola. Evidence of simultaneously induced responses

SummaryFeeding by the homopteranPsylla pyricola on leaves of pear trees induces the production of volatile compounds, such as (E,E)-α-farnesene and methyl-salicylate, as well as the production of polyphenols. The inference on induction is based on GC-MS and HPLC chromatograms from the same samples ofPsylla infested leaves, leaves from the same pear tree beforePsylla infestation and uninfested leaves from other pear trees.Psylla infestation greatly enhanced the production of volatiles ((E,E)-α-farnesene, methyl-salicylate and others) and triggered the production of new polyphenols, characterized by much longer retention times.However, the responses to infestation depend critically on leaf age (defined by leaf distance to apex). With respect to the leaf volatiles it appears that infested, old leaves produce fewer compounds and lower amounts of the volatiles than infested, young leaves. Moreover, there seem to be differences in pattern. Relative to (E,E)-α-farnesene, methyl-salicylate was found in much lower amounts in heavily infested, old leaves. With respect to polyphenols it was found that infested old leaves collected in August have polyphenols with the same retention times, but more or less equal amounts as uninfested young leaves collected in May. This shows thatPsylla infestation causes the induced response mostly in young leaves.The induced leaf volatiles may act as synomones to heteropteran bugs. As shown elsewhere,Anthocoris nemoralis responds significantly to (E,E)-α-farnesene and methyl-salicylate when offered in pure form against clean air in a Y-tube olfactometer. The effect of polyphenols on the performance ofP. pyricola is not yet known. Hence, a role in direct defence is still to be investigated.

INDUCTION OF A p-COUMAROYL TRIHYDROXY TRITERPENE ACID IN Psylla-INFESTED AND MECHANICALLY DAMAGED PEAR TREES

The pattern of induction and the chemical structure of phenolic compounds in pear trees (Pyrus communis, cv. Conference) that were either infested by pear leaf suckers Psylla pyricola and P. pyri or mechanically damaged, or both, were studied. Chromatographic (HPLC) and mass spectral analysis performed on extracts of leaf samples collected at various time intervals from trees subjected to three treatments demonstrated the induction (and/or amplification) of a phenolic compound, identified as 3-O-trans-p-coumaroyltormentic acid (I). New mass spectrometric data on this phenolic compound are presented. HPLC revealed different peak patterns in the course of the period of Psylla infestation and the lapse of time since mechanical damage was inflicted, compared to a control tree. The new phenolic compound became apparent after 12 hr and reached the highest level 30 days after damage by pear leaf suckers. It was also observed after 24 hr at lower intensity in samples from a mechanically damaged tree and exclusively on day 30 at very low intensity in the leaf extracts from the uninfested control trees. We conclude that damage by pear leaf suckers, and to a lesser extent also mechanical damage, induce the synthesis of the new, late-eluting phenolic compound. We propose that this compound is involved in plant defense against pear leaf suckers.

Ratio of Nutrient and Minerals to Defensive Compounds Indicative of Plant Quality and Tolerance to Herbivory in Pear Trees

ABSTRACT The phenolic ester 3-0-trans-p-coumaroyltormentic acid and the volatile compounds (E,E)-α-farnesene and methyl salicylate have been identified previously in pear tree cultivars ‘Bartlett,’ ‘NY10355,’ and ‘Conference.’ Here, using the same dried extract of leaf samples, the contents of total protein, chlorophyll, and the minerals calcium (Ca), magnesium (Mg), phosphorus (P), potassium (K), sodium (Na), carbon (C), and nitrogen (N) were determined. Differences between pear cultivars were found with regard to their absolute content in Ca, K, chlorophyll, and total protein, and to carbon/nitrogen values in uninfested leaves compared with leaves infested by Cacopsylla pyricola (Foester) and C. pyri L. (Homoptera, Psyllidae) on partially and completely infested trees. The ratio of nutrients and minerals to the constitutive and Cacopsylla-induced phenolic and two volatiles reflects a diminution of the leaf quality as food for Cacopsylla nymphs in infested leaves of all cultivars. In uninfested leaves, the magnitude of this ratio in partially infested trees of cv. ‘Conference’ with local response, together with Ca ion content and chlorophyll loss, point to classification of this cultivar as tolerant to herbivory.