Differential effects of jasmonic acid treatment of Brassica nigra on the attraction of pollinators, parasitoids, and butterflies

Herbivore-induced plant defences influence the behaviour of herbivores as well as that of their natural enemies. Jasmonic acid is one of the key hormones involved in both these direct and indirect induced defences. Jasmonic acid treatment of plants changes the composition of defence chemicals in the plants, induces volatile emission, and increases the production of extrafloral nectar. However, few studies have addressed the potential influence of induced defences on flower nectar chemistry and pollinator behaviour. These have shown that herbivore damage can affect pollination rates and plant fitness. Here, we have investigated the effect of jasmonic acid treatment on floral nectar production and the attraction of pollinators, as well as the effect on the behaviour of an herbivore and its natural enemy. The study system consisted of black mustard plants, Brassica nigra L. (Brassicaceae), pollinators of Brassica nigra (i.e., honeybees and syrphid flies), a specialist herbivore, Pieris rapae L. (Lepidoptera: Pieridae), and a parasitoid wasp that uses Pieris larvae as hosts, Cotesia glomerata L. (Hymenoptera: Braconidae). We show that different trophic levels are differentially affected by jasmonic acid-induced changes. While the herbivore prefers control leaves over jasmonic acid-treated leaves for oviposition, the parasitoid C. glomerata is more attracted to jasmonic acid-treated plants than to control plants. We did not observe differences in pollinator preference, the rates of flower visitation by honeybees and syrphid flies were similar for control and jasmonic acid-treated plants. Plants treated with jasmonic acid secreted less nectar than control plants and the concentrations of glucose and fructose tended to be lower than in nectar from control plants. Jasmonic acid treatment resulted in a lower nectar production than actual feeding damage by P. rapae caterpillars

Sensory and nutritional effects of amino acids and phenolic plant compounds on the caterpillars of two Pieris species

The relationships between caterpillars of Pierisbrassicae L. and Pierisrapae L. (Lepidoptera: Pieridae) and a common host plant Brassicaoleracea L. were studied using chemosensory and nutritional techniques. Attention was focussed on amino acids, which are in part essential nutrients, and on phenolic and flavonoid derivatives of two aromatic amino acids, that are products of the secondary metabolism in the host plant.An electrophysiological study of amino acid gustation showed that in both species 14 out of 22 amino acids were stimulants to a receptor cell in a maxillary sensillum. The nutritionally essential amino acids were generally stronger stimuli than dispensable ones. A correlation analysis provided indirect evidence that the amino acid receptor possessed four sites, one less specific and three or possibly four specific ones. A comparison of data on free amino acid concentrations in B.oleracea with dose-response relations of the amino acid cell showed that this cell can quantitatively sense foliar amino acids.Phenolic acids and an anthocyanin that naturally occur in B.oleracea elicited neural responses from two to three maxillary gustatory cells. Chlorogenic and protocatechuic acids, both carrying ortho-substituted hydroxyl groups on the aromatic ring, were the most effective stimulants. A steep increase in responsiveness was occurring with increasing concentrations in the range 0.2 - 5.0 mM. P.rapae was the less sensitive of both species. Flavonols were ineffective. The predominant anthocyanin in B.oleracea , cyanin, evoked neural activity in some cells but inhibited the activity in gustatory cells sensitive to sugars, amino acids and glucosinolates in P.brassicae . Chemosensory responsiveness was reflected in preference behaviour. Naturally occurring levels of phenolic acids in B.oleracea as found in phytochemical studies are able to affect sensory processes in the caterpillars (Chapter 3).Assessment of possible metabolic effects of dietary phenolics and some other dietary variations was performed using a flow-through respirometer. This was designed to monitor continuously the gas exchange of feeding caterpillars during the complete final instar. The results of these measurements were compared to the results obtained using standard gravimetric techniques that make use of the measurement of food intake to calculate metabolic efficiency. Respirometric results yielded small effects on the energetic efficiency of growth, which was in contrast to gravimetric results. The causes of the discrepancies between both methods and the consequences of these findings for studies on insect food utilization in general are discussed (Chapter 4).The nutritional utilization of amino acids and nitrogen was studied comparatively for caterpillars of both species on an artificial diet and on B.oleracea . Food consumption in the final instar was lower on the artificial diet. More food was consumed when leaf amino acid content was lower. Relationships were found between food consumption and the absorption efficiencies of most of the essential amino acids. Absorption efficiencies for glycine, cystein and serine were lower on the artificial diet, differences for other amino acids were small between the diets. Amino acid utilization patterns were similar for both species. Balance sheet calculations showed that an extensive conversion from phenylalanine to tyrosine occurred. For both species indications were obtained that tyrosine and cystein may become limiting for growth when dietary protein levels are low (Chapter 5).Phenolic acids (caffeic and chlorogenic acids) and flavonoids (oenin and quercetin-3-rutinoside) inhibited survival, development and growth when larvae of both species were continuously exposed to these compounds present in an artificial diet. P.brassicae was distinctly more sensitive at lower levels of the compounds (0.4 and 1.0 mM). Final instars of both species were much less sensitive than earlier instars. Growth inhibition in final instars was primarily due to reduced food consumption. The results suggest a potential role of phenolic acids and flavonoids, normal constituents of leaves of B.oleracea , in defence against Pieris caterpillars (Chapter 6).Seven cultivars of B.oleracea were offered as food to study their suitability as a host plant for larvae of both caterpillar species. Parameters of larval performance showed differences between cultivars. Highperformance liquid chromatography was used to analyse leaf tissues of five cultivars with respect to concentrations of phenolic acids and flavonoids. Each of the cultivars was found to have its own quantitative pattern of these compounds. Unidentified polar flavonoid components were detected in highly variable amounts. These preliminary results further support a potential role of phenolic and flavonoid compounds in resistance of B.oleracea against Pieris (Chapter 7).</TT

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

Edible insects unlikely to contribute to transmission of coronavirus SARS-CoV-2

In the context of food safety, edible insects are evaluated for biological hazards such as microbial pathogens according to regulations currently in place. When the European Food Safety Authority evaluated the hazards of edible insects as a potential source of pathogenic viruses for humans and livestock, the novel zoonotic coronavirus SARS-CoV-2 had not yet emerged but other pathogenic coronaviruses such as SARS (SARS-CoV) and MERS (MERS-CoV) were known. As a result of the COVID-19 pandemic, animal sources of protein for human consumption are being evaluated for the risks of being a transmission vector of coronaviruses, like SARS-CoV-2. Insects lack a receptor that can bind SARS-CoV-2, thus preventing the virus from replicating in insects, unlike some vertebrate livestock species and companion animals. Despite extensive monitoring, coronaviruses have never been recorded in insect microbiomes. Contamination of insects produced for food or feed may occur during the production process, resulting from rearing substrate or from insect farmers. However, the currently permitted rearing substrates do not include animal products and the farming process is highly automated, thus limiting interactions between farmers and insects. If contamination would still occur, the fact that the insects in production are not hosts to SARS-CoV-2 precludes virus replication and the further processing of the insects will destroy the contamination. We conclude that the hazard of edible insects being a transmission vector of SARS-CoV-2 is extremely low.</p

Herbivore-induced plant volatiles mediate in-flight host discrimination by parasitoids

Herbivore feeding induces plants to emit volatiles that are detectable and reliable cues for foraging parasitoids, which allows them to perform oriented host searching. We investigated whether these plant volatiles play a role in avoiding parasitoid competition by discriminating parasitized from unparasitized hosts in flight. In a wind tunnel set-up, we used mechanically damaged plants treated with regurgitant containing elicitors to simulate and standardize herbivore feeding. The solitary parasitoid Cotesia rubecula discriminated among volatile blends from Brussels sprouts plants treated with regurgitant of unparasitized Pieris rapae or P. brassicae caterpillars over blends emitted by plants treated with regurgitant of parasitized caterpillars. The gregarious Cotesia glomerata discriminated between volatiles induced by regurgitant from parasitized and unparasitized caterpillars of its major host species, P. brassicae. Gas chromatography-mass spectrometry analysis of headspace odors revealed that cabbage plants treated with regurgitant of parasitized P. brassicae caterpillars emitted lower amounts of volatiles than plants treated with unparasitized caterpillars. We demonstrate (1) that parasitoids can detect, in flight, whether their hosts contain competitors, and (2) that plants reduce the production of specific herbivore-induced volatiles after a successful recruitment of their bodyguards. As the induced volatiles bear biosynthetic and ecological costs to plants, downregulation of their production has adaptive value. These findings add a new level of intricacy to plant¿parasitoid interaction