Paolo Fanti

Development and nutrition of the braconid wasp, Aphidius ervi in aposymbiotic host aphids

This study aims at evaluating whether the bacterial endosymbionts of the pea aphid, Acyrthosiphon pisum (Harris), may be of any nutritional importance to the endophagous braconid parasitoid Aphidius ervi Haliday. Aposymbiotic aphids, obtained by rifampicin treatment on artificial diet, were parasitized, and both parasitoid development and major changes in the nitrogen-derived nutrients in the host hemolymph were observed. A. ervi larvae developing in aposymbiotic pea aphids showed a significant delay in reaching the adult stage and the emerging adults showed a 50% weight reduction compared to those emerging from symbiotic aphids. In the hemolymph of parasitized symbiotic aphids, parasitism-specific proteins of approximately 15 and 45 kD were detected, and their level increased on day 5 and 6 after parasitoid oviposition. At that stage, a significant increase in the free amino acid level was observed, with glutamine showing the highest relative abundance. In contrast, the hemolymph of aposymbiotic aphids did not contain the two parasitism-specific proteins and no increase in free amino acids was observed. The results indicate that the bacterial endosymbionts in parasitized pea aphids are of pivotal importance for the development of A. ervi. Aposymbiotic aphids have a lower nutritional suitability, showing a significant reduction in the hemolymph of nitrogen-derived compounds of nutritional importance to the developing parasitoid larva. The possible role of Buchnera in nitrogen recycling is also discussed as a possible strategy for optimizing the utilization of dietary nitrogen by the parasitoid larva. Arch. Insect Biochem. Physiol. 40:53–63, 1999.

Functional bases of host-acceptance behaviour in the aphid parasitoid Aphidius ervi

Abstract The host acceptance behaviour in Aphidius ervi is investigated, assessing the role of both external and internal host‐associated cues, offered to the experimental parasitoids with parafilm‐made aphid dummies. The reaction to internal cues present in the host haemolymph is clearly evident, and its intensity is enhanced by external cues. Parasitoid females lay few eggs in aphid dummies filled with host haemolymph. A significant increase in the number of both oviposition reactions (host stinging) and egg laying is observed only when these dummies are coated with cornicle secretion. However, this enhancement is not observed when the aphid dummies contain distilled water. Thus, the host acceptance behaviour of A. ervi females appears to be controlled by the integration of both external and internal chemical cues. The physiological basis of this behavioural response is investigated with a detailed study on the anatomy and ultrastructure of A. ervi ovipositor. The detection of chemical cues present in the host haemolymph that act as kairomones is made possible by the presence of gustatory sensilla on the tip of the ovipositor. These sensilla consist of porous areas, reached by unbranched dendrites running inside both the lower valves (i.e. first valvulae) and the unpaired upper valve (i.e. second valvulae). The mechanosensory function during oviposition appears to be provided mostly by the basiconic sensilla found on the tip of external valves (i.e. third valvulae). A tentative functional model accounting for the observed oviposition behaviour of A. ervi is proposed.

The expression in tobacco plants of Aedes aegypti Trypsin Modulating Oostatic Factor (Aea-TMOF) alters growth and development of the tobacco budworm, Heliothis virescens

The production and characterisation of transgenic tobacco plantsexpressing a precursor of a regulatory peptide from Aedesaegypti (Trypsin Modulating and Oostatic Factor, Aea-TMOF) whichinterferes with the development of tobacco budworm larvae is described. Tobaccoplants were transformed with a synthetic gene containing 6 TMOF units spaced bydibasic residues, Arg-Arg, as potential post-translational cleavage sites.Peptide extracts from transgenic plants had TMOF activity and inhibitedin vitro the biosynthesis of serine proteases. Thisactivity was consistently present in T1 plants and absent in control plants.Tobacco budworm larvae, fed with transgenic leaves showed a reduced growth ratecompared to those fed with control plants. The low rather than acute toxicityofthis low impact gene is discussed in the context of alternative integrated pestmanagement strategies.

An insect peptide engineered into the tomato prosystemin gene is released in transgenic tobacco plants and exerts biological activity.

Tomato systemin is a signalling peptide produced in response to wounding that locally and systemically activates several defence genes. The peptide is released from the C-terminus of prosystemin, the 200 amino acid precursor, following post-translational modifications involving unknown events and enzymes. In tobacco, two systemin molecules have been recently isolated, neither sharing any sequence homologies with the tomato prosystemin gene/protein, but performing similar functions. We modified the tomato prosystemin gene by replacing the systemin-encoding region with a synthetic sequence encoding TMOF (trypsin-modulating oostatic factor), a 10 amino acid insect peptide hormone toxic to Heliothis virescens larvae, and expressed the chimeric gene in tobacco. The results reported here show that transformed leaves contain the TMOF peptide and exert toxic activity against insect larvae reared on them. In addition, subcellular localization studies showed the cytoplasmic location of the released TMOF, suggesting that in tobacco the enzymes responsible for the post-translational modifications of the tomato precursor protein are present and act in the cytoplasm to recognise the modified prohormone. The molecular engineering of the precursor, beside supplying new clues towards the understanding of prosystemin processing, constitutes an useful tool for plant genetic manipulation, by enabling the delivery of short biological active peptides.

A viral chitinase enhances oral activity of TMOF.

In this study we investigate the combined effect on Heliothis virescens (Lepidoptera, Noctuidae) larvae of Aedes aegypti-Trypsin Modulating Oostatic Factor (Aea-TMOF), a peptide that inhibits trypsin synthesis by the gut, impairing insect digestive function, and Autographa californica nucleopolyhedrovirus Chitinase A (AcMNPV ChiA), an enzyme that is able to alter the permeability of the peritrophic membrane (PM). Aea-TMOF and AcMNPV ChiA were provided to the larvae by administering transgenic tobacco plants, co-expressing both molecules. Experimental larvae feeding on these plants, compared to those alimented on plants expressing only one of the two molecules considered, showed significantly stronger negative effects on growth rate, developmental time and mortality. The impact of AcMNPV ChiA on the PM of H. virescens larvae, measured as increased permeability to molecules, was evident after five days of feeding on transgenic plants expressing ChiA. This result was confirmed by in vitro treatment of PM with recombinant ChiA, extracted from the transgenic plants used for the feeding experiments. Collectively, these data indicate the occurrence of a positive interaction between the two transgenes concurrently expressed in the same plant. The hydrolytic activity of ChiA on the PM of tobacco budworm larvae enhances the permeation of TMOF molecules to the ectoperitrophic space, and its subsequent absorption. The permeation through the paracellular route of Aea-TMOF resulted in a spotted accumulation on the basolateral domain of enterocytes, which suggests the occurrence of a receptor on the gut side facing the haemocoel. The binding of the peptide, permeating at increased rates due to the ChiA activity, is considered responsible for the enhanced insecticide activity of the transgenic plants expressing both molecules. These data corroborate the idea that ChiA can be effectively used as gut permeation enhancer in oral delivery strategies of bioinsecticides targeting haemocoelic receptors.

An ‘artificial aphid’ for Aphidius ervi (Hym., Braconidae)

Abstract:  An artificial aphid was developed by making parafilm® bubbles filled with aphid haemolymph and its suitability for parasitoid oviposition was tested. Aphidius ervi accepted the parafilm® aphid and laid eggs on it. By stretching, the thickness of the parafilm could be changed. The thicker the parafilm, the higher was the mean number of ovipositional stings and the lower the mean number of eggs released by A. ervi.

Tomato belowground–aboveground interactions: Rhizophagus irregularis affects foraging behavior and life history traits of the predator Macrolophus pygmaeus (Hemiptera: Miridae)

In recent years, studies on arbuscular mycorrhizal fungi (AMF) have been revealing that the belowground symbiosis can influence the performance of aboveground herbivores and their natural enemies through its effects on the host plant. In this study, we tested whether the colonization of tomato plants by the arbuscular mycorrhizal fungus Rhizophagus irregularis (Syn. Glomus intraradices Schenk and Smith) (Glomeromycota: Glomeraceae) affects the performance of the zoophytophagous mirid bug Macrolophus pygmaeus Rambur (Hemiptera: Miridae). Mycorrhizal colonization in tomato plants positively influenced the predator host-plant acceptance for feeding and oviposition, as well as nymphal survival and female weight. We hypothesize that AMF can modify mirid bug foraging behavior and performance.

The aphid Acyrthosiphon pisum exhibits a greater survival after a heat shock when parasitized by the wasp Aphidius ervi

The pea aphid Acyrthosiphon pisum is a common pest of many species of legumes and its parasitoid Aphidius ervi is regarded as a successful biocontrol agent. In this study, we report a greater survival rate of parasitized aphids compared with unparasitized ones, after exposure to a very high temperature (39°C for 30min). After the heat shock, the survival of unparasitized aphids decreases according to their age at the heat shock treatment, suggesting a different adaptation of the aphid life stage to the different microclimatic conditions they experience. Survival of parasitized aphids does not change according to the time of the heat shock treatment, but it is always significantly higher compared with the unparasitized ones. Parasitized aphids are very quickly subjected to a wide range of physiological modifications and the observed increased survival could be a consequence of these modifications before the heat shock treatment. The possible explanations as well as the possible adaptive nature of the observed phenomenon are discussed.

Aphids: Biology, Ecology, and Management

Abstract Past studies have identified 18 species of aphids attacking tomato in open-field agriculture and greenhouses. However an in-depth review of the literature reveals only two species, Macrosiphum euphorbiae (Thomas) and Myzus persicae (Sulzer), as frequent and common aphid pests of tomato throughout the world. In this chapter, we review the identification, general biology, distribution, host range, and seasonal occurrence of these two aphids and discuss how they damage plants. We present research that has attempted to develop sampling strategies and devise economic thresholds. Furthermore, we review the history and current status of strategies used to manage M. euphorbiae and M. persicae. These strategies are presented under the broad headings of chemical control, biological control, host-plant resistance, cultural control, and others. With the wealth of information contained in this chapter, the reader will gain valuable insight into where we have been, and where we are likely to proceed in dealing with aphids on tomato.