There is increasing evidence that chemical cues play a pivotal role in host selection by the natural enemies of aphids. We use Vinson's (1976) division of the host selection process into habitat location, host location and host acceptance for both parasitoids and predators and review what is known about the role of semiochemicals in aphid selection by natural enemies. For habitat location (i.e. detection of the host plant), volatiles emitted by plants after aphid attack have been described for a number of plant-aphid interactions. These synomones indicate not only the presence of an aphid host plant to the predator or parasitoid, but also the presence of aphids. Volatiles emitted from undamaged host plants are often attractive to aphid parasitoids, but less so for predators. Host location by the natural enemy on the food plant is guided by semiochemicals that mostly originate from the aphids, in particular aphid alarm pheromone, honeydew, or the smell of the aphid itself. Host acceptance is guided by contact chemicals for both predators and parasitoids. In parasitoids, host recognition may be based on visual cues or on contact chemicals on the aphid's cuticle, whereas host acceptance is ultimately based on as yet unknown substances within the aphid's hemolymph. While it appears that many predators and parasitoids are attracted to the same semiochemicals, synergistic and antagonistic interactions among chemical substances have only rarely been investigated. More research into model systems is needed, not only to identify important semiochemicals, but also to determine their range of attraction. Recent progress in the development of analytical techniques has created new opportunities to improve our understanding of the chemical ecology of aphid-natural enemy interactions in the coming years.
Chemical cues involved in both host location and oviposition by Harmonia axyridis Pallas (Coleoptera: Coccinellidae) were investigated in laboratory and field experiments. Among the five volatiles tested in a four-arm olfactometer ((E)-β-farnesene, β-pinene, β-caryophyllene, cis-3-hexen-1-ol, and limonene), gravid H. axyridis females were significantly attracted to limonene and β-caryophyllene. These two chemicals also increased H. axyridis oviposition on plants. Attraction of H. axyridis towards limonene was further tested under field conditions using controlled-release dispensers. (E)-β-farnesene, a major aphid alarm pheromone and a potential predator kaironome, was also tested in this study. Field data confirmed those from laboratory studies concerning the attraction of H. axyridis towards limonene and a lack of response to (E)-β-farnesene. In contrast, the aphid predator Episyrphus balteatus DeGeer (Diptera: Syrphidae) was attracted to (E)-β-farnesene but not to limonene. These findings indicate that H. axyridis uses chemicals, such as limonene, for prey location and oviposition, and support the potential use of volatiles in the management of H. axyridis.
The performance of the aphid-specific fungal pathogen Pandora neoaphidis was studied in relation to changes in herbivore resources for the pea aphid, Acyrthosiphon pisum, on different host plant species. Dose-response bioassays were conducted with A. pisum which had been reared on dwarf bean then inoculated with P. neoaphidis and returned to dwarf bean or inoculated and transferred to field bean, pea or lucerne. The smallest estimated median lethal concentration (LC50) was 7.7 conidia mm-2 (95% confidence interval 5.4-11.2) for aphids returned to dwarf bean, with LC50s of 13.0 (9.2-19.1) and 14.6 (10.2-21.5) conidia mm-2 for aphids transferred to field bean or pea, respectively. The LC50 when aphids were transferred to lucerne [2941.0 conidia mm-2 (237.3-2.1x109)] was greater than for the other three plants. In a subsequent experiment, A. pisum were reared on pea as well as dwarf bean for four generations before bioassays. The LC50 was 7.3 conidia mm-2 (4.4-12.4) for aphids reared and incubated on dwarf bean, compared to 13.3 (8.0-23.9) and 15.3 (8.8-29.9) conidia mm-2 when aphids were transferred between dwarf bean and pea, and vice versa, respectively. The LC50 for aphids reared then incubated on pea plants was 27.9 (15.8-57.3) conidia mm-2. Hence, the virulence of P. neoaphidis, measured by LC50, was greatest when A. pisum was reared and maintained on dwarf bean, the plant used for long-term routine culturing of the aphid in our facilities. In conclusion, virulence of P. neoaphidis was greater on plant species to which A. pisum had become adapted during long-term laboratory rearing. Plant resources may affect infection by P. neoaphidis and the fungal entomopathogen will have a greater impact on aphid herbivores which are not suffering physiological stress related to a change in host plant.
1_The maritime pine bast scale, Matsucoccus feytaudi Ducasse (Hemiptera: Matsucoccidae), occurs in the western part of the Mediterranean basin and is a sap sucking insect that feeds only on maritime pine (Pinus pinaster Aiton). It causes damage in SE France and Italy, where it was accidentally introduced. In Spain information is scarce and, moreover, almost nothing is known about the predators of this species. This study was designed to determine the seasonal trends in abundance of M. feytaudi and its major predators, which might help to improve the biological control of this pest in other areas. Natural P. pinaster stands in the Valencian Community (Spain) were surveyed in 2004. In addition, the seasonal trends in abundance of M. feytaudi and its natural enemies were monitored in three stands over a period of three years (2002, 2005 and 2006). The monitoring was carried by wrapping sticky tapes around tree trunks and using delta traps baited with sexual pheromone. The maritime pine bast scale was detected in all the stands surveyed. At almost all the sites surveyed, three species of predators were captured: Elatophilus nigricornis Zetterstedt (Hemiptera: Anthocoridae), Hemerobius stigma Stephens (Neuroptera: Hemerobiidae) and Malachiomimus pectinatus (Kiesenwetter) (Coleoptera: Malachiidae). The presence of M. pectinatus is noteworthy as this is the first record of this species as a possible predator of M. feytaudi. The results show that M. feytaudi, although differing in its phenology depending on the location, is univoltine in the study area. The prepupae, pupae and adults of M. feytaudi appeared between December and March in colder areas and between October and February in warmer areas. E. nigricornis nymphs are important predators of M. feytaudi, and were abundant when the scale insect (crawlers, prepupae, pupae, male and female adults) was present., 2_The flight period of E. nigricornis and the hemerobiid H. stigma ranged from May to October. However, these flight patterns did not correlate with the presence of the different stages of the bast scale (crawlers, prepupae, pupae, male and female adults) on the surface of tree trunks. The presence of M. pectinatus in large numbers in some stands suggests it might be an important natural regulator, which helps to keep M. feytaudi populations at low densities in the areas of Spain studied. This malachiid shows a strong kairomonal attraction to the sexual pheromone of M. feytaudi and its flight activity is significantly correlated with the presence of crawlers of bast scale., Eugenia Rodrigo ... [et al.]., and Obsahuje seznam literatury
Phorids parasitizing Coccinella septempunctata, the seven spot ladybird, attend host prepupae and parasitize them at the point of ecdysis to the pupal stage. In this system, oviposition rates would be maximized through the choice of older pre-pupal hosts in preference to young ones. Field study revealed that old pre-pupal hosts were indeed more likely to be attended by phorids than young pre-pupae. We tested the hypothesis that this was due to a preference by simultaneously offering phorids an old and a young prepupal host in a choice test. The results suggest that phorids do indeed distinguish between host prepupae on the basis of age, choosing the older prepupa.