Jean-Claude Grégoire

Research on Parasitoids and Predators of Scolytidae – A Review

Scolytidae are major forest pests in Europe. For example, Ips typographus (L.) is considered the main pest problem in forestry in many central and northern European countries. Consequently, there is a long tradition of forest entomology studying various aspects of bark beetle ecology, including their natural enemy complexes, with a view to developing control methods. In recent years, the need for the development of sustainable pest management methods, taking into account the whole forest ecosystem, has enhanced the interest in natural mortality factors. Mills (1983) provided an extensive review of the natural enemies of conifer feeding bark beetles in Europe. In contrast, natural enemies of broadleaf-feeding species have never been reviewed. Furthermore, much research has been carried out in the last 20 years, in particular in fields such as host/prey location or tritrophic interactions. Other noteworthy reviews include two books by Hedqvist (1963; 1998) on chalcid and braconid parasitoids of Scolytidae in Sweden, and a publication by Nuorteva (1957) on parasitoids of bark beetles in Finland. Data are also available in general parasitoid and predator catalogues such as Thompson (1943), Herting (1973), and Noyes (2001), although these often repeat errors contained in primary publications. This review will focus primarily on parasitoids and predators of bark beetle species considered to be pests of living trees in Europe, although it may also consider relevant research in other continents. A list of these European species is given in chapter 1. Pathogens of scolytids are reviewed in another chapter (Wegensteiner, chapter 12).

Population dynamics in changing environments: the case of an eruptive forest pest species.

In recent decades we have seen rapid and co‐occurring changes in landscape structure, species distributions and even climate as consequences of human activity. Such changes affect the dynamics of the interaction between major forest pest species, such as bark beetles (Coleoptera: Curculionidae, Scolytinae), and their host trees. Normally breeding mostly in broken or severely stressed spruce; at high population densities some bark beetle species can colonise and kill healthy trees on scales ranging from single trees in a stand to multi‐annual landscape‐wide outbreaks. In Eurasia, the largest outbreaks are caused by the spruce bark beetle, Ips typographus (Linnaeus), which is common and shares a wide distribution with its main host, Norway spruce (Picea abies Karst.). A large literature is now available, from which this review aims to synthesize research relevant for the population dynamics of I. typographus and co‐occurring species under changing conditions.

The Greater European Spruce Beetle

The greater European spruce bark beetle, Dendroctonus micans (Kugelann) (Coleoptera: Scolytidae), is, with the Chinese species Dendroctonus armandi Tsai and Li, the only palaearctic representative of its genus. It appears to have moved to Eurasia in relatively recent times, and its closest relative is the Alaskan D. punctatus LeConte, the two species being doubtfully distinct.79

Kinetics of larval gregarious behavior in the bark beetle Dendroctonus micans (Coleoptera: Scolytidae)

This paper provides a simple mathematical model of the aggregation behavior of the gregarious intracortical-feeding larvae of the bark beetle, Dendroctonus micans. The models assumptions are that each individual produces pheromones at a constant rate and reacts positively to a pheromone gradient. These hypotheses have been tested by comparing experiments and simulations, both of which showed that (1) homogeneously distributed individuals aggregate rapidly at the center of the experimental or theoretical arena and (2) eccentric, preformed groups succeed in attracting dispersed individuals, provided that the initial size of these groups is sufficiently high. There is good agreement between most of the experimental and theoretical results, providing a link among chemical communication, density of larvae, and random events occurring during the development of a spatial structure.

Rapid increase in dispersal during range expansion in the invasive ladybird Harmonia axyridis

The evolutionary trajectories associated with demographic, genetic and spatial disequilibrium have become an issue of growing interest in population biology. Invasive species provide unique opportunities to explore the impact of recent range expansion on life‐history traits, making it possible to test for a spatial arrangement of dispersal abilities along the expanding range, in particular. We carried out controlled experiments in laboratory conditions to test the hypothesis of an increase in dispersal capacity with range expansion in Harmonia axyridis, a ladybird that has been invading Europe since 2001. We found a marked increase in the flight speed of the insects from the core to the front of the invasion range in two independent sampling transects. By contrast, we found that two other traits associated with dispersal (endurance and motivation to fly off) did not follow the same spatial gradient. Our results provide a striking illustration of the way in which predictable directional genetic changes may occur rapidly for some traits associated with dispersal during biological invasions. We discuss the consequences of our results for invasion dynamics and the evolutionary outcomes of spatially expanding populations.

Volatile compounds in the larval frass ofDendroctonus valens andDendroctonus micans (Coleoptera: Scolytidae) in relation to oviposition by the predator,Rhizophagus grandis (Coleoptera: Rhizophagidae)

During a laboratory study evaluatingRhizophagus grandis (a specific native predator of the Eurasian bark beetle,Dendroctonus micans), as a potential biocontrol agent against the North American bark beetle,Dendroctonus valens, it was found that feeding larvae and laboratory-produced frass of the potential prey elicited very high oviposition responses in the predator. Comparative chemical analysis of this laboratory-produced larval frass revealed that one major volatile compound, (-)-fenchone, is associated with the larvae of bothDendroctonus species.D. micans also generated pinocamphone while oxygenated monoterpenes in the frass ofD. valens were camphor,cis-4-thujanol, fenchol, terpinen-4-ol, myrtenal, pinocarvone, borneol, verbenone, piperitone, campholenaldehyde,trans-myrtanol,cis-myrtanol,p-cymen-8-ol and 5-oxo-camphor. This range of prey-produced compounds with a possible biological effect onR. grandis was narrowed down subsequent to comparative analysis of field-collected larval frass. (-)-Fenchone, pinocamphone, camphor, terpinen-4-ol, borneol, fenchol, and verbenone were found to be common to both prey species. A mixture of these seven components was tested in a bioassay, where it elicited as much oviposition as did larval frass ofD. micans. The oviposition stimulants forR. grandis are thus clearly among the mixtures constituents.

Intraguild predation by Harmonia axyridis on coccinellids revealed by exogenous alkaloid sequestration

Summary.Under laboratory conditions, the multicolored Asian lady beetle, Harmonia axyridis is well known as an intraguild predator of other ladybirds. However the real impact of this exotic species on native species was poorly investigated in the field. Because many ladybird species produce alkaloids as defensive compounds, we propose here a new method of intraguild predation monitoring in coccinellids based on alkaloid quantification by GC-MS. In laboratory experiments, adaline was unambiguously detected in fourth instar larvae of H. axyridis having ingested one egg or one first instar larva of Adalia bipunctata. Although prey alkaloids in the predator decreased with time, traces were still detected in pupae, exuviae and imagines of H. axyridis having ingested one prey when they were fourth instar larvae. Analysis of H. axyridis larvae collected in two potato fields shows for the first time in Europe the presence of exogenous alkaloids in 9 out of 28 individuals tested. This new method of intraguild predation detection could be used more widely to follow the interactions between predators and potential chemically defended insect preys.

Selective predation on chemically defended chrysomelid larvae: a conditioning process

Laboratory experiments showed that femaleTenthredo olivacea prefer to prey upon insects of a previously encountered species, instead of upon unknown ones. This has been observed when comparing two natural prey of the sawfly, the larvae ofPhratora vitellinae and those ofPlagiodera versicolora. The two species secrete copious amounts of defensive secretion, the first salicylaldehyde, and the latter a mixture of cyclopentanic monoterpenes. The predator appears less reluctant when encountering a species whose secretion has been previously experienced. A selective pressure might thus exist favoring rare secretions, which is consistant with the well-known diversity of defensive compounds among sympatric insects.

Post-storm surveys reveal large-scale spatial patterns and influences of site factors, forest structure and diversity in endemic bark-beetle populations

The storm that struck France on december 26th and 28th 1999 felled 140 million m3 of timber and had a high economic, social and landscape impact. This event offered the opportunity to study large-scale patterns in populations of forest insect pests that would benefit from the abundant breeding material. A large-scale survey was carried out in France in 2000 to sample the most frequently observed species developing on spruce (Ips typographus, Pityogene schalcographus) and pine (Tomicus piniperda, Ips sexdentatus) in 898 locations distributed throughout wind-damaged areas. The local abundance of each species scored on a 0 to 5 scale was analysed using geostatistical estimators to explore the extent and intensity of spatial autocorrelation, and was related to site, stand, and neighbourhood landscape metrics of the forest cover (in particular the interconnection with broadleaf forest patches) found within dispersal distance. All species but I. sexdentatus, which was much less abundant, displayed large-scale spatial dependence and regional variations in abundance. Lower infestation levels per tree (windfalls and standing trees) were observed in stands with a high proportion of wind-damaged trees, which was interpreted as the result of beetles distributing themselves among the available breeding material. More infestations were observed in wind-broken trees as compared to wind-felled trees. More importantly, populations showed significant relationships with the structure of coniferous stands (in particular with the number of coniferous patches). T. piniperda population levels were negatively correlated to the amount of coniferous edge shared with broadleaf forest patches, possibly because of the disruptive effect of non-host volatiles on host-finding processes at the landscape-scale. The differences observed between species regarding patterns and relationships to site, stand, and forest cover characteristics are discussed in relation to the ecological characteristics of each species.

Visual, semi-quantitative assessments allow accurate estimates of leafminer population densities: An example comparing image processing and visual evaluation of damage by the horse chestnut leafminer Cameraria ohridella (Lep., Gracillariidae)

Abstract: Qualitative or semi‐quantitative visual assessments are most often used for estimating population size of herbivorous insects. The precision of these estimates, however, is often difficult to establish. A ‘simulation game’ with the horse chestnut leafminer, Cameraria ohridella Deschka & Dimic (Lep., Gracillariidae) shows that visual, semi‐quantitative assessments can provide accurate information. Damaged areas of 411 horse chestnut leaves collected in 100 sites were closely related to mine numbers despite some variability in mine and leaf size (R2 = 0.915; n = 411; P < 0.001). On the basis of this relationship, two methods of population assessment are compared: (i) digital image processing of leaf damage and (ii) visual assessment using a damage key reflecting the relative infested area on each leaf (0, 0%; 1, 0–2%; 2, 2–5%; 3, 5–10%; 4, 10–25%; 5, 25–50%; 6, 50–75%; 7, 75–100%). Both methods used to estimate damage presented a similar, close relationship to the ‘real’ numbers of mines (R2 = 0.858; n = 777; P < 0.001 for image processing and R2 = 0.905; n = 777; P < 0.001 for visual assessment). The potential of using visual assessments as an accurate and fast method in situ at the tree scale is discussed.