Enemies of the elm bark beetle

In Table 14 different Scolytus species and a number of their Hymenoptera parasitoides (superfamily Ichneumonoidea and Chalcidoidea) are listed. Grissell {[761]} reported that Cerocephala eccoptogastri  (Pteromalidae) may either be a primary parasite associated with Scolytidae or a secondary parasite associated with Braconidae.

Beaver {[470]} identified 23 beetle species associated with S. scolytus.  The majority of the beetles acts as scavengers in the Scolytid galleries living on mites, small insects, fungi  or  sap. Only eight of these species (Dromius quadrimaculatus, Microlomalus flavicornis, Phloeopora testacea, Denticollis linearis, Malthodes marginatus, Thanasimus formicarius, Epuraea biguttata and Librodor quadriguttatus) play a role as predator in the population dynamics of the large elm bark beetle. However, predation of S. scolytus (larvae and/or adults) by the eight species appears to be slight.

Von Keyserlingk {[347]} showed that infection of S. scolytus with the fungus Beauvaria bassiana, a fungus naturally associated with elm bark, killed the beetle 8-14 days after contamination. Nevertheless, infected females are sometimes able to oviposit and produce healthy offspring. B. bassiana has been observed to attack nearly every insect order (including Coeloides sp.). Colonization of dying elm bark by the fungus Phomopsis oblonga has a detrimental effect on Scolytus reproduction by decreasing the number of viable offspring in brood trees or reducing the potential breeding material {[483],[522],[539],[548]}. Laboratory experiments characterized several pathogenic fungi that are able to infect larvae and/or adults of the elm bark beetle in vitro, e.g. Metarhizum anisopliae, Trichoderma harzianum, T. polysporum, Scytalidium lignicola  and Paecilomyces farinosus {[597],[602],[603]}.

Nematode infection can cause (partial) sterility in female beetles. Fransen {[381]} reported a delay in egg development for female S. scolytus  infected with Rhabditolaimus leuckarti, which prolonged the twig crotch feeding stage (up to 31 days). The thread worm withdraws nutrition from the beetle. So far a number of nematodes have been found that are either internally or externally associated with elm bark beetles. In addition, the worms can live as inhabitants of bark beetle tunnels (Table 15). Nematodes like Cryptaphelenchoides scolyti appear to be closely attached to the beetle cuticle at one stage in its development but free living at another stage {[393]}. Neoplectana carpocapsae infects all three major DED vectors: S. scolytus, S. multistriatus and H. rufipes {[523],[524],[601]}.

Table 14          Elm bark beetle enemies that belong to the superfamilies Ichneumonoidea and Chalcidoidea (order Hymenoptera) {[56],[349],[399],[496],[743],[761]}

Doberski {[525]} analysed the mite populations present in elm logs infested by S. multistriatus and S. scolytus in England and reported the infrequent appearance of the parasitic mite Pyemotes scolyti (Pyemotidae: Prostigmata). This mite causes high mortality (up to 70-80%) among Scolytus populations in Poland {[574]}.

Protozoa are single-celled organisms that often sponge on animals of more complex structure. S. scolytus, S. multistriatus, S. pygmaeus and S. ensifer serve as hosts for the protozoa Stempellia scolyti , Plistophora scolyti (within the gut) and Nosema scolyti (infects the blood cells and the beetles' excretory organs) {[485],[486],[487]}. However, the transmission of DED by these beetles is not retarded by infection {[393]}.

Pesson et al. {[488]} isolated two bacteria from elm bark beetles in France: Aerobacter scolyti and Escherichia klebsiellaeformis. Although they may be present in the normal bacterial flora of the beetle's alimentary system, they can kill the insect through septicemia. Infection of S. scolytus and H. rufipes with the non-specific nematode Neoaplectana carpocapsae may subsequently result in infection with the nematode-associated bacterium Achromobacter nematophilus . The latter infection can kill the beetle as well {[523],[524]}. Jassim et al. {[773]} report on the pathogenicity of the gram-positive soil bacterium Bacillus thuriengsis towards larvae of S. scolytus  and S. multistriatus in vitro. B. thuriengsis produces insecticidal crystal proteins (d-endotoxins) of 27 to 140 kDa that appear to interact with specific high-affinity binding sites on the midgut epithelium of susceptible insects {[780],[781]}. The toxins disturb the osmotic balance of the insect by generating pores in the epithelial cell membrane that result in swelling and subsequent lysis of the cells. Consequently, the larva will stop feeding and die {[782],[783]}. In contrast to Bacillus megaterium, B.cereus and B. alvei  are also observed to be pathogenic to the larvae of the small and the large European elm bark beetle in vitro. Similar experiments in which S. scolytus larvae were exposed to cultures of Pseudomonas syringae  resulted in a mortality rate of 24-30% {[773]}.

Table 15:        Nematodes associated with Scolytid elm bark beetles {[381],[393],[489],[490],[491],[523],[524],[597],[601],[784]}

n.d.      no data available