Fungi are one of the most common types of pathogen observed causing disease in insects in the field. They usually infect when a mobile spore or conidium lands on the cuticle of the insect and a germ tube actively penetrates into the haemocoel. However, there have been few successful commercial microbial insecticides developed mostly due to a lack of cost effective production methods (Frederici, 1999). Three fungi have been identified for controlling beetles – Beauveria, Metarhizium and Zoophthora. B. bassiana may be useful in controlling beetle larvae in soil as it has a very broad host range (Hajek and Butler, 2000). Fungi are less promising in in-hive control or stored combs due to adverse effects in bees e.g. M. anisopliae is effective against a range of coleopteran and lepidopteran pests (Frederici, 1999) but has been shown to cause mortality in honeybees. Therefore it is likely that fungi will only be applicable to the control of SHB larvae and pupae in soil. The problem with the use of fungi is the amount of material required to achieve an acceptable level of control, which for beetle larvae in soil would require an application rate of 20-30 kg of dried conidia per hectare for B. bassiana (Frederici, 1999). Hokkanen (1993) determined the effectiveness of B. bassiana against the related pollen beetle Meligethes aeneus and suggested that overwintering survival of the beetles in soil may be reduced by 50% after treatment although the study was not conclusive.
B. bassiana strain ATCC 74040 is effective against a range of soft-bodied coleopteran pests; tradename Naturalis-L, Naturalis-O and Naturalis-T (Troy) (BCPC 2001a).
Summary of susceptibility of coleopteran species to Bt strains and their reported effects on honeybees (families listed in two columns show species differences) (after Glare and O’Callaghan, 2000)
Steinernematid and heterorhabditid nematodes are small terrestrial nematodes (less than 3mm) that are parasites of soil inhabiting insects (Bedding et al., 1993). It is not usually the nematodes that kill the insects but the symbiotic bacteria they carry within their alimentary tracts. The nematodes enter the insect’s haemocoel and release their bacterial symbiont which proliferates and kills the host. Conditions caused by the fatal septicaemia by the symbionts favour nematode reproduction by producing nutrients and antibiotics that inhibit the growth of other microflora on the cadaver. The host range of these nematodes is broad and includes more than 200 species of beetle, lepidopteran and orthopteran pests as well as other insects such as cockroaches. This raises issues about their specificity and effects on non-target invertebrates as well as introduction of non-indigenous species. Entomopathogenic nematodes are widely distributed throughout the world with 14 species of Steinernema and 8 species of Heterorhabditis described (Frederici, 1999). The natural host range of S.carpocapsae is broad and includes four insect orders and ten families, natural infections have been most commonly observed in coleoptera and lepidoptera. S.feltiae has been detected mainly in soil dwelling coleoptera (Scarabaeidae and Curculionidae), lepidoptera and diptera. Although these nematodes have been isolated from a wide range of habitats little is known of their biology in natural or agricultural ecosystems. They do not survive well in environments that are dry, although the definition of a suitable moisture content depends on soil type. Nematodes work best against insects breeding in soil and shaded moist environments.
Glazer et al. (1999) assessed the efficacy of differing strains of entomopathogenic nematodes in controlling third instar larvae of sap beetles (Coleoptera: Nitidulidae) which are serious pests of date palms. Heterorhabditis strain IS-5 resulted in more than 65% mortality of Carpophilus humeralis and strains HP88 and IS25 gave 95-100% mortality of C. humeralis, but efficacy was far lower against C. hemipterus.
A nematode has been isolated from three species of sap beetles in the USA. Psammomermis nitidulensis causes 80% parasitism and death in the dusky sap beetle (Carpophilus lugubris) (Poinar and Dowd, 1997). However, it is unclear if this nematode is present outside the USA which would limit its usefulness in Europe.
H. bacteriophora is effective against a wide range of insect pests and is very effective for the control of soil dwelling beetles (BCPC 2001a). Trademarks include Heteromask (BioLogic), Cruiser (Ecogen), Lawn Patrol (Hydro-Gardens) and Larvanem (Koppert).
H. megidis is effective against soil insects (BCPC 2001a). Trademarks include Nemasys H (MicroBio, Becker Underwood, Westgro Sales, Plant Products, BCP, Brinkman)
Taber and Hood (2000) suggested that the presence of fire ants around the colonies may reduce the build up of populations due to control of larvae pupating in the soil.