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L.P.S. VAN DER GEEST
glands (Ongus, 2006). It was possible to isolate the virus and to determine the base
sequence of the virus genome. The virus (Varroa destructor virus 1) is a single
stranded RNA genome and, based on the base sequence, it was decided that it belongs
to the genus Iflavirus (Ongus et al., 2004). Viruses in this genus belong also to the
picorna-like viruses. The virus is closely related to deformed wing virus, known from
honey bees. The latter virus causes morphological anomalies in wings of bees.
3. DISEASES CAUSED BY BACTERIA
The most widely studied bacterium in invertebrate pathology is Bacillus
thuringiensis. It was first described in 1915 by Berliner, who isolated it from soil
samples in the Thuringian Forest in Germany. Simultaneously with spore formation,
a crystalline body is formed in the bacterium. Upon ingestion by an insect, this
crystal (δ-endotoxin) falls apart into toxic subunits that may cause paralysis of the
alimentary tract, resulting in the death of the insect. Most varieties (serotypes) of B.
thuringiensis show an effect on larvae of Lepidoptera, but some also on other groups
of insects, e.g. Coleoptera and Diptera.
Very comprehensive research has been carried out on B. thuringiensis that has
resulted in the development of several commercial preparations that are mainly used
against lepidopterous pests. Also, the gene encoding for the crystalline toxic body
has been isolated and transferred into crop plants, e.g. corn and cotton, making these
crops resistant towards a number of lepidopterous pests. Several serotypes of B.
thuringiensis produce in addition an exotoxin, the β-exotoxin, named thuringiensin.
This exotoxin is excreted by the bacterium into the culture medium. It has a
nucleotide-like structure and inhibits DNA-dependent RNA polymerase. This results
in a blockage of mitosis. When thuringiensin is applied to young holometabolous
insects, morphological deformations may occur in the adult stage.
Field applications of thuringiensin were successful against the citrus red mite P.
citri (Hall, Hunter, & Arakawa, 1971) and Tetranychus pacificus (Hoy & Ouyang,
1987). Later, Royalty, Hall, and Taylor (1990) conducted experiments by testing two
different formulations of thuringiensin against the twospotted spider mite T. urticae.
The results indicated that thuringiensin might be a potential acaricide. In particular
young instars are susceptible, since these have a high growth rate. Various
physiological processes in young organisms require higher RNA synthesis than in
the older slower growing stages. A major drawback is that thuringiensin is toxic for
a wide range of organisms. Not only are spider mites affected, but also beneficial
mites, such as Phytoseiulus persimilis: oviposition starts to decline after 2 days and
ceases completely after 3–4 days in both predator and spider mite T. urticae (Guo,
Zuo, Zhao, Wang, & Jiang, 1993). The chemical is apparently a nonselective
acaricide that should not be used in combination with predatory mites.
The spore-crystal complex of B. thuringiensis has been tested on spider mites
by Krieg (1972), but no mortality was observed. However, Chapman and Hoy
(1991) conducted experiments in which T. urticae and Metaseiulus occidentalis
were treated with a commercial preparation of B. thuringiensis var. tenebrionis. This
variety of B. thuringiensis shows an effect on beetles and is recommended for use
against the Colorado Potato Beetle, Leptinotarsa decemlineata. No effect was noted