50
uygun parçasını taĢıyacak Ģekilde genetik olarak iĢlem görmüĢlerdir. Bazı yönlerden
bu, yalnızca bu ürünlere zarar veren böceklerin Bt toksinlerinin etkisine uğramasını
güvence altına aldığından, diğer tip böcekler için hiçbir riski olmadığından önemli bir
teknolojik ve pratik geliĢmedir.
Bu uygulama
Bt kullanımını, köklerde beslenen veya bitki dokularında delikler
açan -örneğin Avrupa mısır delicisi- dolayısıyla bitki yüzeylerine püskürtülen
Bt
süspansiyonları ile kontrol edilemeyen böceklere geniĢletmektedir. Diğer yandan,
burada olumsuz bir gidiĢ de vardır. Çünkü hedef böcekler sürekli olarak bu
toksinlerin etkisine uğrayacak ve bu durum, bu toksinlere karĢı direnç geliĢimi için
çok kuvvetli bir seleksiyon baskısı yaratacaktır. ÇeĢitli ürün idaresi stratejileri, bu
riski azaltmak için geliĢtirilmektedir.
Biological Control - Bacillus
popilliae
Produced
by Jim Deacon
Institute of Cell and Molecular Biology, The University
of Edinburgh
This is one of 8 Biocontrol Profiles. It introduces the topic of biocontrol and deals with the
commercial use of the milky disease bacterium
Bacillus popilliae to control the Japanese
beetle, a serious pest of turf, fruit crops and garden ornamentals in the USA.
Biological Control
Natural environments tend to be balanced
environments, where organisms depend on one
another and also constrain one another by competition for resources or by parasitism,
predation, etc. But human influences can upset these balances, and this is most evident when
an exotic organism is introduced on purpose or by accident. Many of the most serious pests,
crop diseases or invasive weeds are the result of "introductions" from foreign lands.
The newly
introduced organisms find a favourable environment, free from their previous constraints, and
they proliferate to achieve "pest" status. Entomologists has a useful term for this - they refer to
the constraining organisms in the region of origin as "the natural enemy complex".
We can define Biological control (biocontrol) as:
the practice or process by which an undesirable organism is controlled by means of
another (beneficial) organism.
In other words, biocontrol is both a naturally occurring process (which we can exploit) and the
purposeful use of one organism to control another.
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In practice, biocontrol can be achieved by three methods.
Inundative release (also termed "classical biocontrol") in which a
natural enemy of a
target pest, pathogen or weed is introduced to a region from which it is absent, to give
long-term control of the problem. An example of this is the use of
Bacillus popilliae to
control the Japanese beetle in the USA, discussed below.
The
biopesticide approach in which a biocontrol agent is applied as and when
required (often repeatedly), in the same way as a chemical control agent is used.
Examples of this include the use of
Bacillus thuringiensis,
Phlebiopsis gigantea and
Agrobacterium radiobacter.
Management and manipulation of the environment to favour
the activities of naturally
occurring control agents. An example of this is seen in take-all control in grass turf.
Control of the Japanese beetle
In this section we discuss the use of a bacterium,
Bacillus popilliae, to control a major introduced
pest in the USA.
Much of the text below has been copied and updated from a book now out of print [JW Deacon,
1983.
Microbial Control of Plant Pests and Diseases. Van Nostrand Reinhold, Wokingham]
===============================================================
The Japanese beetle,
Popillia japonica (Figure A), was accidentally introduced into
the USA early
this century. Although it is not a problem in its area of origin, the beetle causes serious damage in
the USA. It spread rapidly from the initial sightings in New Jersey (1916) and today it is found over
roughly half of the country, in almost every state east of the Mississippi. It is a problem as an adult
beetle because it feeds on a wide range of plants, eating out the leaf tissues between the leaf
veins (Figure B), and it accumulates on ripening fruit causing substantial damage. It is also a
problem in the larval stage because the adult beetles lay their eggs in grass turf and the grubs
destroy the grass roots, especially on new housing estates where natural enemies are absent.
Figure A. Adult
Japanese beetles, about 1-2 cm long. Figure B, feeding damage on foliage. Based
on slides provided by Fairfax Biological Laboratory.
By the 1930s the beetle problem had become so serious that a search was begun for a control
measure. This led to the discovery of some naturally occurring diseased larvae. The disease was
termed
milky disease because of the milky white appearance of the grubs, due to a large number
of refractile bacterial spores in the haemolymph (insect blood) (Figures C, D). Two types of
bacterium were subsequently isolated from two types of milky disease. Type A disease was
characterized by a pure white appearance of the grubs and the bacterium in this case was named
B. popilliae. Type B disease differed in that the grubs showed a transition from white to brown over
winter and the bacterium causing
this disease was named B. lentimorbus. A range of other milky
disease bacteria were isolated from beetle hosts throughout the world, but the trend now is to
regard all of these as varieties of
B. popilliae because
they
are more closely related to one another