Session 6 - Biological control of the GGB

Topic: Biological Control of the GGB
Duration: 1 hour 30 minutes
Objectives:

At the end of this session, the participants should:

i) know the meaning of the term Biological Control

ii) know the important advantages and disadvantages of biological control techniques

iii) recognize and identify Teretriosoma nigrescens (T.n.), a predator of the GGB which has been introduced in Kenya in an effort to control the GGB.

Target Group:

- Subject Matter Specialists (SMS's)
- Technical Assistants (TA's)
- Farmers
- Local leaders

Training Materials:

i) Well-preserved samples of T.n. and its larvae
ii) Well-preserved samples of the GGB, its larvae and pupae
iii) BB with chalk stick or FC with paper and felt pen
iv) Magnifying glasses (lenses).

Process:

15 minutes

Introduction to Biological Control

a) The trainer should first of all explain the principles of biological control, the difference between biological control and chemical control and the main groups of organisms used as biological control agents as there are:

- pathogens
- parasites
- parasitoids
- predators

b) Biological control should then be defined as: Manipulation of beneficial organisms with the goal of reducing pest populations on an economically acceptable level.

5 minutes

The Preconditions for Effective Biological Control

c) The trainer should then explain the following prerequisites for effective biological control using predators:

i) Both the predator and the host must be capable of existing in the same surroundings (habitat).

ii) The predator must be sufficiently strong or adapted to kill either the adult host or its young ones.

iii) Ideally, the reproduction rate of the host should be higher than that of the predator otherwise both the predator and the host will soon become extinct.

d) The trainer should then hand to each participant, samples of the GGB beetles, eggs, larvae and pupae to examine by means of a lens. All participants should be able to recognize the GGB immediately. Thereafter, samples of T.n. beetles and larvae should be handed to each participant to examine. The trainer should then identify and explain T.n. as a predator of the GGB. The following points should be explained:

30 minutes

The Suitability of T.n. as a Biological Control Means for GGB

i) The GGB has relatively few known effective enemies/predators. The best known predator for GGB in the world is T.n.

ii) T.n. does not naturally occur in Africa. It has only been recently introduced from Central America to Togo, Benin and to certain parts of Kenya by KARI in an effort to control the GGB biologically.

iii) T.n. is well adapted to antagonize and control the GGB to a certain extent through the following means:

1) It lives in the same surroundings (maize, cassava, etc.) and climatic zones as the GGB.

2) Its size is such that it can easily go through the pores or bores made in maize, cassava, wood, etc. by the GGB.

3) In order to penetrate the bore, it uses its relatively strong legs to sweep away the frass left in the passages by the GGB.

4) It is sensitive to the aggregation pheromone (smell sent out by the GGB beetle to attract other GGB's). So it is easy for it to locate the GGB wherever it may hide.

5) The T.n. beetle feeds on the eggs, larvae and pupae of the GGB. Each adult T.n. eats about one to 1.5 GGB larvae per day.

6) The T.n. larvae also feeds on GGB eggs, larvae and pupae. T.n.'s larvae are even more ferocious than the adult beetles, each larva of T.n. is capable of consuming about 4.5 GGB larvae per day.

7) In an experiment carried out to determine the effectiveness of T.n. to control the GGB biologically, the predator was introduced to loose white maize and to maize left on cobs. In each case, the same quantity of maize was left free to act as a control. The four quantities of maize were observed for a period of 110 days (nearly four months). The results are summarized in the following table:

	Loose Maize	Maize on Cobs
Population of GGB present in maize where T.n. was NOT introduced	640	1,109
Population of GGB present in maize where T.n. WAS introduced	85	315
Relative reduction in population of GGB	87%	72%

8) The same experiment showed that, in the case of loose maize, losses were reduced by 76% when T.n. was introduced. The corresponding reduction in losses for maize still on cobs was 63%.

9) It can be shown that, up to a point, the higher the population of T.n. in the maize, the lower the losses that may be incurred by the farmer. However, after a certain optimum T.n. population level, no further effect may be observed due to shortage of food for T.n. and cannibalism which sets in.

e) The trainer should allow participants' questions, after which he/she may ask some questions to ensure that the main points have been learnt.

f) The trainer should then summarize the session's lessons by:

i) Defining the term 'biological control';
ii) Briefly highlighting points i), ii) and iii) of c) above;
iii) Naming T.n. as the main predator of the GGB;
iv) Very quickly listing points 1 to 8 of d) iii) above.