The discovery of Cephalonomia stephanoderis
BETREM, a new parasite of the coffee berry borer Hypothenemus hampei
FERR. (syn. Stephanoderes hampei
Ferr.), seemed to open new prospects for biological control of the pest.
However the phenomenon that C.stephanoderis
uses H. hampei
both as Prey and host for oviposition made us suppose that the parasite had particular relations with its host, in order to enable both insects to survive. The main reason of this study was to examine this relationship.
To estimate the effect of the parasite on the population of H. hampei
and therefore on the attack of coffee, a study of component factors in this attack was necessary and therefore of the relationship between hostplant and harmful insect.
TICHELER (1961) had shown that the attack on coffee berries depended on the population density of H. hampei
in the field, but the susceptibility of different coffee varieties to attack by this insect was hardly known.
In our field trials at Institut Français de Café et Cacao (IFCC) in the Ivory Coast, the progress of the attack on the berries was studied in three varieties (Robusta INEAC, Robusta Ebobo and Kouilou) during two subsequent coffee seasons (chapter 4.).
Particular attention was paid to the influence of regular harvesting of the ripe berries (every 2 or 4 weeks) on the progress of the attack. The same was studied in a part of the plots where no berries were harvested.
FACTORS IN THE RELATIONSHIP: HOSTPLANT TO PEST INSECT. (CHAPTER 5.)
Of the three varieties, Kouilou was known to be most susceptible to attack by H. hampei
(LEEFMANS, 1923a; PORTERES, 1959). However, in our trials Kouilou turned out to be definitely less susceptible than the two other varieties.
To assess the differences in susceptibility of the varieties, use was made of 'monocyclic' and 'polycyclic' tests (ZADOKS, 1972). 'Cyclic' stands for the sequence of processes from the entrance of the insect into the coffee berry through egg laying to the emergence of its offspring from the berry. To compare the results of the different tests, the most susceptable variety was used as control. The results of the tests measuring the components of susceptibility were used to compose an index of susceptibility.
In the polycyclic tests, the susceptibility of the variety was assessed in the field where the host (fruits of a tree or of a plantation) were infested by several generations of beetles.
Certain components contribute less to the progress of attack in the field in Kouilou than in the two other varieties. This proved that partial resistance of Kouilou to attack by H. hampei
is due to certain properties of the seeds in the berries.
The seeds of the variety Kouilou are less suitable for the scolytid, than seeds of the varieties R. INEAC and R. Ebobo.
FACTORS IN THE RELATIONSHIP OF THE PEST INSECT TO ITS PARASITE (CHAPTER 6.)
In the study of the bionomy of Cephalonomia stephanoderis,
the conditions were investigated for predation and oviposition. These two activities influenced one another only when prey was scarce. The parasitic and predatory activities of C. stephanoderis
were confined to the population of H. hampei
on only one berry.
The rate of development and longevity of the adult parasite accord well with those of the host.
Field trials showed that the parasite was most abundant when the host population was at its peak. There is only a short period when C. stephanoderis is
abundant, since the population of the host H. hampei
decreases because of food scarcity and lack of appropriate conditions for egg laying (chapter 7).
EFFECT OF THE PARASITE ON THE PEST
The effect of presence of the parasite on the population of living pest hosts was studied in parts of the plots that were not disturbed by picking the coffee berries. This effect was estimated in two different ways:
- By using the proportion of infested berries containing living parasites (%Pa).
- By estimating the mortality of beetles in the berries.
Both estimates show that the population of H. hampei
decreases by 20-30 %, because of the presence of C.stephanoderis
at the end of the coffee season, but this effect falls to 5 % in the period between seasons.
In conclusion, the presence of the parasite in a coffee plantation could exert a favourable influence on the persistance of a population of H. hampei