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M.S.T. ABBAS
from that of males. Abraham, Faleiro, Shuaibi, and Alabdan (2001), however,
reported that R. ferrugineus captured by pheromone traps were most likely
female dominated, as the sex ratio in captured weevils was 1 male:2.7 females.
El-Garhy (1996) found this ratio to be 1 male:2 females (in Ismaelyia,
Egypt) while Abbas et al. (2006) found that out of 18,047 RPW captured from
five date palm plantations (Ras Al-Khaima, UAE) during 2000 and 2001, the
females represented 60.2% of the total catch, while males represented 39.8%
(almost 1 male:1.51 females). Finally, Abd-Allah and Al-Khatri (2005) compared
the catch of different colours of pheromone traps hung on the palm trees at 1 m
height in Sultanate of Oman. These authors found that the orange or red traps
captured almost 2-fold RPW compared to the blue ones. However, sex ratio in
the captured weevils was almost 1 male:1.1 females, in the 3 traps.
5.3. Entomopathogenic Nematodes
EPN from the families Steinernematidae and Heterorhabditidae are widely
regarded as being excellent biological control agents for a number of insect pests
in soil and cryptic habitats (Gaugler & Kaya, 1990; Kaya & Gaugler, 1993).
They possess many positive attributes including their wide range hosts, safety to
vertebrates, plants as well as non target organisms, exemption from registration
in many countries, ease of in vitro production and application using standard
spray equipments. The two families bear mutualistic bacteria in the intestine,
belonging to the genera Xenorhabdus (in Steinernematidae) and Photorhabdus
(in Heterorhabditidae).
The free-living, non-feeding 3rd instars (infective juveniles) of these
nematodes possess attributes of both insect parasitoids or predators and
entomopathogens. Like parasitoids and predators, they have chemo-receptors
and are motile; like pathogens, they are highly virulent, killing their host victims
within 24–48 h.
5.3.1. Pathogenicity to RPW
Several laboratory studies were carried out to evaluate the efficiency of EPNs
against larvae and adults of R. ferrugineus. Abbas and Hanounik (1999) tested
the virulence of Steinernema riobravis, S. carpocapsae (All strain) and
Heterorhabditis sp. (Egyptian isolate) against larvae and adults of RPW. They
found that the three nematode species caused 10–100% mortality in the larvae,
at concentrations ranging from 30 to 240 infective juveniles (IJs) per larva. The
trial was carried out in Petri-dishes lined with filter paper. The LC 50 values were
51, 61 and 56.6 IJs per larva for S. riobravis, S. carpocapsae and
Heterorhabditis sp., respectively. Adults RPW were less susceptible to the
nematode infection, as the corresponding LC 50 values for the three species were
900, 1,100 and 1,416 IJs per adult. However, such infected adults produced
2,000–242,000 IJs per weevil, but no correlation was found between dose and
IJs production.