Abstract:
Housefly, Musca domestica Linnaeus (Muscidae) is a household pest that is primarily controlled
by chemical insecticides that often affect the environment and promote insecticide resistance. As
a safer alternative to the chemical insecticides, plants of Cupressaceae family are often used locally
to repel houseflies (HFs) in different parts of Rwanda and Uganda. However, studies on their
chemical characterization and bioactivity are limited. This study aimed at assessing the chemical
variability and insecticidal efficacy of essential oils (EOs) from C. lusitanica Mill. (Cupressaceae)
growing in three agro-ecological zones (AEZs) of Rwanda. The specific objectives were to: (i)
Compare the yields of EOs from C. lusitanica leaves collected from three AEZs in Rwanda, (ii)
analyze the major chemical components of obtained EOs, and (iii) evaluate their insecticidal
activity against houseflies. Fresh leaves of C. lusitanica were judgmentally collected from Burera,
Huye and Kayonza districts representing highlands (HLZ), midlands (MLZ) and lowlands (LLZ)
zones, respectively. Steam distillation was used to extract the EOs and the average yields as per
AEZ were calculated. The chemical components of EOs were analyzed using Fourier Transform
Infrared (FT-IR) Spectroscopy and Gas Chromatography-Mass Spectrometry (GC-MS), while
insecticidal activity was evaluated by exposing adult houseflies separately to test solutions (90, 70,
50 and 30% v/v) for 6, 12 and 24 h. Acetone and Dichlorvos (DDVP, 77%) insecticide served as
test controls. The lethal doses (LD50 and LD90) of tested solutions obtained via Probit analysis and
Repellency percentages (PR) were recorded. Significantly higher (P= 0.02) EOs yield was from
LLZ (Kayonza) (0.39 ± 0.01% v/w) when compared to HLZ (Burera) (0.27 ± 0.02% v/w) that was
not significantly different (P= 0.13) from MLZ (Huye) EOs yield (0.34 ± 0.02% v/w). FT-IR
results indicated the presence of C-H stretch for alkanes (2950-2850 cm-1
), C=O bend for
aldehydes (1720-1740 cm-1
), Ketones (1720-1705 cm-1
), and Carboxylic acids (1725-1700 cm-1
),
C=C for alkenes (̰~1640 cm-1
) and C-C stretch for Aromatics (900-800 cm-1
) in the EOs. GC-MS
results revealed the dominance of Sabinene, Myrcene and α-Pinene for EOs from HLZ;
Umbellulone, δ-3-Carene and Sabinene for EOs from MLZ; and γ-Terpinene, Umbellulone and
Bornyl acetate for EOs from LLZ. DDVP insecticide showed significantly higher fumigant (LD50
of 0.015 to 0.002 ppm and LD90 of 0.52 to 0.20 ppm) and contact (LD50 of 0.01 to 0.001 ppm and
LD90 of 1.03 to 0.19 ppm) toxicities against adult houseflies exposed for 6 to 24h. EOs from LLZ
showed the highest contact toxicity on houseflies exposed for 24 h (LD50=0.08 ppm; LD90=16.26
ppm), while EOs from HLZ showed the lowest toxicity in 6 h of exposure (LD50 = 0.64 ppm; LD90
= 706.21ppm). EOs from MLZ showed toxicity with LD50 values of 0.41 to 0.15 ppm and LD90 of
453.24 to 22.01 ppm in 6 to 24 h of exposure. Fumigant toxicity followed a similar trend (LLZ:
LD50=0.15 ppm; LD90=24.79 ppm for houseflies exposed for 24 h; HLZ: LD50=1.90 ppm;
LD90=1250.21 ppm after 6 h of exposure; and MLZ EOs with LD50 of 0.66 to 0.28 ppm; and LD90
of 521.36 to 80.65 ppm in 6 to 24 h of houseflies’ exposure. The repellent activity indicated that
fewer adult houseflies were repelled at low dose (30% v/v) of EOs for 6 h, with MLZ EOs
recording higher PR (42.77 ± 2.79) than LLZ (40.47 ± 0.62) or HLZ (36.81 ± 2.39). However,
over 70% of houseflies were repelled at high dose (90% v/v) of EOs for 24 h (MLZ: 79.52 ± 3.30,
LLZ: 75.62 ± 1.37, and HLZ: 70.38 ± 3.20%). At all tested doses, less than 14% and over 97% of
houseflies were repelled by Acetone and DDVP, respectively. The variation of yields and chemical
components as well as promising insecticidal activities were observed for EOs of C. lusitanica
from the three study-based AEZs. The promising findings of this study call for further research
into the biosafety of C. lusitanica EOs for use as safer insecticide against housefly.