Abstract:
Dacus vertebratus (Dv) and Zeugodacus cucurbitae (Zc) are important polyphagous
Fruit Flies (FF) causing considerable yield loss in watermelon (Citrullus lanatus)
worldwide. Entomopathogens have been used extensively for controlling insect pests
due to environmental concerns to replace the use of chemical pesticides. However, information on the use of entomopathogens for FF control is scanty. Therefore, the life
cycles of Dv and Zc, and their management with entomopathogens were investigated. Life cycles of male and female Dv and Zc on watermelon were studied to determine
Development Time-DT (days), morphometrics (mm) and survival (%) of immature
stages following standard procedures. Eight commonly grown fruits and vegetables:
watermelon, pawpaw, melon, cucumber, pickled-cucumber, caserta-zucchini, magda- zucchini and fordhook-zucchini were evaluated for susceptibility to Dv only due to
non-susceptibility of Zc to other vegetable hosts apart from watermelon following
standard methods. Fecundity and DT of Dv were assessed. Soil Samples (SS) were
randomly collected from six main watermelon producing areas in Southern Benin
Republic to identify and characterise local entomopathogens with capacity to control
FF using standard procedures. Two commercial isolates of Beauveria bassiana (Bb337, Bb338) and Metarhizium anisopliae (Ma) were bio-assayed each at 10
5
, 10
6
, 10
7
, 10
8
, 10
9 and 10
10 conidia/mL against Dv and Zc larvae for mortality. Probit analysis was
computed to determine Lethal Concentration (LC50). Efficacy of four isolated fungi
from SS were evaluated at 10
10 conidia/mL on both FF; mortality was assessed. Effectiveness of field application of Ma (50g/ha), Bb337 (50g/ha), Bb338 (50g/ha), ICIPE-69 (200mL/ha) and a synthetic insecticide (K-optimal, 10
3mL/ha) was
evaluated on watermelon (25,000 plants/ha). Treatments were applied twice a week
from three-weeks after sowing to two-weeks before harvest and yield (t/ha) data was
collected. Data were analysed using descriptive statistics and ANOVA at α0.05. Development of Dv and Zc was holometabolous with four stages. Male DT (16.6±0.3)
and female DT (16.7±0.3) of Dv were not significantly different from Zc (male =
17.5±0.3; female = 18.8±0.4). Dacus vertebratus was smaller (female = 6.3±0.3 mm;
male = 6.3±0.2 mm) than Zc (female = 6.4±0.3 mm; male = 8.2±0.2 mm). Survival
rate of Dv and Zc were not significantly different for egg (Dv = 78.5%; Zc = 79.2%), first-instar (Dv = 72.7%; Zc = 71.7%), second-instar (Dv = 92.7%; Zc = 90.6%), third-
ii
instar (Dv = 92.5%; Zc = 93.1%) and pupae (Dv = 85.5%; Zc = 80.7%). Eggs laid per
mated-female of Dv were significantly higher on watermelon (650.5±50.2) followed
by pawpaw (537.6±47.3), fordhook-zucchini (537.5 ± 46.3), cucumber (528.6±38.6), pickled-cucumber (528.1±39.2), melon (521.7±39.1), magda-zucchini (515.1±38.9)
and caserta-zucchini (510.0±37.3). The DT of Dv was shortest on watermelon
(16.6±1.3) < 16.7±1.6 (magda-zucchini) < 18.3±1.6 (caserta-zucchini) < 19.1±1.4
(fordhook-zucchini) < 19.6±1.9 (pawpaw) < 19.7±1.2 (cucumber) < 20.0±1.1 (melon
and pickled-cucumber) suggesting host suitability. Four fungi species: Aspergillus
niger, A. flavus, Botryotricum sp. and Fusarium verticillioides with entomopathogenic
potential were identified from SS. Larval mortality was significantly higher with Ma
(Dv = 37.5%; Zc = 63.1%), Bb337 (Dv = 31.8%; Zc = 39.2%) and least with Bb338
(Dv = 26.1%; Zc = 35.6%). The LC50 of larval stage of FF subjected to
entomopathogens was 10
10 conidia/mL. Highest mortalities were recorded on Zc
(44.0%, 33.0%, 28.0%, 22.0%) treated with A. niger, A. flavus, F. verticillioides and
Botryotricum sp. compared to mortalities on Dv (4.0%, 5.0%, 2.0%, 8.0%, respectively). Watermelon yield was superior with Ma application and was in the order:
148.8±25.2t/ha (Ma) > 145.8±25.2 t/ha (K-optimal) > 136.7±23.6 t/ha (Bb337) >
119.2±20.5 t/ha (Bb338) > 115.8±20.9 t/ha (ICIPE69). Watermelon was the most susceptible host fruit of Dacus vertebratus and Zeugodacus
cucurbitae. Metarhizium anisopliae at 50g/ha was sufficient to control the insect, thus
increasing watermelon production above conventional pesticide
