Cucumis sativus Genotypes Resistance to Dacus ciliatus (Diptera: Tephritidae)

Cucumber, Cucumis sativus is a widely cultivated plant in the Cucurbitaceae family, which is used as a vegetable. Various pests at different stages of growth cause economic damage to it. Lesser pumpkin fly, Dacus ciliatus Loew (Diptera: Tephritidae) could be considered as one of the most important pests of cucurbits worldwide, which direct feeding of larva on fruit making it unusable. Farmers often spray their fields with various pesticides and in some cases is not very effective. Owing to many problems in the chemical control of fruit flies, using resistant plants is one of the most important components in integrated pest management. Host plant resistance is an important component for the management of lesser pumpkin fly, due to difficulties associated with its chemical and biological control. Resistant genotypes can be used to manage this pest.

Seeds were sown in 2018 with ten replicates (blocks) for each genotype following a completely randomized block design. The area of each bed was 4 m × 4 m and the plant-to-plant distance was maintained at 50 cm with a drip irrigation system. All the recommended agronomic practices (e.g. weeding, fertilization, hoeing, etc.) were performed equally in each experimental bed. The infested fruits were sorted and the percent fruit infestation was calculated. Different genotypes by degree of infection into completely resistant (no contamination), very resistant (1-10%), resistant (11-20%), relatively resistant (21-50%), susceptible (51-75%) and high susceptible (100-75%) were sorted. In this experiment, morphological and biochemical characteristics related to resistance were measured in six different genotypes of cucumber (Hayek, Surina, Maximus, Kish, Armenian cucumber, and a local genotype of Kermanshah). The effect of these traits on fruit infestation and larval density was determined. Ten fresh fruits were selected from each genotype and fruit length, fruit diameter, fruit rind thickness, and rind trichomes density were measured and recorded. To measure biochemical traits two fresh fruits from each genotype were selected and the number of chemical compounds such as phenols, flavonoids, tannins, and total alkaloids was measured.  Percentage of fruit infestation, larval density, biophysical, and biochemical traits values were analyzed by Two-way analysis of variance using SPSS 24 software. The means of significant parameters, among tested genotypes were compared using Tukey’s tests for paired comparisons at the probability level of 5%. Correlations between biophysical and biochemical fruit traits and fruit fly parameters (percent fruit infestation and larval density per fruit) were determined using correlation analysis at the 95% significance level.

The larval density per fruit increased with an increase in percentage of fruit infestation and there was a significant positive correlation between percent fruit infestation and larval density per fruit. Among the studied genotypes, Hayek resistant, Surina, Maximus, and Kish moderately resistant, Armenian cucumber susceptible and local genotype was highly susceptible. Phenol content (r = 0.77), tannin (r = 0.89), total alkaloids (r = 0.93) and flavonoids (r = 0.87) were statistically correlated with the percentage of fruit infestation. The percent fruit infestation had significant positive correlation with fruit length (r = 0.55), fruit diameter (r = 0.48), fruit rind trichomes density (r = 0.81) and rind thickness (r = 0.87). Maximum variation in fruit infestation (86.50%) and larval density (63.70%) was explained by the alkaloids. Stepwise regression analysis of our data showed that the highest variation in fruit infestation (75.30) and larval density (61.40) was explained by the trichomes density. Therefore, it can be argued that the reduction of fruit fly infestation on resistant cultivars could be due to physical and biochemical characteristics.

Reduction of fruit fly infestations on resistant genotypes could be due to antibiosis (adverse effect of host plant on the development and reproduction of insect pests, which feed on the resistant plant) and antixenosis (operates by disrupting normal arthropod behavior). Our results suggest that biochemical and biophysical fruit traits could contribute to these mechanisms of resistance. Rind thickness, rind trichomes density, total alkaloids, and tannin were playing an important role in pest resistance. In summary, certain biochemical (tannins, phenols, alkaloids, flavonoid) and biophysical (rind thickness, fruit rind trichomes density, fruit length, fruit diameter) traits were linked to the resistance of cucumber against D. ciliatus and therefore can be used as marker traits in plant breeding programs to select resistant genotypes. By using Hayek genotypes, which have less susceptibility to the lesser pumpkin fly, farmers can produce safe products with less residual insecticides.