Document Type : Research Paper

Authors

• ُSheida Darvishi ¹
• Shahram Hesami ²
• Mehdi Gheibi ¹

¹ Department of Entomology, Islamic Azad University, Shiraz, Shiraz, Iran

² Department of Entomology, Shi. C., Islamic Azad University, Shiraz, Iran

Abstract

Keywords

• the lettuce aphid
• ladybeetle
• biological control
• Life table

Extended Abstract

Introduction

    The lettuce aphid Nasonovia ribisnigri Mosely (Hemiptera: Aphididae) is recognized globally as one of the most destructive pests of lettuce crops. By colonizing the inner leaves of lettuce heads—where contact insecticides penetrate poorly—this aphid can cause severe yield loss, sometimes reducing production by more than 40%. The pest has expanded widely in Europe, North America, and parts of Asia, including Iran, where it has become a recurring challenge in lettuce-producing regions. Given the limited effectiveness of chemical control, natural enemies play a critical role in managing N. ribisnigri populations within integrated pest management (IPM) programs.

Among the predators of N. ribisnigri, the variegated ladybeetle Hippodamia variegata Goeze (Coleoptera: Coccinellidae) has particular importance. This species is widely distributed throughout the Palearctic and has been reported from a variety of agroecosystems, including cereals, vegetables, and ornamentals. In Iran, H. variegata occurs abundantly and preys on several aphid species. Previous studies have examined various biological aspects of this predator, showing that temperature, prey availability, and host species significantly influence its development and fecundity. However, no detailed age-stage, two-sex life table analysis had been conducted to quantify its population growth potential when feeding specifically on the lettuce aphid N. ribisnigri.

This study aimed to evaluate the developmental duration, survival, reproduction, and life table parameters of H. variegata feeding on N. ribisnigri under three constant temperatures—20, 25, and 30 ± 1°C. Determining the optimal temperature range for predator performance is essential for predicting its biocontrol potential in lettuce fields, facilitating better planning of augmentative releases and compatible IPM strategies.

Materials and Methods

Colonies of N. ribisnigri were collected from lettuce fields around Shiraz and maintained on fresh lettuce leaves in ventilated acrylic cylinders under controlled conditions. A stock colony of H. variegata adults was also collected from the same fields and reared for one generation to ensure acclimation to laboratory conditions.

For life-table experiments, one hundred eggs of H. variegata (less than 6 hours old) were individually placed on lettuce leaf discs positioned on agar gel inside Petri dishes. Three constant temperature regimes (20, 25, and 30 ± 1°C), 70 ± 10% RH, and a photoperiod of 16:8 (L:D) were used. Newly hatched larvae were isolated individually to prevent cannibalism, and fed ad libitum with all developmental stages of N. ribisnigri. Larvae, pupae, and adults were checked every 12 hours to record survival and developmental duration.

After adult emergence, one male and one female (each <6 hours old) were paired in new Petri dishes and monitored daily. The pre-oviposition period (APOP and TPOP), oviposition period, fecundity, and adult longevity were recorded until all individuals died.

Life table parameters were calculated using the age-stage, two-sex life table approach (Chi & Liu 1985; Chi & Su 2006) with TWOSEX-MSChart software. Parameters estimated included intrinsic rate of increase (rm), finite rate of increase (λ), net reproductive rate (R₀), and mean generation time (T). Variances and standard errors were computed via bootstrap resampling (10,000 iterations), and means were compared using Tukey’s test at the 5% significance level.

Results

 Development and Survival

    Temperature had a significant effect on all developmental stages. Egg development decreased from 5. 0 days at 20°C to 1. 6 days at 30°C. Larval development across four instars and pupal duration also shortened significantly with increasing temperature. Total pre-adult development required 22.38 days at 20°C, 15 days at 25°C, and only 12.62 days at 30°C.

Pre-adult mortality was lowest at 25°C (14%), moderate at 20°C (20%), and highest at 30°C (31%), indicating some detrimental effects of elevated temperature despite faster development.

Adult Reproduction and Longevity

    Temperature strongly influenced reproductive performance. The adult pre-oviposition period (APOP) was shortest at 30°C (1.77 days) and longest at 20°C (4.0 days). The total pre-oviposition period (TPOP) also decreased markedly with increasing temperature.

Fecundity exhibited a non-linear trend: the highest fecundity occurred at 25°C, where females laid 1253.61 ± 17.8 eggs, significantly more than at 20°C (1102.14 eggs) or 30°C (587.86 eggs). The oviposition period also decreased at higher temperatures, spanning 61.81 days at 20°C, 50.06 days at 25°C, and 35.41 days at 30°C.

Adult longevity declined sharply at high temperature. Females lived 68.0 days at 20°C but only 40.1 days at 30°C. Males showed similar trends but generally shorter lifespans.

(a)                 Life Table Parameters

Life table analysis revealed 25°C as the optimal temperature for population growth:

• Intrinsic rate of increase (r_m):
  • 1452 ± 0.0031 at 20°C
  • 2143 ± 0.0039 at 25°C (highest)
  • 2131 ± 0.0093 at 30°C
• Finite rate of increase (λ):
  • 1564 ± 0.0035 at 20°C
  • 239 ± 0.0048 at 25°C
  • 2376 ± 0.0115 at 30°C
• Net reproductive rate (R₀):
  • 69 ± 67.7 offspring at 20°C
  • 03 ± 68.9 offspring at 25°C (highest)
  • 39 ± 89.1 offspring at 30°C
• Mean generation time (T):
  • 94 ± 0.44 days at 20°C
  • 27 ± 0.31 days at 25°C
  • 50 ± 0.25 days at 30°C (shortest)

Although development was fastest at 30°C, fecundity and survival declined, causing overall population growth to peak at 25°C.

Conclusion

    This study demonstrates that temperature profoundly influences the biology and population dynamics of H. variegata feeding on the lettuce aphid N. ribisnigri. While higher temperatures accelerate development, very warm conditions (30°C) reduce fecundity and survival. The optimal temperature range for predator performance lies between 25 and 30°C, with 25°C providing the highest population growth potential, as reflected by maximal rm, λ, and R₀ values.

These findings indicate that H. variegata has strong potential as a biological control agent against N. ribisnigri under warm spring and early summer conditions typical of many lettuce-growing regions in Iran. Adoption of temperature-aware release strategies and integration of this predator into IPM programs can substantially enhance the sustainable management of lettuce aphid populations.

Author Contributions

Conceptualization, Sh.D. and Sh.H.; methodology, Sh.D and Sh.H. and M.Gh.; Research & Investigation, Sh. D.; software, Sh.D.; writing—original draft preparation, Sh.D.; writing—review and editing, Sh.H. and M.Gh.; All authors have read and agreed to the published version of the manuscript. Sh.H. is supervisor and M. Gh. is advisor of the project. All authors contributed equally to the conceptualization of the article and writing of the original and subsequent drafts.

Data availability statement

Data generated or analyzed during this study are provided in full within the published article.

Ethical considerations

All research followed ethical guidelines to minimize harm to insects and avoid disruption of natural habitats. Sampling was limited to the minimum necessary, and no endangered or protected species were collected. All activities complied with relevant regulations, and data were recorded and reported with integrity.

Conflicts of Interest

The authors declare that they have no conflicts of interest