We studied the effect of temperature and supplementary nutrition (honey water) on development, longevity and oviposition of the litchi fruit borer, Conopomorpha sinensis Bradley (Lepidoptera: Gracillariidae). Temperature had significant effects on the duration of the pupal period, pupal emergence rate, adult longevity and oviposition. The pupal period increased as the temperature declined. Pupal emergence rates were significantly higher at 20 °C, 25 °C and 30 °C than at 15 °C and 35 °C. When the temperature was lower, adult longevity was longer. The provision of supplementary nutrition significantly increased adult longevity, but there was no significant difference in longevity among a series of concentrations. In addition, temperature had a significant effect on oviposition, with the most eggs being laid at 25 °C. There was no significant difference in the numbers of eggs laid at supplementary nutrition levels of 5, 10, 20, 30, 40 or 50% honey water, although the number laid was approximately 6.33–7.56 fold greater than in the control. We found that the biological characteristics of C. sinensis change with temperature or concentration of honey water and these results provide a reference for rearing C. sinensis artificially and for forecasting.
The litchi fruit borer, Conopomorpha sinensis Bradley (Lepidoptera: Gracillariidae), is a destructive pest of litchi (Litchi chinensis Sonn.; Sapindales: Sapindaceae) and longan (Dimocarpus longan Lour.; Sapindales: Sapindaceae), causing significant economic losses (Zhang et al. 2011). Little is known about its biological characteristics because of its boring habit, and artificial rearing technology is not mature (Tsang & Liang 2007). We have been studying C. sinensis for several years (Chen et al. 2010; Chen et al. 2011; Zhao et al. 2012). According to our laboratory-based observations, the growth and behavior of C. sinensis is influenced by external temperature, humidity and supplementary nutrition conditions. However, little is known about these factors may affect natural populations of C. sinensis.
Until now, there are no reports on biological characteristics of C. sinensis under different temperatures and supplementary nutrition (honey water). Temperature and concentration of honey water were 2 major factors influencing the growth and behavior of C. sinensis. Temperature influences the whole life cycle of the pest, including the pupal, adult, egg and larval stages (Du et al. 2009; Zhang et al. 2011; Shi et al. 2011). Honey solutions used as adult food have been reported to increase longevity and the length of the oviposition period of the cabbage looper, Trichoplusia ni (Hübner) (Noctuidae), and soybean looper, Chrysodeixis includens Walker (Noctuidae) (Shorey 1963; Jensen et al. 1974), pink bollworm, Pectinophora gossypiella Saunders (Gelechiidae) (Lukefahr & Griffin 1956), tobacco budworm, Heliothis virescens (F.) (Noctuidae), corn earworm, Helicoverpa zea (Boddie) (Noctuidae), and cotton leafworm, Alabama argillacea (Hübner) (Noctuidae) (Lukefahr & Martin, 1964).
In the current study, we examined the effects of different temperatures on pupal duration, emergence rate, adult longevity and oviposition. We also compared water-fed adults with those fed with different concentrations of honey water in regard to their longevity and length of oviposition. This study provides a reference for rearing C. sinensis artificially and for forecasting populations in the field.
MATERIALS AND METHODS
Individuals of C. sinensis were collected from litchi or longan orchards at the Guangdong Academy of Agricultural Sciences. The insects were kept in constant temperature 50 × 50 × 210 cm incubators (GXZ380B, Ningbo Jiangnan Instrument Factory, China) at 85 ± 5% RH and 14:10 h L:D. Honey used in the experiment was sourced from the local market. Diluted honey with water at 0, 5, 10, 20, 30, 40, 50% (v:v) was provided to moths on a cotton wick, respectively.
Pupal Duration and Emergence Rate
We collected litchi fruit every 5 days from the orchards from 1 Jul 1 to 11 Aug 2011. The fruits were placed in white porcelain dishes (30 × 40 cm) and covered with paper, as described by Tsang & Liang (2007). The dishes containing the fruit were then placed in an insectary at 85 ± 5% RH and 14:10 h L:D. We gathered abundant numbers of C. sinensis pupa from the paper every day. Each pupa was then immediately transferred to a glass tube (1 × 10 cm), which was then put in 1 of 5 constant temperature incubators, set at 15 °C, 20 °C, 25 °C, 30 °C, or 35 °C. Ten glass tubes were used in each treatment and each treatment was repeated 3 times. Each tube was observed on a daily basis until pupal eclosion. Pupal duration and adult emergence rates were then determined.
Conopomorpha sinensis pupae were collected by the same method mentioned above from a longan orchard every 3 days from 1 Jul to 21 Aug 2011. After the pupa eclosed, male and female moths were distinguished by the abdomen terminal. The abdomen terminal of the female is blunt or rounded, while that of the male is pointed. The moths were put together in pairs (1 male and 1 female) in white, transparent mineral water bottles (2 L) and fed with 1 concentration of honey water (either 5, 10, 20, 30, 40, or 50%), or with water as a blank control (0%). The bottles were then placed in constant temperature incubators at 15 °C, 20 °C, 25 °C, 30 °C, or 35 °C, respectively. In total, 20 females and 20 males were used in each treatment, which was repeated 3 times. To determine longevity, each bottle was observed on a daily basis until the moths had died.
Conopomorpha sinensis moths were collected as mentioned above from a longan orchard every 3 days from 1 Jul to 21 Sep 2012. In total, 20 males and 20 females were placed in each oviposition cage (50 × 50 cm) and provided with fresh longan fruit. The female moths are known to oviposit on the pericarp of longan fruit. When testing the effect of temperature (15 °C, 20 °C, 25 °C, 30 °C, or 35 °C) on oviposition rate, supplementary nutrition was provided in the form of 10% honey water. When testing the effect of the concentration of honey water (5, 10, 20, 30, 40, or 50%), the experiments were conducted in insectary at 25 °C, 85 ± 5% RH and 14:10 h L:D. Treatments were replicated 3 times. The number of eggs laid each day was recorded until all the females had died.
Effect of Temperature on Pupal Duration and Emergence Rate
As shown in Table 1, pupal duration and eclosion rate of C. sinensis were significantly different depending on the temperature. Pupal duration was longest at 15 °C, and then declined with temperatures > 15 °C, and the shortest pupal duration occurred at 35 °C. The highest emergence rates were at 25 °C, 20 °C and 30 °C, which were significantly higher than at either 15 °C or 35 °C. Therefore, 15 °C was found to be suitable for extending pupal duration, whereas 25 °C was most suitable for maximum eclosion.
Effect of Temperature and Supplementary Nutrition on Longevity of Females
As the temperature declined (Table 2), female longevity was gradually extended to reach the maximum at 15 °C and 5% honey water, while the minimum longevity occurred at 35 °C and 0% honey water. Female longevities at 30 °C, 25 °C and 20 °C were not significant different (P > 0.05), but these longevities were significantly longer than at 35 °C and significantly shorter than at 15 °C.
As the concentrations of honey water provided to the females were increased (Table 2), the longevities of females increased from 1.33 to 28.29 days. Although there was no significant difference in female longevities at the different concentrations of honey water, all groups of females that received honey water regardless of concentration lived significantly longer than females fed only water (blank control) except that females fed 10% honey water lived an average of 11.55 days, which was not significantly longer than the 2.59 days lived on average by those fed 0% honey water.
Effect of Temperature and Supplementary Nutrition on Longevity of Males
As the temperatures increased (Table 3), the longevities of C. sinensis males gradually decreased, just as the longevities of females decreased. Also there was no significant difference (P > 0.05) in male longevities at 30 °C, 25 °C and 20 °C. Male longevity at 35 °C was significant higher than at 30 °C, 25 °C and 20 °C, and male longevity at 15 °C was significant less than at 30 °C, 25 °C and 20 °C.
THE DURATIONS OF PUPAL DEVELOPMENT AND ADULT EMERGENCE RATES OF CONOPOMORPHA SINENSIS AT DIFFERENT TEMPERATURES.
THE LONGEVITIES OF CONOPOMORPHA SINENSIS FEMALES AT VARIOUS COMBINATIONS OF TEMPERATURE AND VARIOUS CONCENTRATIONS OF HONEY PROVIDED AS SUPPLEMENTARY NUTRITION.
THE LONGEVITIES OF CONOPOMORPHA SINENSIS MALES AT VARIOUS COMBINATIONS OF TEMPERATURE AND VARIOUS CONCENTRATIONS OF HONEY PROVIDED AS SUPPLEMENTARY NUTRITION.
As the concentrations of honey water provided to the males were increased (Table 3), the longevities of males increased from 1.00 to 30.50 days. Although there was no significant difference in male longevities at the different concentrations of honey water, all groups of females that received honey water regardless of concentration lived significantly longer than males fed only water (blank control).
Effect of Temperature and Supplementary Nutrition on Oviposition Rate
At 25 °C (Fig. 1) C. sinensis females laid significantly more eggs than females kept at any other temperature. Oviposition was almost extinguished at 15°C, whereas there was no significant difference in the numbers of eggs laid at 20, 30 or 35 °C.
At all honey water concentrations of 5–50% females oviposited significantly more eggs than females fed only water (Fig. 2). However, there was no significant difference among the number of eggs laid at the different concentrations of honey water.
We found that longevity of C. sinensis moths was significantly extended by temperature. In terms of the relation between phytophagous pests and their host plants, the longevity of the pests is an important factor as it contributes to the size of the pest population. Increased longevity also means that the pests have more opportunity to damage the host plant, and also to produce more offspring. But, low temperature also affects the activity level, and less active adults at low temperatures might damage the host plant less, even though they live longer. Temperature is an important factor that is known to affect insect longevity. Similar results have been reported by Dyer & Landis (1996), McDougall & Mills (1997), Uckan & Ergin (2003) and Chen et al. (2005).
The results of the current study show that there is no linear relation between the concentration of honey water and adult longevity, and similar results reported by Gu et al. (2010). In fact, high concentrations of sugar solution appeared to have an adverse effect on the longevity of C. sinensis, although this was not statistically significant. The reasons might be that consuming a higher concentration of honey water results in increased osmotic pressure, which may have a negative impact on the physiology of the insect. In addition, these results indicate that honey water should be used to feed C. sinensis in laboratory cultures to assure and extend their survival. Similar results were reported by Elmer & Barber (1992), Jacob & Evans (2000), Leatemia et al. (1995), Leius (1961), Olson & Andow (1998), Spafford & Evans (2004) and Uckan & Ergin (2002). Jensen et al. (1974) showed that the soybean looper, C. includens, laid significantly more eggs, lived longer and mated more frequently when adults were fed either 10% honey or nectar from cotton blossoms than when fed only water. Based on the number of spermatophores found in the bursa copulatrix, soybean loopers fed honey for just 2 days mated more often and laid vastly more eggs than those fed water only.
Further research into the optimum temperature for the development, emergence and oviposition of this important economic pest is both necessary and vital to its artificial rearing control and forecasting of populations in the field.
This study was funded by China Litchi and Longan Research System Foundation (award no. CARS-33) from the Ministry of Agriculture of China. We thank Keming Li, Xiaowei Zhang and Zhiting Zhao for collecting the litchi fruit borers and technical assistance.