Potential influence of carbohydrate and amino acid intake by adults on the population dynamics of Cnaphalocrocis medinalis(Lepidoptera:Crambidae)

Ll Chuan-ming ,XU Jian ,LIU Qin,HAN Guang-jie ,XU Bin,YANG Yi-zhongLlU Xian-jin

1 College of Horticulture and Plant Protection,Yangzhou University,Yangzhou 225009,P.R.China

2 Institute of Agricultural Sciences for Lixiahe Region in Jiangsu,Yangzhou 225007,P.R.China

3 Key Laboratory of Food Quality and Safety of Jiangsu Province,Institute of Food Safety and Nutrition,Jiangsu Academy of Agricultural Sciences,Nanjing 210014,P.R.China

Abstract Cnaphalocrocis medinalis is a key lepidopteran pest of rice.However,little is known about the nutritional requirements of the adult or the effects of adult-derived nutrients on reproduction.The aim of the present study was to evaluate the effects of carbohydrates and amino acids on the reproductive and demographic parameters of C.medinalis.Different feeding solutions significantly influenced adult survival and reproduction.All the sources of carbohydrates used in the treatments(fructose,glucose,and sucrose) were sufficient to increase adult longevity and fecundity,and benefited the development of ovaries in the adult stage.The positive impact of carbohydrates on lifetime fecundity was due to the prolonged oviposition period and the increased daily fecundity.The intrinsic rate of increase (rm) of C.medinalis increased from 0.103 in waterfed individuals to approximately 0.138 when adults were fed with solutions containing carbohydrates.In contrast,amino acid intake by adult insects exhibited no effect on the longevity,fecundity,ovarian development or population growth,even showing an impact of decreasing longevity of females.As nectar secreted by the flowering plant is generally rich in sugars,the potential effects of nectar on the adults of C.medinalis and other pests have to be considered during the development of biological control by applying flowering plants as a microhabitat and food source for natural enemies in rice fields.

Keywords:Cnaphalocrocis medinalis,carbohydrate-feeding,adult nutrition,reproduction,population dynamics

1.lntroduction

It is well known that resource availability influences the life history traits of invertebrates,particularly for phytophagous insects.The relative intake of nutritional resources significantly impacts the survival,growth,development and reproduction of individuals (Cahenzli and Erhardt 2013;Zhouet al.2019).Lepidopteran adults are believed to rely primarily on nutrients accumulated during the larval stage for the output of somatic maintenance and reproduction(Colasurdoet al.2009;Barroset al.2010;Darraghet al.2019).In fact,among herbivorous insects,the larval and adult stages usually differ considerably with regard to their nutritional requirements and food ecology.Previous studies conducted on specific species of butterflies have shown that the adult diet significantly contributes to their reproductive capacity (Fischeret al.2004;Bauerfeindet al.2007;Geisteret al.2008).Nutrients ingested during the adult stage affect various parameters including longevity,oviposition period,and lifetime fecundity (Marchioro and Foerster 2013;Dindoet al.2019;Jaumann and Snell-Rood 2019).For example,consumption of sugars by adults significantly increased the oviposition period,fecundity,fertility and longevity ofAnticarsiagemmatalis(Lepidoptera:Noctuidae) (Weiet al.1998),while certain species preferentially fed on nectars with high amino acid contents (Jervis and Boggs 2005).Nectar is a sugar-rich plant secretion that contains low levels of amino acids,proteins,lipids,vitamins,and secondary plant compounds,as well as other organic compounds and minerals.Flowering plants that secrete nectars are major food sources for adult insects,notably in the case of Lepidoptera (Gilbert and Singer 1975).Flowering plants also serve as food sources for adult parasitoids,benefiting their survival,host-searching activity and reproduction (Heimpelet al.1997;Wanget al.2014;Chauet al.2019).Thus,such plants can be utilized to improve the effectiveness of biological control agents (Settele and Settle 2018;Vamsiet al.2019),and this strategy has been shown to be practical in various agroecosystems,as adopted through increasing the variety and flowering time of the flowering plants (Gurret al.2012,2016).For example,by planting sesame at the edge of rice fields,the number of parasitic wasps could be increased and the natural control of rice insect pests was enhanced,thereby reducing applications of chemical pesticides (Zhuet al.2014).

The rice leaffolder,Cnaphalocrocismedinalis(Guenée)(Lepidoptera:Crambidae),is one of the most important pests in Asian rice-growing systems,as are the brown planthopper,Nilaparvatalugens,and white backed planthopper,Sogatella furcifera(Hemiptera:Delphacidae) (Khanet al.1988).In China,the control ofC.medinalisand other rice insect pests in the last decades has relied heavily on frequent applications of chemical insecticides,although the negative side effects on natural enemies,environment and food safety have been of significant concern (Hanet al.2018;Xuet al.2019).These problems highlight a growing need for sustainable pest management in order to reduce the negative effects of pesticides in rice fields (Gurret al.2012;Selvarajet al.2012).As an intervention approach,it was reported that growing nectar-producing plants around rice fields could improve the effectiveness of natural enemies,as it led to significant reductions of the populations of brown planthopper and whitebacked planthopper,contributing to the ecological intensification of the agricultural systems(Gurret al.2016).However,the provision of flowering plants for natural enemies may also serve as a food source for insect pests,which,in the case of lepidopteran species,can be an important factor for determining their abundance and population dynamics (Miller 1996;Clausenet al.2001;Wäckerset al.2007).Access to nectar by adult insects is considered a major cause of outbreaks in some species of Lepidoptera (Weiet al.1998;Marchioro and Foerster 2012;Guoet al.2018).As a typical lepidopteran pest,C.medinalisrequires complementary nutrition after emergence,and its adults can suck nectars fromPaspalumconjugatum,Amaranthusviridis,LigustrumlucidumandGossypiumin the field (Zhanget al.1980).The role of adult-derived nutrition with respect to the reproduction ofC.medinalishas not been investigated.Our previous studies have shown that larval diet affected the growth,reproduction and feeding preferences ofC.medinalis(Xuet al.2012;Liet al.2017).However,little is known regarding the nutritional requirements of the adults or the effects of adult-derived nutrients to their reproduction.Evidently,obtaining such information is quite critical to the rational evaluation of using flowering plants as a strategy for improving natural enemy effectiveness.The present study focused on evaluating the effects of carbohydrates and amino acids on the reproductive and demographic parameters ofC.medinalis.The experiments aimed to expand our understanding of the nutritional requirements of adults and the resulting population performance of the pests.

2.Materials and methods

2.1.lnsects

A laboratory colony ofC.medinaliswas maintained for more than 20 generations based on the methods described by Shono and Hirano (1989).Adults from this stock population were used in the present study.Founder adults were caught from the rice paddies in the suburb of Yangzhou City,China (32°24´N,119°26´E).The adults were provided with 10% honey solution and the larvae were reared with corn seedlings.Both adults and larvae were maintained at (25±1)°C,70% relative humidity and a 16 h L:8 h D photocycle.

2.2.Feeding solution for adults

Complex sugar solution,amino acid solution and combined solution of both carbohydrates and amino acids were prepared for adult nutrition tests.The types and concentrations of sugars and amino acids were determined based on their contents in the nectar of flowers commonly visited by lepidopteran species (Wattet al.1974;Heyneman 1983).The complex sugar solution was composed of fructose,glucose and sucrose at a ratio of 2:2:1.The mixture was dissolved in distilled water at a concentration of 25% (w/w).The amino acid solution contained alanine,arginine,glycine,lysine,proline and serine,and was diluted in distilled water at a concentration of 0.167 mol L–1(for each component).The combined solution consisted of the complex sugar solution supplemented with the six amino acids (at 0.167 mol L–1).To compare the effectiveness of different carbohydrates,each the three pure sugars in the sugar mixture was also dissolved at a concentration of 25%(w/w).Distilled water was used as control.

2.3.Experimental procedures

Newly emerged adults obtained from the laboratory colony were used in the experiments.A total of 30 replicates were performed for each treatment.Each replicate comprised one adult couple,which was kept in a 125-mL plastic cup and sealed with plastic film.Adult food was supplied through a hole (5 mm in diameter) in the cup,filled with cotton which was soaked in different feeding solutions.The feeding solutions were replaced daily with the same treatment to ensure that adults ingested only the diet.All the treatments were maintained in the climate chamber under a 16 h L:8 h D photocycle,at (25±1)°C and (70±10)% relative humidity.Dead adults were recorded each day until the day that none had survived.Adult survival and longevity of male and female insects were calculated.Mated adults laid eggs on the wall of the plastic cup.The cup was replaced daily following oviposition initiation and the eggs were counted.The oviposition periods of females were recorded for each feeding solution.Egg viability was determined based on the number of hatched larvae among laid eggs.

In addition,another 30 females for each feeding solution were reared separately under the same conditions for ovary dissection.On days 1,2,and 3 following feeding,the adults were dissected and the developmental stages of their ovaries were determined according to the method of Zhanget al.(1980),where ovaries were divided into grades 1 to 5.The ovarian stages were averaged among individuals within the same nutrient treatment and subsequently compared between treatments.

From the biological parameters obtained in the adult stages,fertility life tables were constructed for each feeding solution according to Carey (1993).The reproductive parameters calculated were:net reproductive rate (R0),intrinsic rate of increase (rm) and finite rate of increase (λ).

2.4.Statistical analysis

The experimental layout included a randomized design.Differences between treatments for adult longevity,reproductive capacity and egg viability were assessed by univariate analysis and one-way analysis of variance(ANOVA).To compare longevity between males and females,a factorial ANOVA was employed considering diet and sex as sources of variation.Additionally,survival curves were constructed for each adult diet according to the Kaplan–Meier function (Kaplan and Meier 1958).The differences of ovarian development over time were assessed by the two-way analysis of variance,considering diet as an independent variable and detection time as a dependent variable.Multiple comparisons of the means between treatments were determined using the Fisher’s protected least significant difference (PLSD) test.Differences between means were considered significant atP≤0.05.All statistical analyses were performed with the PASW Statistics,version 18.0.

3.Results

3.1.Longevity and survivorship

Adult longevity ofC.medinaliswas significantly affected by the feeding treatment (F6,419=70.66,P＜0.001),without significant difference between sexes (F1,419=12.002,P=0.074).The interaction between diet and sex was significant(F6,419=15.943,P＜0.001).Supplies of each type of carbohydrate sources led to an increase of adult longevity(male:F6,209=80.746,P＜0.001;female:F6,209=14.172,P＜0.001).The feeding of amino acids exhibited no effect on adult longevity,but exerted a negative effect in the females(F1,58=13.628,P＜0.001).However,the treatment of the combined solution of both carbohydrates and amino acids resulted in a prolonged adult longevity compared with the control (Fig.1).Different types of sugars demonstrated differential effectiveness on longevity of male insects.The greatest longevity was found in the complex sugar solution and in glucose-fed males,which was significantly different from those of fructose and sucrose (F3,119=35.091,P＜0.001).No significant differences were noted among different sugar sources for the female insects (F3,119=0.382,P=0.766).

Fig.1 Adult longevity of Cnaphalocrocis medinalis fed on different solutions.Values (mean±SE;n=30) followed by different letters on the bars indicate significant differences in the cluster of male or female,according to PLSD test (P＜0.05).

Kaplan–Meier analysis of the survival curves indicated no significant differences in survivorship among different adult feeding solutions for either males (χ2=264.778,df=6,P＜0.001) or females (χ2=96.685,df=6,P＜0.001).The survival curves of the male and female insects exhibited similar patterns,with carbohydrate intake increasing adult longevities ofC.medinalis.The maximum survival of certain insects was determined by the presence of carbohydrates and was extended from 5 days (male) and 10 days (female)to over 15 days (male) and 12 days (female).Survival analysis further indicated that the amino acid intake exhibited no effect or negative effects on male or female longevity(Fig.2).

3.2.Reproduction

The reproductive capacity ofC.medinaliswas affected by consumption of different nutrients (F6,209=12.882,P=0.001),with the number of eggs laid ranging from 26.7 to 117.2 per female (Table 1).In all of the treatments where adultderived diets containing carbohydrates were used,the total fecundity increased compared with that of the control.Multivariate tests indicated no significant difference between the treatments of different pure sugars to those of total fecundity (F3,119=1.293,P=0.28).By contrast,in females that consumed amino acids the fecundity did not increase compared with diets containing only distilled water.Similarly,carbohydrate intake prolonged the period of egg laying(F6,209=13.811,P＜0.001) with increased oviposition rate(F6,209=2.714,P=0.015).However,no significant effects were noted with regard to the amino acid content.Although the diet containing amino acids resulted in a higher rate of oviposition than that of the control group,this difference was not statistically significant.Egg viability analysis indicated there was no significant difference between different adult diets (F6,209=0.377,P=0.893).

3.3.Fertility life table

The fertility life table parameters differed among different diet treatments with regard to theR0(F6,209=11.082,P＜0.001),rm(F6,209=6.856,P＜0.001),andλ(F6,209=6.811,P＜0.001).The capacity of increase in the population of each generation,was 24.8-fold on the diet containing distilled water,while values for the diets containing carbohydrates were 73.17-to 98.33-fold (Table 2).In contrast,when adults were supplied with only amino acids as adult food,theR0was 34.17-fold,which was not significantly different from that of the control groups (F1,57=3.712,P=0.059).Similar toR0,theλandrmwere higher than those of the control group when the female insects consumed carbohydrates.No significant differences were noted between the different sugar nutrients.Adult insects fed on amino acids exhibited no effects with regard toλ,although a relatively higher value ofrmwas noted compared with that of the control group (F1,58=13.628,P＜0.001).

Fig.2 Kaplan–Meier survival curves of Cnaphalocrocis medinalis males and females fed on different solutions.

Table 1 Reproductive parameters of Cnaphalocrocis medinalis adults fed on different soultions

Table 2 Demographic parameters of Cnaphalocrocis medinalis adults fed on different solutions

3.4.The ovarian development grades

Ovarian development ofC.medinaliswas influenced by different diets (F6,189=71.582,P＜0.001) following the detection time(F2,189=95.934,P＜0.001).No significant interaction between adult diet and time was found (F12,189=0.674,P=0.775).The ingestion of carbohydrates improved the development of the ovary,although no significant effects of adult-derived amino acids on ovarian development were observed (Fig.3).On the first day following feeding with distilled water,approximately 80% of the ovaries were developed to the grade 2 stage,while 20% were still in grade 1.These effects were similar to those noted in the adult insects that consumed amino acids.However,when the adult insects consumed diets containing carbohydrates,the majority of the ovaries were developed into grades 2 to 3,while none of them was in grade 1.Following the feeding time,the ovarian stages of different treatments developed gradually,maintaining the tendency that the ovaries developed faster with access to carbohydrates than those of amino acids and water control (2 days:F5,59=2.542P=0.039;3 days:F5,59=5.788,P＜0.001).The complex sugar solution showed the greatest effect on ovarian development especially in the third day,which was significantly different to the treatment of fructose (F4,49=1.731;P=0.16).

4.Discussion

The present study indicated that the adult diet significantly influenced the survival and reproduction ofC.medinalis,in the same manner as previously reported for other Lepidoptera species which required adult nutrition to reach sexual maturity (Bauerfeind and Fisher 2005;Bauerfeindet al.2007;Marchioro and Foerster 2013).These findings demonstrated that the full reproductive potential ofC.medinalisis dependent on adult-derived nutrients.This result can help us to understand the fact that adults ofC.medinalisconsume the nectars of various floral plants as complementary nutrition in the field (Zhanget al.1980).

Previous studies with Lepidoptera species have found that carbohydrates exert positive effects on the longevity of adults (Marchioro and Foerster 2013).The present study confirmed that glucose,sucrose and fructose significantly increased the longevity of both males and females ofC.medinalis.The prolonged lifetime of adults may benefit the mating and oviposition period of the adults.According to the longevity difference between males and females,adult males seemed to require more nutrients than females,as males supplied with carbohydrates exhibited a longer lifespan (Fig.1).This is consistent with previous findings demonstrating that males required sufficient calories from carbohydrates for migrating and mating;in addition,males would transfer energy to females through mating to fulfill the nutritional requirement of egg production (Boggs 1990;Zhouet al.2019).

Fig.3 Comparison of the ovarian development grade of Cnaphalocrocis medinalis adults fed on different solutions for 1,2 or 3 days.Values (mean±SE;n=30) followed by different letters indicate significant differences between treatments according to the PLSD test (P＜0.05).

Carbohydrate intake further affected the reproduction ofC.medinalisand is considered a beneficial supply to the development of the ovaries at the adult stage.The results demonstrated thatC.medinalishas a carbohydrate nutrient which was required for the species to develop their full productivity at the adult stage.Similar results were also noted in certain lepidopteran adults as demonstrated in previous reports,with several important life table parameters found to be positively associated with the consumption of carbohydrates (Geisteret al.2008;Mollemanet al.2009;Alexandreet al.2010).Carbohydrates not only provided benefits to the oviposition period of adults,but also to their fecundity.The positive impact on lifetime fecundity was due to the prolonged oviposition period and the increased daily fecundity (Table 1).Furthermore,owing to the carbohydrate supply,the ovary development was further promoted.The rapid ovarian development would allow females to shorten the preovipositional period.

It was postulated that amino acids derived from the adult diet may also play an important role in development and reproduction (Jervis and Boggs 2005;Cahenzli and Erhardt 2013).Certain species of Lepidoptera were found to prefer nectar mimics with amino acids over nectars lacking amino acids (Jervis and Boggs 2005).However,in the present study,carbohydrates in the adult diets seemed to be the most important component that affected adult development,while the amino acid solution for adults did not improve the lifetime ofC.medinalis.Furthermore,the lifetime of females fed on amino acids was shorter than that of the distilled water diet,suggesting that only the amino acid solution exhibited a negative effect on the longevity of females.Likewise,adultderived amino acids did not affect the oviposition period,total number of eggs or ovarian development.Although the life table parameter of thermvalue was slightly higher,theR0andλvalues were not significantly different from those of the distilled water group.This result was similar to those reported from other studies.Specifically,no significant effects were caused by amino acid diets on the fecundity of various lepidopteran species as previously reported (Hill and Pierce 1989;Mevi-Schutz and Erhardt 2003;Bauerfeind and Fischer 2005;Mollemanet al.2008).However,this result should be interpreted with caution since the experiments were performed under laboratory conditions.Early studies have shown that essential amino acids are derived only from the larval diet,placing an upper limit on the use of adult dietary resources to enhance reproductive success (O’Brienet al.2000,2002).In recent studies,it was found that nectar-derived amino acids affected the quality of butterfly offspring (Cahenzli and Erhardt 2013).Both essential and non-essential amino acids that adults feed on were allocated to eggs and were still present even in the early larval stages(Eranet al.2017).Therefore,the effects of amino acids on adults need further study.The ratios or concentrations best suited for the female feeding should be considered (Borzouiet al.2018;Reiferet al.2018;Deanset al.2019).

Cnaphalocrocis medinalisis one of the most important pests that disrupt the development of rice.It is well known that the serious damage is caused by the rapid increases in the pest population (Chakraborty and Deb 2011).The present study clarified that the presence of carbohydrates in the diet ofC.medinalissignificantly improved the longevity and reproductive activity of adult insects,while it exhibited important consequences forC.medinalispopulation dynamics.Thermrates ofC.medinalisincreased from 0.103 in water-fed individuals to 0.138 when adults were fed on solutions containing carbohydrates,which suggested that after two generations (about 60 days) when a source of carbohydrate was available,the population would increase more than 23 times compared to the water-fed treatment group.These amplified generations would be considered a high-risk for the potential outbreak of pests in rice paddies.In a previous study examining the embedding of flowering buckwheatFagopyrumesculentum(Fagopyrum:Polygonaceae) in cabbage plots in order to increase the biological control by enhancing the parasitoid effect,this plant did not lowerTrichoplusiani(Lepidoptera:Noctuidae)densities even though parasitism byVoriaruralis(Diptera:Tachinidae) on larvae was elevated (Lee and Heimpel 2005).Lepidopteran pests may also take advantage of the buckwheat nectar (Baggenet al.2000).Toonasinensisis a suitable summer nectar source that potentially influences migration and reproduction ofMythimnaseparatamoths and thus contributes to outbreaks of this pest (Guoet al.2018).The use of plant biodiversity to enhance the efficiency of natural enemies for suppressing pests has been the subject of recent research in rice (Gurret al.2012;Zhenget al.2017).Sesame (Sesamumindicum) was selected as a border plant to reduce the damage by hemipteran pests in rice paddy (Zhuet al.2013).Although it was reported thatS.indicumfloral resources did not enhance the longevity or fecundity of the two tested rice Lepidoptera pests,Sesamia inferens(Lepidoptera:Noctuidae) andChilosuppressalis(Lepidoptera:Pyralidae) (Zhuet al.2015),the impact of the flowering plant onC.medinalisand other lepidopteran pests should not be overlooked.The current study revealed thatC.medinalismay exhibit a carbohydrate-feeding strategy in the adult stage.An essential concentration of carbohydrates may satisfy the adult demand for energy (Pavliket al.2018).The rich component of sugars in the nectar of flowing plants would benefit adult longevity and reproduction,thereby shifting the dynamics of the pest population.Furthermore,the composition of nectar and other available sources of nutrients in the field is more complex than that of pure sugars(Wäckerset al.2007;Pavliket al.2018).The development of conservation requires the implementation of biological control programs that aim to increase the abundance and efficiency of natural enemies by providing alternative flowering plants in the rice field.In addition,the increase in the availability of food to the adults ofC.medinalisand other pests should be taken seriously into account.

5.Conclusion

We found that different carbohydrates in feeding solutions significantly influenced the survival and reproduction ofC.medinalisadults.Carbohydrate intake increased adult longevities and prolonged the period of egg laying with an increased oviposition rate,and thermrates increased from 0.103 to approximately 0.138.Thus,the potential effects of nectar on the adults ofC.medinalisand other pests have to be considered during the development of biological control practices by applying flowering plants as a microhabitat and food source for the natural enemies in rice fields.

Acknowledgements

This work was supported by the Natural Science Foundation of Jiangsu Province,China (BK20191216),the Jiangsu Agricultural Science and Technology Innovation Fund(ZX(17)2002),the International Cooperation and Exchanges Projects of Jiangsu Province (BZ2020039),and the Opening Fund of Jiangsu Key Laboratory for Food Quality and Safety-State Key Laboratory Cultivation Base (028074911709).

Declaration of competing interest

The authors declare that they have no conflict of interest.