Biology and management of the litchi stink bug, Tessaratoma papillosa(Hemiptera: Tessaratomidae):Progress and prospects

YAO Qiong, QUAN Lin-Fa, XU Shu, DONG Yi-Zhi,LI Wen-Jing, CHEN Bing-Xu

(Plant Protection Research Institute, Guangdong Academy of Agricultural Sciences, Guangdong Provincial Key Laboratory of New High Technology for Plant Protection, Guangzhou 510640, China)

Abstract: Litchi stink bug, Tessaratoma papillosa(Hemiptera: Pentatomidae), is one of the most widespread and destructive pest species on litchi(Litchi chinensis)and longan(Euphoria longan)in South China and Southeast Asia.T. papillosa feeds on the buds, tender branchlets, flowers, and fruits of host plants.Furthermore, the nymph over the 3rd instar and adult of T. papillosa are the vectors of witches’ broom pathogen on longan tree.In this article, we provide a detailed review on T. papillosa based on the research over the past 60 years in China, in order to provide references for the further study and development of green control technology of this pest species.T. papillosa is a hemimetabola insect which occurs one generation a year.Its nymphal duration and adult longevity are about 80 d and 203-371 d, respectively.Moreover, averagely one female adult of T. papillosa can deposit 190 eggs in its lifetime.Regarding the life habits, T. papillosa has the aggregation behavior, possesses phototaxis and chromatics tropism, and prefers tender branchlets.By the classification and distribution observation of antennal sensilla, RNA-seq analysis of antennae, dissection and observation of scent gland and comparative analysis of secretary components from scent gland, the two organs of antennae and scent gland of T. papillosa have been more deeply studied and understood.To date, the occurrence of T. papillosa is mainly forecasted through its ovarian development of female adults.Together with the pest forecasting technology, management of T. papillosa mainly depends on chemical control, supplemented by cultural, physical and biological control measures so far.The biological control technology has been reported the most, with aspects of the protection and utilization of natural enemies and the utilization of botanic pesticide.Owing to the seasonal restriction of experimental insect source and geographical limitation of infestation, the research progress of T. papillosa is relatively slow, the in-depth studies are limited and the research fields are relatively narrow.In future study, we can explore the selection mechanism of host plants, the interaction among host plants, natural enemies and symbiotic bacteria, and pesticide resistance from the perspectives of omics, molecular biology, cell biology and other aspects, to provide new clues for green pest control technology of T. papillosa.

Key words: Tessaratoma papillosa; biological characteristics; antenna; scent glands; forecasting method; pest management

1 INTRODUCTION

Litchichinensis(litchi)andEuphorialongan(longan)(Sapindaceae)are valuable fruits native to China(Chenetal., 2013).In 2018, the cultivated areas of litchi and longan in China are 551 733 and 275 933 ha, respectively; with the production outputs of 3.03 and 1.57 million metric tons, respectively(Chen, 2018).The litchi stink bug,Tessaratomapapillosa(Hemiptera: Tessaratomidae), is one major pest on litchi and longan in South China and Southeast Asia(Chen, 2017).In China,T.papillosais widespread in Guangdong, Guangxi, Hainan, Fujian, Yunnan, Guizhou, Jiangxi and Taiwan areas(Zheng, 2014).The nymph and adult ofT.papillosahave piercing-sucking mouthparts for sucking buds, tender branchlets, flowers, and fruits, causing damage, wilting, and fruit-and flower-drop of host plants(Chen, 2017).The wounds they leave provide convenience for some fruit diseases, such as litchi downy blight and anthracnose.Furthermore, the nymph over the 3rd instar and adult ofT.papillosaare the vectors of witches’ broom pathogen on longan tree(Xuetal., 1993).T.papillosareleases foul-smelling, irritant, and corrosive defensive liquid when it is disturbed.The liquid leads brown burns to host plants, and even burns people’s eye and skin, causing ache, irritability and inflammation(Mo, 2007; Chenetal., 2009).Besides litchi and longan, the host plants forT.papillosainclude loquat, shaddock, citrus, lemon and other fruit trees.T.papillosahas serious impacts on fruit output and quality, and causes a large economic loss in China.So far, application of synthetic insecticides is still the most effective tactic againstT.papillosa.However, frequent and large quantity usages of chemical pesticides cause some adverse impacts, such as environmental pollution, pest resistance, toxicity to non-target organisms, and limitation of the green cultivation of litchi and longan.Owing to the seasonal restriction of experimental insect source and geographical limitation of infestation, the research progress ofT.papillosais relatively slow.In this article, we gave a detailed review onT.papillosabased on the research over the past 60 years in China, hoping to provide references for the further study and management of this pest species.

2 BIOLOGY CHARACTERISTICS

2.1 Life history and developmental stages

T.papillosais a hemimetabola insect which occurs one generation a year in host plants.The life stages ofT.papillosaconsist of egg, nymphal and adult stages(Fig.1).The duration of its egg stage is directly related to the temperature, ranging from 8 d to 25 d; the nymph undergoes 5 instars; and the adult longevity is 203-371 d(Liu, 1965; Liu, 1996)(Table 1).T.papillosaadult mates and lays eggs during February-August, with its oviposition peak in April-May(Liu and Gu, 1998).Adult eclosion often happens in July and August.The newly emerged adults usually choose host plants with tender branches as their feeding sites and overwinter as sexual immature adults.Their overwintering sites are not only limited to litchi and longan trees, but also include wampee, mango, grapefruit trees and others.Overwintering adults prefer to the blade back of the current-year shoot with high density, sufficient sunshine, and out of wind(Liu, 1965; Liuetal., 2012).After the early spring in the following year, post-overwintering adults fly to host plants for feeding, mating, and egg-laying, and then eventually die during June-July(Yangetal., 1994; Liu and Gu, 1998).

Fig.1 Stages of Tessaratoma papillosa metamorphosis

2.2 Mating and ovipostion

T.papillosamates and lays eggs for multiple times in its lifetime.Mating happens in day or at night and lasts at least 10 h(Wangetal., 2010).The eggs ofT.papillosaare oval in shape with a smooth shell and a wax coat for protection, and their diameter ranging 2.5-2.9 mm.The red eyespot on the egg shell gradually becomes macroscopic at 2-3 d before hatching(Liuetal., 1966).The reproductive period of female adult ofT.papillosais as long as 18 weeks consisting of 5-10 oviposition periods.Each female can deposit 14-28 eggs at a time and its average lifetime fecundity is 190 eggs, which is highly affected by host plant parts(Table 1).The 2nd to 5th weeks after the first time of egg-laying is the peak oviposition period for a female adult, when it accounts for 46.8% of the total egg-laying amount in female lifetime(Liu, 1965; Liu and Gu, 1998; Xieetal., 2004; Liuetal., 2012).According to the field investigation, blade back of host plant is the favorite egg-laying site of litchi stink bug(Liu, 1965; Weng and Lin, 1994).

2.3 Life habits

Regarding the life habits,T.papillosahas the aggregation behavior, possesses phototaxis and chromatics tropism, and prefers tender branchlets.

2.3.1Aggregation behavior: It is common that adults aggregate with nymphs of different instars in orchards.There are dozens of or even hundreds of litchi stink bugs in one aggregation.Besides, newly emerged adults have preference for upper branchlets(Lietal., 2013a).Insects mainly rely on aggregation pheromone to attract other individuals of the same species for aggregation, joint habitation, feeding and attacking other species(Mori, 2014).For Pentatomidae insects, aggregation pheromone comes from their scent gland, body volatile matter, and excreta, and is mainly perceived by their antennae(Mo, 2007; Tillman and Cottrell, 2019).BothT.papillosawith antennae cut andT.papillosawith intact antenna could aggregate, but the aggregation speed and efficiency of the former were relatively slower than the latter(Lietal., 2013a).Therefore, the antenna plays an alternative role in the aggregation pheromone perception ofT.papillosa.

2.3.2Tropism: In a study concerning chromatics tropism ofT.papillosa, female adults tended to choose the red, while male adults tended to the blue; and both sexes had the lowest selection probabilities for green color, in spite of the green leaves of host plants(Wangetal., 2011b).In addition, the adult has a strong phototactic response to light.The selection probabilities ofT.papillosato light reached 100% in a test contrasting host plant in dark versus light only(Wangetal., 2011b).Intriguingly, the aggregation behavior of litchi stink bug was observed both in light and dark environment(Lietal., 2013a).These results suggest that chemical communication plays a vital role in aggregation behavior, but visual stimulation is more important in host localization inT.papillosa.Besides,T.papillosaprefers tender branchlets.According to the investigation in orchards, 79.7% ofT.papillosaadults, 77.6% of nymphs and 51.8% of eggs were found in fruit trees with multiple fruits or tender branchlets(Liu, 1965).As for the tropism to branchlet height, most of the overwintering adults aggregated in the lower part of branchlets of host plant(66.8%), followed by middle part of branchlets(30.6%), and the least in upper branchlets(2.6%)(Xu and Ye, 1991).Such research results are conducive to a comprehensive understanding on the life habits ofT.papillosaand provide theoretical and practical foundation for the research and application of pollution-free control technology ofT.papillosa.

3 ANTENNAE AND SCENT GLAND

3.1 Antennae

The antennae are the primary sites of olfactory reception and also serve as active sensors in many insect species(Elgaretal., 2018).Through numerous sensilla on the antennae, legs, mouthparts and other parts, insects can perceive chemical information in the environment, between the same species or from other species, and then make a series of behavioral responses such as location, mating, feeding and avoiding dangers(Hansson, 2014).So far, there has been a preliminary understanding aboutT.papillosaantennae.In the research of the types, amount and distribution of the antennal sensilla ofT.papillosaby using scanning electron microscope, five sensilla were observed, including sensillum trichoid, sensillum chaeticum, sensillum basiconicum, sensillum moniliform, and sensillum coeloconicum.Among them, the sensilla trichoid occupy the largest part.Besides, there are obvious differences in the amount and distribution of sensilla between male and female.Sensillum moniliform is peculiar to male antennae, while sensillum coeloconicum is peculiar to female antennae(Zhaoetal., 2006).By using RNA-seq analysis, 59 odorant receptors, 14 ionotropic receptors and 33 odorant binding proteins were identified from the male and female antennal transcriptomes inT.papillosa(Wuetal., 2017).These results provide a comprehensive resource to future analysis of olfactory sense inT.papillosa.

3.2 Scent gland

The scent gland is one important feature that differentiates Pentatomidae from other hemipteran taxa(Schaefer, 1972).The main physiological role of the scent gland secretions includes defense, alarming, aggregation and sex attraction(Bryanetal., 1999; Martínezetal., 2017).In Pentatomidae, the scent gland of stink bugs consists of abdominal scent glands(ASG)and metathoracic scent glands(MTG)(Tillman and Cottrell, 2019).Normally, the ASG only has function in nymphs, as well as in adults of few Pentatomidae insects.In contrast, MTG is peculiar to adults in Pentatomidae insects(Aldrichetal., 1995).The nymphs ofT.papillosahave 4 pairs of ASGs with cystic structure, which are located in the central of the back of the 1st, 2nd, 3rd and 4th abdominal segments.But the ASGs in the 1st and 4th abdominal segments lose function in the 4th and 5th instar nymphs.The MTG inT.papillosaadults is located at the ventral part between the metathorax and the 1st abdominal segment.The MTG is composed of a single bivalve cystic-shaped structure, orange in color(Mo, 2007).

With the help of gas chromatography mass spectrometry(GC-MS)and solid-phase microextraction-gas chromatography mass spectrometry(SPME-GC-MS), a comparative analysis was performed with the secreta from scent gland ofT.papillosaadults before and after they were disturbed, together with a comparison with volatile components of the flowers and leaves of the litchi tree, it was concluded that the secreta of adult scent gland is synthesized byT.papillosainstead of being obtained from the host plant(Zhangetal., 2009).According to the GC-MS result of secreta from ASG and MTG inT.papillosa, there are 8 common compounds in the ASG secreta of all instars of nymphs, including tridecane,(E)-2-octenal, 5-ethyl-2(5H)-furanone,(E)-2-decenal,(E,E)-2-4-decadienal, dodecane, undecane and nonenal, and 9 common compounds in the MTG secreta of male and female adults, including tridecane, dodecane, pentadecane, 1-tridecene,(E)-2-hexenal,(E)-2-octenal,(E)-2-hexyl acetate, octyl-cyclohexane and benzophenone.Among the 9 common MTG components, the tridecane content was the highest, accounting for 51.8% and 47.1% in the scent gland secreta of male and female adults, respectively(Wangetal., 2011a, 2015b; Zhaoetal., 2012).Some MTG components, such as(E)-2-octenal dodecane, undecane and tridecane, could elicit electroantennogram(EAG)responses of male and female adults ofT.papillosa.Intriguingly, male adults were attracted or repelled to the MTG components at different concentrations in the Y-tube olfactometer bioassay(Wangetal., 2011a).

There are additional studies on the scent gland secreta ofT.papillosa.The scent gland extracts ofT.papillosapossess inhibition effect on plant and toxicity to other insects.The scent gland extracts had strong inhibition activity against seed germination(including radish, mustard and rice)(Zhaoetal., 2014).In addition, the scent gland extracts had fumigation, contact, and repellent activities to the tested insects(including four aphids and four stored product pest insects)(Jiangetal., 2011a, 2011b).Hence, it is necessary to screen the active substance of scent gland secreta for the development of bio-pesticides and herbicides.

4 PEST FORECASTING

The occurrence of agricultural pests depends on individual development of the constituent population and their interaction with external factors.Such internal and external effects can be manifested by their internal physiological systems, such as the reproductive organs, fat bodies and glands(Qin, 1960).According to the relationship between the development conditions of the internal organs(fat body and ovary)and pest occurrence, anatomy-based pest forecasting methods were proposed in different occurrence periods ofT.papillosa(She and Pan, 1993).To date, the occurrence ofT.papillosais mainly forecasted through the ovarian development of female adults.Based on its morphological changes, the ovarian development ofT.papillosawas divided into 5 stages,i.e.milky white transparent stage, yolk deposition stage, pre-delivery stage, egg-laying stage and the end of egg-laying stage(She and Pan, 1993).It was found that the morphological changes ofT.papillosaovary had close relations to its occurrence(Liu and Gu, 1998).Combined with this method and orchard investigation, the occurrence ofT.papillosahas been accurately forecasted, including its oviposition period, oviposition peak period, and nymph outbreak period, in Guangdong province of China in recent years(unpublished data).The forecasting methods can provide best timing for pest control, so that timely precautions can be conducted to improve the control efficiency ofT.papillosa.

5 PEST MANAGEMENT

Together with the pest forecasting technology, management ofT.papillosamainly depends on chemical control, supplemented by cultural, physical and biological control measures in litchi and longan cultivation so far.

5.1 Cultural control

Recommended farming practices in cultural control ofT.papillosaare:(1)strengthening the orchard management, and timely removing the fallen leaves and weeds in orchards;(2)timely pruning dense, thin, long and diseased branches after harvesting, and then centrally burning these branches;(3)decreasing winter branchlets and flowers, in order to reduce overwintering sites and population density ofT.papillosa; and(4)strengthening irrigation and fertilizer management, in order to improve the resistance of fruit trees and restore the growth of damaged branchlets(Yangetal., 1994; Chenetal., 2009).

5.2 Physical control

The most common method for physical control ofT.papillosais artificial capture.Generally, it is conducted in winter and early spring when the temperature is low, since the overwintering adults aggregate and move slowly during this time.Vigorously shaking the branches can make them fall off the tree, and then burn them centrally.In the different outbreak periods ofT.papillosa, manually removing the leaves with eggs, nymphs and adults can directly reduce its population(Weng and Lin, 1994; Chen, 2008).

5.3 Biological control

To date, there are 2 major biological methods used in control ofT.papillosa,i.e.the protection and utilization of natural enemies and utilization of botanic pesticides.

5.3.1Protection and utilization of natural enemies: Several predatory enemies are identified forT.papillosa, including some spiders, mantids, and birds(Lietal., 2013b; Wangetal., 2015a).But the field observations of predators are limited and the efficacy of possible predators are ambiguous(Johnetal., 2016).Anastatusjaponicas(Hymenoptera: Eupelmidae)and some entomopathogenic fungi are best studied natural enemies specific toA.japonicasin China.However, the efficacy of natural enemy is restricted by artificial production conditions and costs(Chen, 2017).

A.japonicus:A.japonicusis a parasitic natural enemy toT.papillosa.A.japonicuslays eggs in host at any developmental stages ofT.papillosaeggs, and grows by exhausting the nutrition of the host egg and causing the death ofT.papillosa(Puetal., 1962).Since 1960s,A.japonicashas been widely released into field as a biocontrol agent forT.papillosa, and it is reported as the most effective biological control method forT.papillosain Guangdong province of China(Puetal., 1962; Sheetal., 1995).Protection and maintenance of the population advantage ofA.japonicusare crucial for this method.By reducing the use of chemical pesticides, adjusting the release frequency and density, and providing habitat and overwintering sites forA.japonicusvia reasonable thinning the orchards, the field parasitation rate ofA.japonicusinT.papillosaeggs could be as high as 94%(Zhangetal., 2007; Xianetal., 2008; Zou, 2008).

Mermissp.: TheMermissp.(Nematoda: Mermithidae)is another parasitic natural enemy ofT.papillosadescribed in the literature.It was firstly reported in 1988, but with only two reports till now(She and Pan, 1988; Sheetal., 1999).There are four periods in its larval stage: free-living period, pre-parasitic period, parasitic period, and pre-adult period.Larva in the pre-parasitic period entersT.papillosaand larva in the pre-adult period gets out of the host both by penetrating the host body wall, and then plunge into the soil for overwintering.The parasitism rate ofMermissp.reaches 70% in laboratory.The parasitizedT.papillosahas no abnormal phenotypes, but has significantly reduced water content, fat body and haemolymph sugar content than healthyT.papillosahas.Normally,T.papillosadies quickly after parasitism.The internal organs of the survival are damaged so they develop abnormally and have no reproductive capacity.However, there are no update reports forMermissp., and further research(e.g., research on host-finding for parasitoids)is required for application of this nematode.

Entomopathogenic fungi: Entomopathogenic fungi, includingBeauveriabassiana(Hypocreales: Cordycipitaceae),Metarhiziumanisopliae(Hypocreales: Clavicipitaceae)andPaecilomyceslilacinus(Hypocreales: Ophiocordyciptaceae), are alternatives to chemical pesticides and have a broad prospect in biological control ofT.papillosa(Jia, 2005; Fanetal., 2011; Meng, 2017).B.bassianastrain Bbtd1 was isolated from the naturally infectedT.papillosaand reported to have the highest toxicity.The infection rates ofB.bassianaBbtd1 to nymphs and adults ofT.papillosawere reported to be over 90% when used at a concentration of 1.0×107conidia/mL in laboratory tests and field trials.In addition, J813, Ma01 and Ma03 strains ofM.anisopliaehad equal toxicity asB.bassianaand a strong superinfection ability(Liangetal., 2009; Fanetal., 2011; Lin, 2011).TwoP.lilacinusandB.bassianastrains isolated from cadavers of adultT.papillosacould induce the highest mortality in the 2nd instar nymphs ofT.papillosawhen used at a concentration of 1.0×108conidia/mL(Mengetal., 2017).Entomopathogenic fungi offer the advantages of high safety, low pollution, and low risk of pest resistance.However, the efficacy of this method is closely related to strain virulence, developmental stages of target pest, weather and other environmental factors.Furthermore, the target pest could still harm the host plant for a long time after infection of pathogenic fungi until they die.At present, it is believed that the addition of a small amount of pesticides in usage of entomopathogenic fungi can improve the control efficacy againstT.papillosa(Lin, 2005; Huangetal., 2009).

5.3.2Utilization of botanical pesticides: At present, the development and utilization of botanical pesticides for control ofT.papillosamainly focus on the screening of extracts from plants which have repellent effect or insecticidal activity(Chen, 2009).A neem compound(azadirachtin)was applied against >400 species of insects, and proved to be one of the most promising botanical pesticide for integrated pest management(Gemmaetal., 2008).There are only two reports of azadirachtin impact onT.papillosa.Acute exposures of the 1st instar nymphs ofT.papillosato 5.2 mg/mL of neem seed resulted in 51.8% mortality in 24 h(Luetal., 2006).Injection of azadirachtin to litchi tree at a concentration of 0.17 g active ingredient per centimeter trunk diameter resulted in corrected mortality of 45% in nymphs ofT.papillosa(Maireetal., 2006).However, the trunk injection method has many disadvantages.For example, the action concentration of agent is difficult to control, injection really does harm to the tree, and the residue in the fruit may affect the fruit quality(Liuetal., 2007).As one of the most successful botanical pesticides in agricultural use, there is much still to be discovered about azadirachtin to reach most of its potential users.

5.4 Chemical control

Currently, chemical control is the primary tactic to controlT.papillosaby directly reducing the pest population density in China.According to the China Pesticide Information Network, there are 26 pesticide products officially registered with validity in China forT.papillosacontrol in 2020(Table 2).Obviously, the major pesticides forT.papillosacontrol are the pyrethroid with medium and low toxicity, which was proved with better performance than other types of pesticides in chemical control ofT.papillosain laboratory tests(Lietal., 2015).Compared with the conventional spraying method, thermal aerosol spraying of self-developed oiling agent(mixed with 4.5% cypermethrin)was reported to improve the efficiency of prevention ofT.papillosaby 23% in field trials(Wuetal., 2001).However, this method is limited to few chemical agent types and requires a high-level processing technology of the chemical agent.Thus, in terms of the application method of pesticides, it is recommended to use conventional spraying instead of thermal aerosol spraying.

Table 2 Pesticide products officially registered for Tessaratoma papillosa in China

In addition, chemical control timing is important, since the pesticide resistance ofT.papillosavaried seasonally.In the early spring, the overwintering adults gradually stop hibernation, and start to feed and prepare for mating and oviposition.The increased reproductive effort reduces pesticide resistance.Thus, it is the best time for chemical control ofT.papillosaadults in mating and oviposition period.Nymph outbreak period is during April-June.The wax layer and chitin layer ofT.papillosanymphs is thin at this time.It is another good opportunity for chemical application due to high sensitivity of nymphs to pesticide.Adults outbreak in July-August, feed on host plant and prepare for overwintering.In this period, the reduced reproductive output results in increased immunity and detoxification, therefore the control effect forT.papillosais not good(Chen, 2008; Chenetal., 2009; Lietal., 2013b).

6 CONCLUSION AND FUTURE CHALLENGES

The green control technology of pests in orchards becomes an important tactic to develop the pollution-free litchi and longan industry which are the backbone of fruit planting in South China.T.papillosais one major pest that causes huge economic losses in every manufacturing season of litchi and longan.Till now, the studies onT.papillosamainly focus on its biological characteristics, life history, developmental duration, living habits, occurrence feature and spatial distribution.Such studies have laid a foundation for the forecasting and prevention ofT.papillosa.However, the research fields are relatively narrow and in-depth studies are limited, resulting in insufficient understanding ofT.papillosa.More diversified and thorough studies onT.papillosaare required for development of green pest control technology.The development of omics research helps us explore tissues, cells, genes, proteins and the interactions among them, and make systematic analysis of the living body(Karahalil, 2016).However, the information ofT.papillosagenome and transcriptome are poor, with only one transcriptome report currently(Wuetal., 2017).Moreover, there is no report on artificial rearing ofT.papillosacurrently, causing the lack of stable experimental insect and limitation of research process.In future study, we can explore the selection mechanism of host plants, the interaction among host plants, natural enemies and symbiotic bacteria, as well as pesticide resistance from the perspectives of omics, molecular biology, cell biology and other aspects, to provide new clues for green pest control technology ofT.papillosa.