Xie Na

Ecological Forestry Development Center of Daiyue District,Tai’an 271000,China

Abstract Rhynchaenus empopulifolis is a defoliator harming poplar,and its occurrence is reported in Tai’an area for the first time.Through indoor rearing and forest investigation,the morphological,biological and damage characteristics of R.empopulifolis in Tai’an were observed,and color pictures of each instar and local characteristics of adult as well as damage symptoms were provided,in order to lay a theoretical basis for rapid identification and precise control of R.empopulifolis.

Keywords Rhynchaenus empopulifolis;Morphological characteristics;Biological characteristics;Damage status

Rhynchaenus empopulifolis Chen,belonging to Rhynchaenus,Curculionidae,Coleoptera,is a defoliator harming poplar[1].The larvae and adults of R.empopulifolis damage poplar leaves for more than 6 months from poplar leaf bud germination to defoliation each year.Mature forest and young forest are attacked,and 2-4 circular holes with the diameter of 5 mm are generally observed in infected poplar leaves.There are more than 10 holes in infected leaves,or even leaves of the whole plant are full of holes in severe cases.Most infected leaves are fallen off in advance,which affect photosynthesis and normal growth of trees,causing serious damage to forestry production.R.empopulifolis was first discovered in 1985 in Mengyin County,Linyi City,Shandong Province,and in 1986 it was first reported to harm poplar varieties such as Populus simonii and P.Canadensis in China[2].In 1988,the new species was officially named R.empopulifolis by Chen et al.[3].In Heilongjiang Province,R.empopulifolis spread everywhere with expressway as the core.The occurrence number is increasing and the occurrence is changing from occasional to frequent,and from alien species to native species.The occurrence area is over 20 000 hm2,ranking the 19thplace among the 59 major local forestry pests,and belonging to high risk level[4].Currently,R.empopulifolis has been found in Heilongjiang,Jilin,Liaoning,Inner Mongolia,Xinjiang,Gansu,Beijing and Shandong[5-11].In early April 2019,R.empopulifolis was found in Dadoushan Village of Daiyue District,Tai’an City,which was then followed and monitored.By reviewing a large number of literature,it was found that the local characteristics of adult leg,elytron and membranous wing,the morphological characteristics of egg and newly hatched larva,the characters of egg-chamber and the habits of newly hatched larva should be supplemented and explained.Due to differences in geographical regions and climatic conditions,the occurrence time of each instar will be different,which will affect precise prevention and control.Moreover,the life history of R.empopulifolis in Tai’an should be cleared.Taking R.empopulifolis in Tai’an area as the research object,the morphological and biological characteristics and occurrence regularity of the pest were observed,and color pictures of each instar and local characteristics of adult as well as damage symptoms were provided,in order to lay the reference for rapid identification and efficient,precise and convenient prevention and control of R.empopulifolis.

1 Materials and Methods

1.1 Survey of damage status The poplar forest in Dadoushan Village,where R.empopulifolis was first discovered in Tai’an area,was selected as the center point to diffuse to the areas and nurseries with concentrated planting of poplar trees.Spot investigated was mainly adopted,and sample plots were established in areas with severe damage for concentrated investigation.Standard plants in sample plots were sampled by diagonal sampling method.The percentage of complete leaves in standard plants was recorded,and the damage status was analyzed.

1.2 Observation of morphological,biological and damage characteristics Field observation was conducted from April 2019 to April 2020 in poplar forest in Dadoushan Village.Combined with indoor rearing,the morphological characteristics of R.empopulifolis were supplemented via anatomical microscope observation,and the biological characteristics and damage characteristics of R.empopulifolis in Tai’an area were clarified.

2 Results and Analysis

2.1 Damage status R.empopulifolis still occurred sporadically in Tai’an area.However,in Dadoushan Village where R.empopulifolis was first discovered,some poplar trees were seriously damaged,and the percentage of complete leaves was about 50%.

2.2 Supplementary description of morphological characteristics

2.2.1 Adults.Adult(2.5-3.1)mm×(1.3-1.9)mm,shaped like black sesame seeds;forewings translucent,covered with backward appressed white cilia,elytron densely ciliate at margin;unilateral elytron subcircular at apex,length-width ratio ca.2.5∶1;depression zones 8,subparallel,each with nearly uniform 20-30 black punctures(Fig.1a,1b,1c).Underwing membranous,longest-widest ratio ca.3.5∶1,transparent, distinct-veined, flabellatewinged(Fig.1d),suitable for flying.Mesopedes tibia with short brush hairs at inner depression of front end;hind legs stout,suitable for jumping;leg praetarsus with hooked claw at terminal(Fig.1e).

Fig.1 Adults of Rhynchaenus empopulifolis Chen

2.2.2 Eggs.Egg long-elliptic(Fig.2a),ca.0.5 mm×0.27 mm(Fig.2a,2b),transparent,covered with mucus,milky-white at first,becoming pale yellow.

Fig.2 Eggs of Rhynchaenus empopulifolis Chen

2.2.3 Larvae.Newly hatched larvae translucent,with green food remnants(Fig.3a);mature larvae ca.3.5 mm×1.5 mm(widest),attenuate from thorax to abdomen(Fig.3b);concealed in pupal cells(Fig.3c).

Fig.3 Larvae and Pupal cell of Rhynchaenus empopulifolis Chen

2.2.4 Exarate.Exarate pale yellow;compound eyes black brown;antennae,beaks,legs and wing buds clearly visible,folded below thorax and abdomen(Fig.4a);pupae sparsely irregularly spotted(Fig.4b).

Fig.4 Pupa of Rhynchaenus empopulifolis Chen

2.3 Biological characteristics

2.3.1 Life history.R.empopulifolis occurred one generation per year in Tai’an area,and survived winter as adults in poplar litters,clods debris or topsoil.Overwintering adults began to harm trees in middle March of the next year,and entered peak post-hibernation period in late March;all adults post-hibernated in early May.After feeding for about a week,adults could mate and spawn.The egg period was from late March to middle May,and early April was the peak hatching period.The larval period was from early April to middle May.In middle and late April,leaves of poplar trees damaged by R.empopulifolis were full of circular holes and irregular leaf mines(Fig.5a,b).A large number of pupal cells of R.empopulifolis leaped on the ground and then gathered under coverings or litters(Fig.5c).The pupal period was from middle April to late May,and the peak emergence period of adults was from May 5 to May 8.After emergence,adults continued to climb trees to harm and feed on mesophyll.As the temperature dropped,adults gradually got down from trees in late September to go through the winter,and all adults got down from trees in late October to survive the winter.

Fig.5 Damage status of Rhynchaenus empopulifolis Chen and aggregation distribution of pupal cells

2.3.2 Living habits.The eggs,larvae and adults all had hidden life.Adults were good at flying and jumping,but did not make long-distance migration.When the average daily temperature was above 10℃,adults were activated.More than 80% of the adults were on east and north sides of trunk,but less on south and west sides.Adults absorbed the mucus secreted by germinated buds of poplar and further fed on lower epidermis and mesophyll of young leaves,and leaves were dorsally punctuate.After nutritional supply for about a week,adults mated and spawned.Each mating usually lasted about 15 min,and adults mated for several times.Before spawning,adults often bit an egg chamber out of lower epidermis of young leaves near margin with mouthparts,and egg chamber was located in mesophyll.A female adult usually lay an individual egg,2-4 eggs each leaf.Egg chambers usually located on both sides of main vein between widest part of leaf transverse axis and leaf tip,which was close to leaf tip and opened toward tip (Fig.6a,indicated by arrow).At the beginning of April,eggs began to hatch,and larvae dived into mesophyll from egg chamber end to mdivein.After feeding for 1 d,newly hatched larvae grew to 1 mm,and leaf mines reached 10 mm.Newly hatched larvae were small,leading to slender leaf mines.With the increase of larva appetite,leaf mines were broadened,0.5-2.0 mm wide,20-50 mm long.Leaf mines were green in the early stage,with dark green excrement(Fig.6b),and light yellow worm bodies in leaf mines could be seen from dorsal surface of leaves(Fig.6b,indicated by arrow).Leaf mines were blackbrown due to local leaf necrosis and fecal accumulation after feeding by larvae(Fig.6c).Larvae made subcircular pupal cells(diameter 5 mm)at the end of leaf mines,and ate all the mesophyll in the cell;pupal cells dehydrated,and mature larvae fell off with pupal cells(Fig.6d).Mature larvae in falling pupal cells jumped by elongation of insect body.It was the peak shedding period of pupal cells on April 18,and pupal cells jumping on the ground were spectacular.When pupal cells jumped and gathered under coverings or litters,they generally stopped jumping without external interference,entering pre-pupal stage.Mature larvae pupated in pupal cells,and rupture of pupal cells did not affect the pupation.Adults all emerged neatly at night,usually from 20:00 to 23:00,without phototaxis.After emergence,adults could climb trees to feed on leaves.

Fig.6 Egg chambers and leaf mines of Rhynchaenus empopulifolis Chen

3 Discussion

Since R.empopulifolis was recorded in the 1980s,it has been reported to harm poplar trees from Harbin,Heilongjiang Province in the north to Linyi,Shandong Province in the south.In April 2019,R.empopulifolis was first found in Tai’an,and the invasion route currently is unclear.Poplar is a main tree species planted in Tai’an,and most are fast-growing and high-yield forests.More importantly,Gaoqiao forest farm,the largest poplar seedling breeding base and poplar improved seed demonstration base,is located in Ningyang County,Tai’an City[12].According to incomplete statistics,the total area may have exceeded 53 000 hm2,so it is particularly important to monitor the occurrence of R.empopulifolis,in order to prevent the spread in a wider range.Since the morphological characteristics,biological characteristics and damage characteristics of R.empopulifolis have been reported[3-4,6-8],due to differences in geographical area and climate conditions,the time nodes of post-hibernation of overwintering adults,pupal cells shedding of mature larvae and adult emergence had a difference of about 10 d with previous reports in Liaoning,Inner Mongolia and Beijing.Post-hibernation time of overwintering adults was gradually advanced from north to south,and climatic condition was one of the important factors.Ten poplar trees were randomly selected in sample plots.With the help of sticky packaging tape,it was observed that more than 80% of adults climbed trees on east and north sides of trunks,and fewer on south and west sides.The reason is still unclear,probably related to wind and air flow.

R.empopulifolis adults treated by low temperature were dissected under anatomical lens.On the basis of previous reports,eight subparallel depressions were found on the elytron,and each was composed of 20-30 subuniform black punctures.The underwing was membranous,longest-widest ratio ca.3.5∶1,transparent,distinct-veined and flabellate-winged,suitable for flying over certain distance.In addition,color images of egg,larva,pupa,each instar of adult,leg,elytron and membranous wing were supplemented,providing the reference for rapid identification and accurate control of the defoliator.

Totally 200 leaves with eggs and 100 leaves with newly hatched larvae were observed under anatomical lens.The results showed that the openings of egg chamber were toward leaf tip,and newly hatched larvae dived into mesophyll from egg chamber end to mdivein.Newly hatched larvae were translucent,with green food remnants.The above results further completed the biological characteristics and damage characteristics of R.empopulifolis.

At present,there are four species of parasitic natural enemies of R.empopulifolis,including Pteromalus miyunensis,P.procetus,P.semotus and Triaspis sp.[13].The parasitic condition of natural enemies will be clarified in the follow-up study,and the biological control methods of R.empopulifolis will be further studied.