For. Stud. China, 2009, 11(1): 24–27 DOI 10.1007/s11632-009-0009-2

RESEARCH ARTICLE

Damage characteristics of three boring pests in Artemisia ordosica ZONG Shi-xiang1, LUO You-qing1*, CUI Ya-qin1, WANG Jian-wei1, YAN Wei1, LIU Ai-jie1, Kari HELIÖVAARA2 1

Key Laboratory for Silviculture and Conservation of Ministry of Education, Beijing Forestry University, Beijing 100083, P. R. China Department of Applied Biology, University of Helsinki, Helsinki FI-00014, Finland

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Abstract The population density and distribution of the three major boring pests, Holcocerus artemisiae, Sphenoptera sp. and Adosopius sp. were studied in their host plant Artemisia ordosica. Results show that the larvae of H. artemisiae mainly bore the rhizome of A. ordosica, but also the larvae of Sphenoptera sp. and Adosopius sp. bore the rhizome of A. ordosica. The adults of Sphenoptera sp. and Adosopius sp. feed on the leaves of A. ordosica as a nutritional supplement. The distribution of the larvae of three pests in their host plant is completely different. H. artemisiae and Sphenoptera sp. are distributed in the whole A. ordosica plant. The newly hatched larvae first feed on the phloem and xylem of stem and then burrow to the roots before the winter of the same year. However, Adosopius sp. is distributed mainly in the roots. The newly hatched larvae move from the phloem to the xylem, and gradually damage the lower part of the trunk. The larvae of the three pests sometimes coexist in a single A. ordosica plant. However, the probability of the coexistence of the three pests is very small. The highest probability of coexistence of two pests was observed in Sphenoptera sp. and H. artemisiae, while the probability of coexistence of Sphenoptera sp. and Adosopius sp. as well as H. artemisiae and Adosopius sp. was smaller. The reasons for coexistence or its avoidance are not only related to the amount of food provided nor the volume of larval living space in a single A. ordosica plant, but also related to the development characteristics of different kinds of larvae and interspecific competition. Key words Artemisia ordosica, Holcocerus artemisiae, Sphenoptera sp., Adosopius sp., damage characteristics, population ecology

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Introduction

Artemisia ordosica is a semi-shrub species which belongs to the family Asteraceae. The species is resistant to drought, barren soil, wind erosion and sand. A. ordosica produces a lot of fruits, and it is easy to collect seeds. The species grows rapidly, and it has a strong function in sand fixation and other characteristics. As a result, it is an important plant of desert shrubbery in northwest China, and plays a particularly important role in desertification control (Tang and Yang, 2005). In recent years, due to e.g. the change of the ecological environment, frequent outbreak of disastrous weather, such as the successive drought for 4–5 years, and unique geographical environment in the arid and semi-arid northwestern regions, A. ordosica has been widely damaged by several boring pests in Inner Mongolia, Ningxia regions, Shaanxi and other provinces in China. Attacks have resulted in death of A. ordosica in large areas, and greatly decreased its function in sand fixation in desert and semi-desert areas. This has caused tremendous losses to ecological environment in northwest China. At present, there are not too many research projects dealing with the boring pests in A. ordosica in China or aboard. There are only preliminary reports on its morphological characteristics, generation development, *

Author for correspondence. E-mail: [email protected]

living habits and control methods (Zhen, 1988a, 1988b; Hua et al., 1990; Gao and Tian, 1998; Rong et al., 1999; He and Zhang, 2000; He, 2004; Liu and Hou, 2005; Tang and Yang, 2005; Chen et al., 2006). No report refers to the population distribution and dynamic change of different species of pests in A. ordosica. In order to effectively control the damage, occurrence and further spreading of the pests, we study the damage characteristics and dynamic population changes of the boring pests in A. ordosica in Ningxia.

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Study area

Yanchi County lies in the eastern part of Ningxia, located in the junction of Shaanxi-Gansu-Ningxia-Inner Mongolia provinces (regions) and the southern edge of Mu Us Desert. It belongs to the transitional belt from Erdos platform to the Loess Plateau, where the climate is dry with very little rain in normal years. The average annual rainfall is less than 300 mm, and the average annual evaporation is about seven times higher than the rainfall. Consequently, the area belongs to a typical arid desert region. The desert vegetation covers 1880 hm2 in this area, most of which is covered by A. ordosica (more than 70% of the total area).

ZONG Shi-xiang et al.: Damage characteristics of three boring pests of Artemisia ordosica

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Research methods

Firstly, the species of boring pests and damage degree of A. ordosica plants were investigated in Erdos City and Alashan League of Inner Mongolia Autonomous Region, Yulin City of Shaanxi Province, Yanchi County, Lingwu and Zhongwei cities of Ningxia Hui Autonomous Region in early August of 2006. Secondly, a sample plot (50 m × 50 m) growing A. ordosica was selected in Gaoshawo Town of Yanchi County, where small numbers of Glycyrrhiza uralensis and Sophora alopecuroides were growing as mixed shrubs, the average height of A. ordosica was 45.3 cm and the ground diameter was 2.3 cm. From mid-August till the end of 2006, according to the distribution of A. ordosica, one shrub was selected at equal intervals of 2 m, each of underground roots and overground parts was divided into several parts using 2 cm as an interval and dissected to find individuals of the pest species in details. The number of individuals was counted and their developmental stages within each section in each A. ordosica were defined. In total, 454 plants were analyzed.

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Results

The major species of boring pests in A. ordosica were Holcocerus artemisiae, Sphenoptera sp. and Adosopius sp. Of these insects, H. artemisiae mainly causes damage to roots of A. ordosica during its larvae stage. This damage causes the whole plant to wither especially when most of the roots are bored to be hollow. Sphenoptera sp. and Adosopius sp. not only damage roots of A. ordosica during the larvae stage, but also feed on leaves of A. ordosica as a nutritional supplement during the adult stage. This feeding on the foliage seriously affects the growth of A. ordosica and results in the weak growth of A. ordosica. In addition, a single A. ordosica plant is often harmed by 2–3 different boring pests together, thus the damage extent of A. ordosica worsens. 4.1 Population densities of the boring insects in A. ordosica The statistical results after dissecting 454 A. ordosica plants show that Sphenoptera sp. is the most abundant pest in the insect-infected plants (46.70%). The second most abundant species is H. artemisiae (23.35%), followed by Adosopius sp. (11.67%). The population densities of the larvae of these three pest species in the damaged plants are shown in Fig. 1. In a single plant, the population density of Sphenoptera sp. can be as high as 16 larvae, while that of Adosopius sp. is six, and that of H. artemisiae only four. However, in more than 97% of the cases, the population density of the

Fig. 1 Population density of the larvae of three boring pests species in the damaged A. ordosica plants

three species is less than four larvae per plant. Until now, the three pests have caused 400 hm2 of A. ordosica shrubbery to wither totally in northwest regions of China, and the damage is getting worse year by year. 4.2

Distribution of larvae in A. ordosica

The distribution and aggregation of the three pests in A. ordosica are completely different (Fig. 2). H. artemisiae and Sphenoptera sp. are distributed in the whole plant, but they are showing a trend first to increase and then to decrease from top to bottom. Adosopius sp. is only distributed in the roots. This distribution pattern is mainly defined by the oviposition behavior of the female. The females of H. artemisiae and Sphenoptera sp. often lay eggs to stems of A. ordosica. After hatching, the larvae first feed on the phloem of stem, then bore into xylem and gradually move from the stems to the roots. This behavior leads to the fact that their population density on the overground part of stem showed a gradual increase trend from top to bottom. Female adults of Adosopius sp. often lay eggs in rhizomes growing 2 cm underground. After hatching, the first instar larvae move from the phloem to the xylem. Gradually they burrow downwards in xylem, and finally the larvae are distributed in the underground parts of the stem. Sphenoptera sp. and H. Artemisiae were mainly distributed in the roots within the scope of 14 cm. The population density gradually decreased as the depth increased, and about 95% of the larvae were aggregated in the roots within 8 cm below soil surface. The distribution pattern of Adosopius sp. shows a trend of first increasing, then decreasing and finally reincreasing. The roots damaged by this species were up to 16 cm below soil surface. About 96% of the larvae were distributed in the roots 0–12 cm underground. However, about 30% of the larvae were concentrated in the roots 4–6 cm underground. In the stem above soil surface, the larvae of Sphenoptera sp. were distributed up to 12 cm, while the lar-

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Forestry Studies in China, Vol.11, No.1, 2009

vae of H. artemisiae reached only 6 cm. The population number of the two pests gradually decreased with the increase of the height of plant. According to the statistical data gathered, slightly more than 50% of the pests in the stem were distributed on the surface of rhizome. In the end of December 2006, when investigating the overwintering stages and pest situation in the area, we found that Sphenoptera sp. and H. artemisiae overwintered as larvae. Most of these larvae were distributed in the roots, and only a few stayed in the stem. In other words, from mid-August till the moment before overwintering, the newly hatched larvae occurring in stem gradually burrowed to the roots to overwinter. This movement of the larvae gradually resulted in the decrease in population density in the stems, and, at the same time, increased the population density in the roots. 4.3

Coexistence of three pests in A. ordosica

Sampling in the field showed that the damage degrees caused by the three pests in A. ordosica were very different. The damage rate of Sphenoptera sp. was the

highest (80.68％), followed by H. artemisiae (40.54%) and Adosopius sp. (19.7%). The results of coexistence are shown in Table 1. The number of cases that all the three species were damaging together a single A. ordosica was very small, only 5.68% of all damaged plants. Certain patterns also emerged when the coexistence of two species out of three was analyzed. It was revealed that the proportion of coexistence for Sphenoptera sp. and H. artemisiae (19.70%) as well as Sphenoptera sp. and Adosopius sp. (8.33%) were higher than the proportion of coexistence for H. artemisiae and Adosopius sp. (1.52%). The main explanation for avoiding coexistence is the big size of larvae causing intense interspecific competition for food. The larvae of H. artemisiae and Adosopius sp. are big, and they need a lot of food and living space to complete their life cycle within the plant. However, a single A. ordosica can provide very limited resources of food and living space. Consequently, H. artemisiae and Adosopius sp. rarely harm the same A. ordosica plant together. The larvae of Sphenoptera sp. are smaller, and the requirements for food and living space are relatively small. When it lives together with H. artemisiae or

Fig. 2 Distribution and aggregation of the three boring pests in stems and roots of A. ordosica plants Table 1 Pests

Coexistence of the three boring pests in damaged A. ordosica plants One kind of pest Coexistence of both pests A B C A×B A×C B×C Number of damaged A. ordosica (plants) 124 36 11 52 22 4 Proportion of coexistence (%) 46.97 13.64 4.17 19.70 8.33 1.52 Note: A is Sphenoptera sp.; B is H. artemisiae; C is Adosopius sp.

A×B×C 15 5.68

ZONG Shi-xiang et al.: Damage characteristics of three boring pests of Artemisia ordosica

Adosopius sp., there will be no interspecific competition in the same A. ordosica plant. As a result, a single A. ordosica can fully satisfy their respective requirements of growth and development. In addition, the probability of the coexistence of Sphenoptera sp. and H. artemisiae in the same plant is two times higher than that of Sphenoptera sp. and Adosopius sp. The reasons for this are probably associated with interspecific competition, or A. ordosica itself and the surrounding conditions. However, these issues have still to be studied further.

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Project of Science and Technology for the 11th Five-Year Plan in China (Grant No. 2006BAD08A1001) and the Program for Changjiang Scholars and Innovative Research Team in Universities (PCSIRT0607).

References

Acknowledgements

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This work was financially supported by the National

(Received September 10, 2008

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Discussions

Of the three boring pests in A. ordosica, Sphenoptera sp. has the greatest population density and the widest distribution, followed by H. artemisiae and Adosopius sp. However, because the larvae of H. artemisiae and Adosopius sp. are bigger and need more food and living space, their harm to A. ordosica is much more severe than that caused by Sphenoptera sp. In addition, the three pests sometimes coexist, which causes serious harm to their host plant. The probability of the coexistence of the three pests in a single A. ordosica plant is relatively small. However, the coexistence of two pests, Sphenoptera sp. and H. artemisiae has the greatest probability, while the probability of the coexistence of Sphenoptera sp. and Adosopius sp., H. artemisiae and Adosopius sp. is slightly smaller. The reasons for coexistence or its avoidance are related to not only the amount of available food nor the volume of living space in a single A. ordosica plant, but also interspecific competition between different pests. Yet, further studies on the competition mechanisms between the pests are needed.

Accepted December 6, 2008)