Biology Of The Emerald Ash Borer And Its Biological Control

The emerald ash borer (EAB), Agrilus planipennis Fairmaire (Coleoptera: Buprestidae), (= A. marcopoli Obenberger), is an important wood-boring beetle attacking ash trees (Fraxinus spp.) (Oleceae). It mainly distributed in partial Asia countries (China, Japan, Korea, Mongolia) and Far East Russia, while in China it presented in Heilongjiang, Jilin, Liaoning, Shandong, Inner Mongolia, Hebei, Tianjin, Taiwan, etc. The important timber species F. mandshurica and gardening plantations F. velutina were damaged severely in northern China.EAB larvae feed on the phloem, cambium, and shallow sapwood under ash tree bark. It is, therefore, very difficult to detect and control because of its highly concealed life history. In the 1960s', EAB severely infested F. Americana and F. pennsylvanica var. lanceolata in the northeastern China so that the two species ash trees had to be substituted gradually by other trees. And now the EAB also could damage F. mandshurica. Recently, outbreaks of this pest arose on F. velutina in parts of Tianjin and Hebei, China. In June 2002, the EAB was reported to be seriously infesting the white ash (F. americana), green ash (F. pennsylvanica) and black ash (F. nigra) in parts of North America, including Michigan and Ohio of USA, and Ontario of Canada. Subsequently, a large number of ash trees died in those areas, and as yet the infestation areas were continuously enlarging.Because the information on EAB is extremely limited, the detailed biology and life cycle of this pest are scarce, needless to say the effective control strategies. Therefore, we investigated the EAB from 2003 to 2005 to obtain data about its biological and ecological habits, the main results and findings were as follows.(1) The life cycle and biology of the EAB were studied using methods of regular investigations in the forest and observations in laboratory. Results indicated that the EAB are mostly univoltine in Tianjin, northern China. They overwintered as full-grown larvae in shallow chamber excavated in the outer sapwood by themselves. In late July, some late stage larvae began to build overwintering chambers, and all larvae entered the sapwood for dormancy before early November. The EAB pupated in the overwintering chamber in early April to mid May the following year, and the average pupal duration was about 20 days. Inlate April, some newly eclosed adults could be found in the pupal cells, but they had not emerged from the host tree. Adults began to emerge in early May, with a peak in mid May. The average longevity of adults was about 21 days and the adulthood lasted through early July. The adults fed on ash foliages as a source of nutrition. Usually mating was conducted and completed on the leaf or trunk surface of ash tree. Female adults oviposited in mid May and eggs hatched on average in 15.74 days. The first instar larvae appeared in early June. The larval stage lasted about 300 days to complete an entire generation.(2) The number of larval instars and stadium of each instar of the EAB were determined by sampling periodically in forests. Six variables including the body length, body width, width of the pronotum, width of the peristoma, length of the urogomphus, and anteriormost width of the urogomphus were examined under binocular microscope with an eyepiece micrometer for choosing the optimal morphological characters or index for the separation of larval instars. Results showed that the larvae of emerald ash borer have 4 instars. The urogomphus and its length were the best characteristic and variable for the separation of larval instars, and the width of peristoma was the second best according to the overlapping degree of variable. Body length, body width, width of the prothoracic plate, and anteriormost width of the urogomphus were not reliable due to high variation. Morphological characters on the inner side of larval urogomphus are also helped to separate the instars tersely and conveniently. The average duration of each instar measured by the median method of population in field was as follows: 18.14 days for the 1st instar, 11.23 days for the 2nd instar, 10.71 days for the 3rd instar, and 261.20 days for the 4th instar. The total larval stage duration was 301.28 days naturally.(3) The species of natural enemies attacked EAB in every stage have been investigated in Tianjin, Jilin, Liaoning, and Helongjiang with methods of forest investigation and laboratory rearing. A total of 16 species of natural enemies were discovered primarily, including 6 parasitoids, 2 woodpeckers, 4 ants, 1 spider, 1 mite, and 2 pathogenic microorganisms. The dominant parasitic natural enemies in the host larval stage were Spathius agrili Yang (Hymenoptera: Braconidae) (Tianjin) and Tetrastichus planipennisi Yang (Hymenoptera: Eulophidae) (northeastern China). Both the two parasitoids were new species. In addition, a Scleroderma sp. which parasitized EAB pupa found in Tianjin also showed better utilization prospects in biological control.(4) Spathius agrili Yang is a new important obligatory natural enemy ectoparasitizing the larvae of emerald ash borer, Agrilus planipennis Fairmaire. The wasp generally occurs 3-4 generations, seldom 2 generations, one year in Tianjin, China, and overwinters in host gallery as a full-grown larva in the cocoon. The generations of S. agrili overlaps heavily as their so varied emergence time of overwintered adults. The sex ratios (female: male) are about 3:1. The adults can live 29.14 days for female and 23.64 days for male in average when fed with 20% honey solution. And the longevities decrease sharply without nutrition supplementation. Amphigony is the natural mode of reproduction, but female also can oviposit without mating under certain conditions and the progeny from parthenogenesis are all males. The mean oviposition is 23.25 eggs per female in laboratory. S. agrili has 5 larval instars and larvae feed on the host haemolymph gregariously. The developmental generation time from egg to adult wasp is 27-28 days at room temperature 22-26℃. The developmental threshod of the overwintering cocoon and thermal requirement of the whole generation are 14.12℃ and 386.57 degree-days respectively. The natural parasitism rates of this wasp can reach 40%-60%, which suggests that the parasitoid has excellent potentials for utilization on the biological control of emerald ash borer.(5) The spatial patterns of the larvae of EAB, the parasitoid Spathius agrili Yang, and pecks caused by woodpeckers preying EAB were investigated in forests. Results showed that the larva of EAB were distributed in a negative binomial fashion horizontally in forests, as was the parasitoid S. agrili and woodpecker attacks. In the vertical direction, the EAB larvae were distributed on ash trunks from above ground to 3.37 m in height, with a peak around 1.50 m. Natural enemies also favored searching in this range.(6) The differences of emergence date between overwintering S. agrili and its host, parasitism rates at different periods in forests, relations between parasitism rates and host densities, and relationships between ovipositions of braconid wasp and body sizes of host larvae were investigated using methods of regular surveys in forests and observations in laboratory. Results revealed that the emergence of S. agrili was more than one month later than that of its host, suggesting good synchrony between parasitoid emergence and host availability. The overwintering S. agrili emerged from mid June to mid August with the peakin July in 2003, while the emergence date lasted from late May through late July with peak in late June to early July in 2004. But the EAB emergence period ranged in mid-late May in 2003, and in mid April through mid May in 2004 (in laboratory). The emergence date of natural enemy asynchronously inosculated with the optimum developmental instars of the earliest host larvae, which was the results of a long-term co-evolution between the two species. It was also revealed that the parasitoid S. agrili could be a specialized natural enemy. The natural parasitism rates of EAB larvae attacked by S. agrili gradually increased on the whole at different periods in forests. The body sizes of host larvae, i.e. larval instar, affected the decision of parasitoid S. agrili to lay eggs or not. This wasp oviposited only on those host larvae with a prontum and body width more than 1.5 mm and a body length in excess of 12 mm, i.e. the 3rd or 4th instar larvae. Egg deposition by female wasps showed no significant relations with body sizes of host larvae only if the parasitoids accepted hosts. These findings would consequentially contribute to the further successfully biological control of the borer.(7) The mass rearing techniques of natural enemy Spathius agrili Yang parasitized the EAB, were researched in laboratory using 4 approaches. Also host ranges of the braconid were determined with methods of investigation in forests plus examination in laboratory. Primary results showed that the methods of 'branch segment' and 'paper package' succeeded in artificial production of the parasitoid under room conditions. S. agrili could exactly distinguish host larva which had been parasitized from healthy ones. At present, no any other insect was found to be the host of this braconid except the EAB. The possibilities for the rearing of parasitoid larvae using non-host larvae as substitute feed were also studied and discussed.Through these investigations and researches, the biology and ecology of the emerald ash borer and its important parasitic natural enemy Spathius agrili Yang have been solved primarily as well as the relationships between the two insects. These findings would consequentially contribute to the further successfully biological control of the bark beetle.