Feasibility of Controlling Ixodes Scapularis Ticks (Acari: Ixodidae), the Vector of Lyme Disease, by Parasitoid Augmentation

FORUM Feasibility of Controlling Ixodes scapularis Ticks (Acari: Ixodidae), the Vector of Lyme Disease, by Parasitoid Augmentation E. F. KNIPLING1 AND C. D. STEELMAN2 J. Med. Entomol. 37(5): 645Ð652 (2000) ABSTRACT A theoretical analysis of the feasibility of controlling tick populations (Ixodidae) by Downloaded from https://academic.oup.com/jme/article/37/5/645/954424 by guest on 25 September 2021 the release of reared Ixodiphagus parasitoids in tick ecosystems yielded promising results. The analysis suggested that if reasonable progress could be made in mass-rearing the parasitoids, it would be possible to control the blacklegged tick, Ixodes scapularis (Say), the vector of Lyme disease, by this biological control procedure. Lyme disease has become the most important vector-borne disease in the United States. In a Þeld-release experiment conducted in Africa by members of the Inter- national Center for Insect Physiology and Ecology, effective control of Amblyomma variegatum (F.) was obtained by the release of Ixodiphagus parasitoids in tick habitats. Encouraging theoretical results along with the encouraging results of a Þeld-release experiment indicate the need for civil and political leaders in countries where ticks are a major problem to sponsor strong and well- coordinated research initiatives focused on the development of this new method of dealing with tick problems. KEY WORDS Ixodiphagus hookeri, Ixodes scapularis, Amblyomma variegatum, acaracide, parasitoid augmentation TICKS ARE AMONG the most important pests that affect high rates of parasitism, but parasitoid populations the welfare of humanity. Disease causing organisms seldom provide adequate control of their hosts. transmitted to humans and livestock are a major ob- A comprehensive study of the role that parasitoids stacle to sound agricultural economies in many parts can play in controlling insect pests (Knipling 1992) of the world. In the United States, society is primarily indicated that if insect parasitoids were to play a prom- concerned with the role that ticks play in the trans- inent role in controlling pests it would be necessary to mission of diseases to humans. For the most part, this augment the number of parasitoids in pest host eco- concern involves the blacklegged tick, Ixodes scapu- systems by artiÞcial means. Nature imposes stringent laris (Say), which is the vector of Lyme disease (Den- limits on the rate of parasitism that natural populations nis 1995). This disease has emerged as a major health can cause. Therefore, if parasitoids were to play a problem in areas where high numbers of deer (Odoco- prominent role in managing ticks, the number of para- lieus virginianus Zimmerman) have developed (Ost- sitoids in the natural population must be augmented feld, 1997). Deer are the primary host for adult I. by artiÞcial means. The only feasible way to increase scapularis. the number of parasitoids in natural environments Acaracides have long been the chief method of would be to mass produce key species and release the controlling ticks in various parts of the world. Al- numbers required in the pest habitats. Currently, we though they play an important role in this regard, ticks have little information relative to the number of para- still cause billions of dollars in losses to agriculture sitoids required to achieve adequate control. There- production year after year. Like insects, ticks have fore, deductive procedures designed to estimate how developed resistance to chemical control agents. many parasitoids would have to coexist with their pest Acaracides are costly to use and they can also cause hosts to achieve adequate control were initiated. Such hazards to people and the environment. Therefore, estimates have been made for a wide range of insect there is continuing interest in the development of pest parasitoids previously discussed by Knipling more acceptable ways to cope with tick problems. This (1992). has included the possibility of controlling ticks by the This article presents the potential role that tick use of tick parasitoids. Ticks serve as hosts for a num- parasitoids can play directly in the management of ber of hymenopterous parasitoid species that belong ticks and indirectly in lowering the incidence of the to the family Encyrtidae. The parasitoids cause fairly disease-causing organisms that they transmit. Specif- ically, the potential role of Ixodiphagus parasitoids for the control of Ixodes scapularis (Say), the primary vector of Lyme disease, is examined. However, the 1 Deceased. Former Director, Entomology Division, USDA-ARS, Beltsville, MD. fundamental principles apply for all ticks, including 2 Department of Entomology, Room 320, Agriculture Building, Uni- species that are major obstacles to livestock produc- versity of Arkansas, Fayetteville, AR 72701. tion in various parts of the world. 0022-2585/00/0645Ð0652$02.00/0 ᭧ 2000 Entomological Society of America 646 JOURNAL OF MEDICAL ENTOMOLOGY Vol. 37, no. 5 Tick Parasitoids. Two species of tick parasitoids are behavior is important in the development of proce- known to exist in the United States. They are Ixo- dures for mass-rearing the parasitoids. diphagus (Hunterellus) hookeri, Howard (1907) and I. Ixodiphagus parasitoids are basically effective para- texanus, Howard (1908). A number of other closely sitoids and the natural rates of parasitism are quite related species exist in other parts of the world (Cole high. Data obtained by Mather et al. (1987), Hu et al. 1965). The biology and behavior of the two known (1993), and Stafford et al. (1996) indicated that max- species in the United States have been studied by a imum rates of parasitism of I. scapularis nymphs may number of investigators (Wood 1911, Cooley and exceed 40%. However, the average is probably closer Kohls 1928, Smith and Cole 1943, Bowman et al. 1986, to 25%, as observed by Lyon et al. (1998). In other Mwangi et al. 1993). areas and in other species, natural rates of parasitism Engorged tick larvae seem to be the preferred tick Ͼ50% have been observed in A. variegatum (F.) in stage parasitized by the parasitoids. However, as ob- Africa by Mwangi et al. (1993) and in Hyalomma- served by Cooley and Kohls (1928), the parasitoid anatolicum anatolicum (Koch) in India as recorded by Downloaded from https://academic.oup.com/jme/article/37/5/645/954424 by guest on 25 September 2021 eggs pass into the nymphs during the molting process Shastri (1984). Although these are rather high rates of and then complete their development in engorged natural parasitism, they are not high enough to achieve nymphs on the host. The immature parasitoids com- adequate tick control. plete their development in Ϸ45 d (Wood 1911). Most efforts made to control ticks by the use of The host-searching behavior of the parasitoids is not parasitoids have involved classical biological control clear. Whether the parasitoids actively search for the (Cooley and Kohls 1933, Smith and Cole 1943). But for hosts or wait for the hosts to come to the parasitoid is the most part these efforts have not been successful. not known. It is also not clear if the tick larvae or The feasibility of controlling ticks by releasing large nymphs are parasitized on or off the hosts. Some ob- numbers of parasitoids in tick habitats has received servers report that the parasitoids have been seen little consideration by researchers. However, Mwangi searching for hosts on animals. In the laboratory, how- et al. (1997) undertook a small augmentation exper- ever, many investigators have observed that the para- iment in Africa that yielded encouraging results. The sitoids will readily parasitize engorged larvae or un- results of that important experiment will be discussed engorged nymphs while off the hosts. in more detail later. Lyon et al. (1998) questioned whether the parasi- Estimating the Host-Finding Capability of Parasi- toids normally parasitize the larvae and nymphs when toids. To analyze the feasibility of controlling a pest by on the host. We raise the same question on theoretical the augmentation procedure it is important to know grounds. It is possible that most host searching by the how many hosts a female parasitoid normally parasit- parasitoids is for engorged larvae and unengorged izes during her lifetime. If that is known, it is possible nymphs while they are off the host. The adult para- by extrapolation to estimate the number and propor- sitoids are reported to be weak ßiers. They may be tion of coexisting host populations that can be para- incapable of readily getting on large hosts such as deer sitized by a given number of parasitoids. However, and cattle. In the investigations conducted by Shastri there is no data found relative to the host-Þnding (1984) and Mwangi et al. (1997) the implications are ability of any parasitoid species. Therefore, indirect that engorging nymphs are parasitized while on the procedures have been used to estimate the host Þnd- cattle. This, however, would be inconsistent with the ing of a wide range of parasitoid species (Knipling known parasitism behavior involving the larval and 1992). The assumption was made that a parasitoid unengorged nymphal stages. Thus, important ques- coexisted with its host when the two populations were tions remain about the behavior of tick parasitoids that stable. Thus, the female parasitoids under such con- could inßuence the way the parasitoids should be used ditions would, on average, produce one adult female for maximum effectiveness. progeny. A state of stability is a universal biological The behavior of tick parasitoids differs in several phenomenon for all animal populations. To estimate respects from the behavior of parasitoids of insects. the host-Þnding ability of the tick parasitoids, a life Most parasitoids have solitary parasitism behavior and table was developed (Table 1). The fecundity of Ixo- will deposit only one egg or larvae in a host, and only diphagus parasitoids is not well known; however, in- one parasitoid can normally mature in a host.

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