Biological Control Agent Of Hemlock Research Articles

Establishment of the predator Laricobius nigrinus, introduced as a biological control agent for hemlock woolly adelgid in Virginia, USA

Laricobius nigrinus Fender (Coleoptera: Derodontidae), a predatory beetle native to western North America, has been released since 2003 for management of hemlock woolly adelgid (HWA), Adelges tsugae Annand (Hemiptera: Adelgidae), a non-native pest killing hemlocks in eastern North America. Over 420,000 L. nigrinus have been released in the eastern USA from field and lab-reared sources, 14,000 of which were deployed in the Commonwealth of Virginia, USA. In order to determine the establishment rates of L. nigrinus in Virginia, surveys were conducted in 2017 and 2018 at all release sites within this state. During the study, stand-level HWA densities were estimated, and hemlock tree health and predator–prey ratios were quantified. The identification of Laricobius spp. recoveries were made using microsatellite analysis. During the period of the study, L. nigrinus were found to have established at 82% of Virginia release sites and were the primary species recovered (80%). Both Laricobius rubidus (18%) and hybrids (2%) were also recovered. Stand-level HWA densities varied greatly over sites and years but showed a general decline in year two of the study. Establishment at such a high percentage of release sites suggests that the climate in Virginia is suitable for the predator and the insect is adaptable to the wide variety of site conditions where hemlocks typically grow.

Host Range Specificity ofScymnus camptodromus(Coleoptera: Coccinellidae), A Predator of Hemlock Woolly Adelgid (Hemiptera: Adelgidae)

Scymnus (Neopullus) camptodromus Yu and Liu (Coleoptera: Coccinellidae) was brought to the United States from China as a potential biological control agent for hemlock woolly adelgid (Adelges tsugae Annand) (Hemiptera: Adelgidae). Scymnus camptodromus phenology is closely synchronized with that of A. tsugae and has several characteristics of a promising biological control agent. As a prerequisite to field release, S. camptodromus was evaluated for potential nontarget impacts. In host range studies, the predator was given the choice of sympatric adelgid and nonadelgid prey items. Nontarget testing showed that S. camptodromus will feed to some degree on other adelgid species, but highly prefers A. tsugae. We also evaluated larval development of S. camptodromus on pine bark adelgid (Pineus strobi (Hartig)) (Hemiptera: Adelgidae) and larch adelgid (Adelges laricis Vallot) (Hemiptera: Adelgidae); a small proportion of predator larvae was able to develop to adulthood on P. strobi or A. laricis alone. Scymnus camptodromus showed no interest in feeding on woolly alder aphid (Paraprociphilus tessellatus Fitch) (Hemiptera: Aphididae) or woolly apple aphid (Eriosoma lanigerum (Hausmann)) (Hemiptera: Aphididae), and minimal interest in cotton aphid (Aphis gossypii Glover) (Hemiptera: Aphididae) in choice and no-choice experiments. Scymnus camptodromus females did not oviposit on any host material other than A. tsugae-infested hemlock. Under the circumstances of the study, S. camptodromus appears to be a specific predator of A. tsugae, with minimal risk to nontarget species. Although the predator can develop on P. strobi, the likelihood that S. camptodromus would oviposit on pine hosts of this adelgid is small.

Assessment of the potential for hybridisation between Laricobius nigrinus (Coleoptera: Derodontidae) and Laricobius osakensis, predators of the hemlock woolly adelgid (Hemiptera: Adelgidae)

In 2003, Laricobius nigrinus Fender was introduced into the eastern United States as a biological control agent of the hemlock woolly adelgid (Adelges tsugae Annand). Following its release, it was discovered that L. nigrinus was hybridising and producing viable progeny with Laricobius rubidus LeConte, a species native to eastern North America. Recently, Laricobius osakensis Montgomery and Shiyake was imported from Japan into the USA as a potential biological control agent of hemlock woolly adelgid. Hybridisation between L. nigrinus and L. rubidus led to interest in the outcome of interactions between L. osakensis and the other two Laricobius spp. The purpose of this study was to determine if L. osakensis could mate with L. nigrinus, if they could produce hybrid progeny, and whether mating interferes with reproductive output. Laricobius spp. were observed mating directly following emergence and found to be capable of producing sterile eggs in the absence of a mating event. Laboratory and confined field studies found no evidence that L. osakensis and L. nigrinus could produce hybrid progeny and the interaction between the two species did not result in a lower reproduction associated with interspecific mating attempts. Interbreeding should therefore not have an impact on biological control using these species. Fecundity experiments showed that L. osakensis produced eggs earlier in the season and at a higher rate than L. nigrinus, suggesting that L. osakensis may have the potential to be an even more successful biological control agent than L. nigrinus.

Effects of Temperature and Photoperiod on the Aestivo-Hibernal Egg Diapause of <I>Scymnus camptodromus</I> (Coleoptera: Coccinellidae)

Three sequential studies were conducted on the interacting effects of exposure to low (5°C) temperature for 0, 7, 28, 56, or 84 d followed by incubation at 10, 15, or 20°C on the egg diapause of Scymnus (Neopullus) camptodromus Yu and Liu (Coleoptera: Coccinellidae). This beetle was imported from China as a potential biological control agent for hemlock woolly adelgid, Adelges tsugae (Annand) (Hemiptera: Adelgidae). Very few eggs laid and held at a constant 15 or 20°C showed any indication of development. Only eggs exposed to temperature combinations of 5 and 10°C had >50% hatch. Highest percent hatch and fastest development occurred when eggs were held at 5°C for 56 or 84 d followed by holding at 10°C. A model estimated the lower threshold for postdiapause development to be 2°C. The effect of temperature on egg hatch was similar at photoperiods of 12:12 and 16:8 (L:D) h, suggesting egg development is not governed by photoperiod or light exposure. Collectively these data indicate that S. camptodromus eggs laid in the spring and summer go through an aestivo-hibernal diapause that is maintained by warm temperatures and that development resumes when temperatures drop, in parallel with the development of hemlock woolly adelgid. This concurrent development allows S. camptodromus eggs to hatch while hemlock woolly adelgid is laying eggs. This synchrony between the development of S. camptodromus eggs and the overwintering adelgid suggest this beetle may be a good candidate for the biological control of the hemlock woolly adelgid.

DNA barcodes and molecular diagnostics to distinguish an introduced and native Laricobius (Coleoptera: Derodontidae) species in eastern North America

Molecular diagnostics based on DNA barcodes can be powerful identification tools in the absence of distinctive morphological characters for distinguishing between closely related species. A specific example is distinguishing the endemic species Laricobius rubidus from Laricobius nigrinus, a biological control agent of hemlock woolly adelgid introduced into the eastern United States. This is especially important because their larvae are morphologically similar and are often collected together in stands where pines and hemlock grow together. Diagnostic nucleotide differences to distinguish species were determined using 157 L. nigrinus and 205 L. rubidus cytochrome oxidase I (COI) barcode DNA sequences. Two polymerase chain reaction (PCR) assays were developed: PCR followed by restriction length polymorphism (RFLP) and real-time PCR (qPCR) based on hydrolysis probes. The qPCR assay had limited success when applied to L. nigrinus originating from inland vs. coastal populations in the western United States. PCR–RFLP was successful, regardless of sample origin. These two species-specific assays provide a choice of diagnostic tools to best match the available lab equipment and management objectives for those using L. nigrinus as a biological control agent.

Propagation, Synchrony, and Impact of Introduced and Native <I>Laricobius</I> spp. (Coleoptera: Derodontidae) on Hemlock Woolly Adelgid in Virginia

Synchrony and impact of the predators Laricobius nigrinus Fender and Laricobius rubidus LeConte, on hemlock woolly adelgid, Adelges tsugae Annand, were studied in an eastern hemlock field insectary in Virginia. First, a field insectary for propagation of the introduced L. nigrinus was established by planting hemlocks in 2001, infesting them with hemlock woolly adelgid in 2002 and 2003, followed by releasing 258 L. nigrinus in 2003. Initial sampling showed that the native L. rubidus was present in the area. Hemlock woolly adelgid and both Laricobius species populations increased annually, from which 305 F3 L. nigrinus adults were collected and redistributed to forests in 2007. Second, the phenology of hemlock woolly adelgid and Laricobius spp. life cycles were monitored in 2005 and 2006. Adult L. nigrinus (F2) and L. rubidus were active on hemlock from fall through mid-spring and overlapped with second-instar sistentes nymphs through progredientes eggs. The predators' eggs were oviposited and larvae developed (i.e., F3 L. nigrinus) from late winter to mid-spring on progredientes eggs, indicating synchrony with suitable prey life stages. Third, a predator exclusion experiment was used to examine the relationships between the predators and prey in 2005 and 2006. When exposed to L. nigrinus (F2 adults and F3 larvae) and L. rubidus, hemlock woolly adelgid survival and ovisac density were lower and ovisac disturbance was higher than hemlock woolly adelgid protected in cages. The establishment and production of L. nigrinus at a field insectary, synchronization with, and impacts on hemlock woolly adelgid after a small release 2 yr earlier makes it an important potential biological control agent of hemlock woolly adelgid.

Factors influencing aestivation in Laricobius nigrinus (Coleoptera: Derodontidae), a predator of Adelges tsugae (Hemiptera: Adelgidae)

AbstractLaricobius nigrinus Fender is being reared for release as a biological control agent for hemlock woolly adelgid (HWA), Adelges tsugae Annand. HWA is an introduced insect lethal to hemlock trees (Tsuga canadensis (L.) Carr. and T. caroliniana Engelm.) in the eastern United States. In nature, the predator (Laricobius nigrinus) and its prey (HWA) undergo a dormant period in the summer (aestivation). In the laboratory, the aestivation of L. nigrinus has not been synchronized with that of HWA, resulting in significant predator mortality. Four factors (genetics, temperature, photoperiod, and moisture) were investigated for their effects on aestivation in L. nigrinus. Both the number of individuals and the time at which they emerged from aestivation were measured in response to these factors. Temperature was the most important cue for termination of aestivation, and photoperiod was a modifying factor. High temperature and long day length delayed emergence and high moisture levels resulted in greater emergence but did not affect emergence time. Genetics, as represented by broods, was not a major factor in aestivation termination. These results have led to improvement in rearing L. nigrinus, since emergence from aestivation can now be synchronized with the active period of HWA. Increased success in rearing has expedited field releases of L. nigrinus in the eastern United States.

Comparison of Numerical Response and Predation Effects of Two Coccinellid Species on Hemlock Woolly Adelgid (Homoptera: Adelgidae)

The hemlock woolly adelgid, Adelges tsugae Annand, is an introduced pest in North America that is native to Asia, and is causing extensive damage to eastern hemlock (Tsuga canadensis Carriere) and Carolina hemlock (Tsuga caroliniana Englemann) in the eastern United States. We compared two coccinellids imported for biological control of the adelgid: Scymnus ningshanensis Yu et Yao from China and Pseudoscymnus tsugae Sasaji and McClure from Japan. In a laboratory study, we measured the numerical response of each beetle species to a range of prey densities, and in field studies we examined the reproductive success and ability of the coccinellids to reduce populations of the hemlock woolly adelgid. In the laboratory, S. ningshanensis showed a positive numerical response as hemlock woolly adelgid density increased, and P. tsugae showed a density-independent response. In field cages, the presence of S. ningshanensis resulted in negative hemlock woolly adelgid population growth, in contrast to positive growth in both control cages and cages containing P. tsugae. Both our laboratory and field experiments suggest that S. ningshanensis has good potential as a biological control agent of hemlock woolly adelgid.

Comparison of Numerical Response and Predation Effects of Two Coccinellid Species on Hemlock Woolly Adelgid (Homoptera: Adelgidae)

The hemlock woolly adelgid, Adelges tsugae Annand, is an introduced pest in North America that is native to Asia, and is causing extensive damage to eastern hemlock (Tsuga canadensis Carriere) and Carolina hemlock (Tsuga caroliniana Englemann) in the eastern United States. We compared two coccinellids imported for biological control of the adelgid: Scymnus ningshanensis Yu et Yao from China and Pseudoscymnus tsugae Sasaji and McClure from Japan. In a laboratory study, we measured the numerical response of each beetle species to a range of prey densities, and in field studies we examined the reproductive success and ability of the coccinellids to reduce populations of the hemlock woolly adelgid. In the laboratory, S. ningshanensis showed a positive numerical response as hemlock woolly adelgid density increased, and P. tsugae showed a density-independent response. In field cages, the presence of S. ningshanensis resulted in negative hemlock woolly adelgid population growth, in contrast to positive growth in both control cages and cages containing P. tsugae. Both our laboratory and field experiments suggest that S. ningshanensis has good potential as a biological control agent of hemlock woolly adelgid.