Stewardship Overview: Despite recently increased control from Entomophaga, gypsy moth will probably be a factor in North American forest ecology for the foreseeable future and it can certainly be expected to continue to expand its range by natural dispersal and to be moved to more distant areas by human transport. If gypsy moth populations reach release phase an outbreak will usually develop (but not as certainly as pre-1989 due to Entomophaga) and managers must decide whether to control them or let outbreaks run their course. On preserves such as those of The Nature Conservancy, or state "natural areas" which are actually managed as such, only biodiversity related issues (including possible tree mortality) need consideration. In some other contexts such as state parks, scenic and recreational areas, and residential areas nuisance factors may also be important. Risks associated with standard management tactics are discussed elsewhere. BTK, Gypchekand the several chemical biocides currently used are generally effective at suppressing defoliation. Dimilinoften provides somewhat greater reduction of gypsy moth larvae (and has much greater impact on non-target caterpillars etc.) than BTK or Gypchekbut the results are not immediate because it does not kill until the next naturally occurring molt and larvae feed normally until then. The newer Mimicwould likely kill faster since it induces a premature molt. Carbaryl (Sevinand BTK typically affect larvae at their next feeding, with death within hours from the former, and thus defoliation is greatly reduced byt either treatment within a day or two. Gypcheckcauses a viral disease which takes several days to two weeks to kill, but when applications are timed properly against young larvae it is quite effective at suppressing noticeable defoliation without killing non-targets. Data in Reardon et al. (1996) suggest about an 85% reduction of larvae and about a 50% reduction in defoliation--which is normally enough to prevent significant damage to trees even where heavy defoliation would otherwise be expected. BTK at least as a single application, may not be adequate to prevent significant defoliation in truly extreme situations with thousands of egg masses per acre and the same is probably true of GypchekIt should be stressed that both BTK, which impacts some non-target Lepidoptera, and Gypchekwhich apparently has minimal or no non-target impacts, are normally quite adequate for foliage protection. In such extreme cases where these alternatives would not be adequate, consider the high probability that the outbreak will collapse after the upcoming defoliation without treatment. In some cases it is virtually impossible to assess whether BTK or no action is the preferred alternative in terms of impacts to native biota. Any review of options should always strongly consider no action as a possible alternative.
Concern about defoliation itself is not usually a major issue for managers of biodiversity oriented preserves, but tree mortality sometimes is. Old growth forests should be protected from at least repeated severe defoliation. Gypchekis always acceptable and in most cases BTK should be considered preferable to repeated defoliation in old growth. Old growth forests are not likely to contain Lepidoptera not found in similar adjacent second growth forest and risks to other terrestrial organisms is minimal at worst. For example once surviving specimens at rutgers University were examined and misidentifications such as Heterocampa varia" and a large majority of the Papaipema were corrected, the moth list for old growth Hutcheson Memorial Forest, New Jersey compiled by Moulding and Madenjian (1979) contains no globally rare species among the 410 species encountered in a five year light trapping study. Oligia crytora (2), Cycnia inopinatus (1), Tornos scolopacinarius (1), if correctly identified, are the only possibly state-rare species but the last two are not forest species and C. inopinatus was probably a misidentification for the common C. tenera. One might expect litter feeding Herminiinae to contain unusual species in old growth areas, but there are only common ones in the Moulding and Madenjian list and among the specimens examined and there is no evidence any rare Herminiinae are particularly associated with old growth. There is no reason to believe these findings are highly atypical, so there should be no long-term negative impact from BTK in most old growth areas especially if there are unsprayed similar second growth stands nearby. The Lepidoptera species most likely to be of concern in the context of old growth forests is Diana fritillary but its optimal Appalachian habitats usually are not susceptible to heavy defoliation. It is highly sensitive to BTK (Peacock et al., 1998).
Globally rare plants are almost never a direct management issue in dealing with gypsy moth. Few globally rare plants are vulnerable to defoliation, in large part because gypsy moth larvae seldom eat herbaceous species. A more important concern with globally rare plants might be protecting their habitat if they are especially sensitive to decreased canopy cover caused by tree mortality. Information on susceptibility of new growth of box huckleberry would be useful but other species in that genus are disfavored as foods. Betula uber would seem to be at minimal risk based on its habitat. State-rare species of trees and shrubs could be an issue. For example two state-uncommon peripheral oaks, Quercus nigra and Q. michauxii, were substantially reduced by persistent initial outbreaks in Cumberland County, NJ. The latter may have been eliminated from Bear Swamp West preserve (Stevens Heckscher, pers. comm., 1990) but seems to have been little impacted in adjacent Bear Swamp East where overall tree mortality was lower. Most of the few mature Q. nigra were killed by the multiple defoliations (see above).
Some globally significant natural communities might be at risk of unacceptable levels of alteration from repeated defoliations, especially with initial invasions. Substantial stand changes are most likely in mixed hardwood stands with substantial oaks but not oak dominance. Such stands usually are on moderately acid or near neutral mesic soils. See 1995 FEIS Appendix G for more information and references. Mortality of scrub oak (Quecrus ilicifolia) from gypsy moth defoliation appears to be unreported and seems very unlikely. This shrub is an important component of several rare natural communities and a foodplant for many rare Lepidoptera. Foodplant mortality is not likely to be a significant impact to barrens Lepidoptera.
Generally protection of native fauna from outbreaks will not be a concern since impacts are usually only short-term (see 1995 FEIS Appendix G). Special cases could occur, most likely involving localized occurrences of rare Lepidoptera, perhaps rare salamanders. A tentative decision was made to use partial treatment with BTK to protect disjunct occurrences of Heterocampa varia and Acronicta albarufa on Nature Conservancy lands at Manumuskin, NJ in the 1990s. Both are summer oak feeders. However, while surrounding xeric and mesic forests were mostly 80-100% defoliated once or twice, the ultraxeric oak woodlands these species prefer locally had neither detectable defoliation nor high numbers of gypsy moth larvae so no treatment was needed. Probably delayed phenology of the oaks minimized gypsy moth hatchling establishment.
In some cases a BTK treatment might even be considered to protect occurrences of spring feeding moths id severe defoliation is anticipated and Gypchekis not an option. Or a manager may wish to consider the relative impacts of BTK versus expected defoliation. For spring feeding butterflies BTK impacts will probably exceed defoliation impacts except perhaps if defoliation would lead to virtually 100% starvation. If severe defoliation is likely to occur unusually early posing a risk (or certainty) of mass starvation, moderately BTK-sensitive or insensitive (Peacock et al., 1998) spring feeding and nearly all summer feeding native caterpillars should benefit from BTK treatment as opposed to no action. Beneficiaries probably include a majority of forest moths, but some of the others and most breeding butterflies could be severely reduced or even eradicated. If defoliation occurs at more normal time (e.g. mid or late June for Connecticut) spring species are likely to be less impacted by severe defoliation than by BTK. The benefits from BTK would probably outweigh the risk to a species with lab assay BTK mortality under 80% and good recovery of survivors. Field mortality will probably be lower. The practical problem though is that it is generally impossible to know the sensitivity of the species of concern (see Peacock et al., 1998), unless the actual species (not its family or even genus) is included in published or otherwise available data.
Determining whether or not a particular Lepidoptera species is at risk from BTK is difficult. To be sure, a manager needs to know the phenology of the species and its sensitivity to BTK, but the latter is unknown and unpredictable for many moths. See discussion elsewhere in this document. For now at least it appears butterfly larvae generally are highly sensitive. For other Macrolepidoptera the Peacock et al. (1998) assay data indicate nearly all species will incur significant mortality from BTK applications if they are first or second instars and for half of these species nearly all larvae could be killed. See Peacock et al. Tables 2, 3, 4 to roughly estimate instar present for the genus. Local Lepidopterists might be able to help land stewards determine the precise phenology of species of concern. Dale Schweitzer can be contacted for rare species. The NatureServe website may also be helpful. In some cases published literature would suffice. However consider the ecology of the species: even 100% mortality of larvae present on application day may not be cause for concern if large numbers of eggs will be hatching a week or more later. At this time of year no macromoth eggs can normally develop to hatching in less than 10 days and many need twice that, so sensitive species such as Actias luna, Antheraea polyphemus, spring Zale, Eutrapela clemataria, Lambdina "fervidara" still flying as adults should not be at risk unless they are unusually sensitive to residue for several weeks. Likewise hatchling mortality may not be of much concern if mid instars are less sensitive (which is by no means a safe assumption except with most Xylenini) and are already present. On the other hand species such as Hemileuca maia, Ennomos magnarius, some Catocala would be 100% at risk as first or second instars from eggs laid the previous fall and the first two and some Catocala were highly sensitive in the lab assays. Early spring species that finished oviposition within two weeks before application would also have very high exposure as first or second instars, likely examples include Pyrgus wyandot, Euchloe olympia, Callophrys henrici.
If the species of concern is a xylenine noctuid (=Lithophanini, Antitypini of Forbes, 1954) of a genus other than Lithophane and present in third or later instar, impacts of BTK will very likely be trivial or at least not worse than from severe defoliation. This is based on Peacock et al. (1998) results consistently showing low or no BTK-sensitivity for species of Chaetaglaea, Sericaglaea, Metaxaglaea, Sunira, Eupsilia, Xylotype, and moderate for Xystopeplus. The four species of Lithophane were more variable, with L. grotei extremely sensitive in a mid instar. On the other hand more than half of treated second instars of the globally rare L. lemmeri produced normal adults making it possibly the least sensitive species assayed as first or second instars. Catocala is another group with multiple state-rare species in the eastern USA that occasionally are at risk of severe mortality in gypsy moth outbreaks (Gall, 1984). The species assayed varied greatly in sensitivity to BTK. However impact to this genus appeared unexpectedly minimal from the Highlands, North Carolina gypsy moth eradication project (Schweitzer, 2000), although the data were not extensive.
Protection of native Lepidoptera or other organisms from impacts of established biocontrols is essentially impossible. A land steward might decide to decline releases of future biological control species on a preserve. However if the organism establishes elsewhere in the area it will soon be present anyway. Serious impacts to native species are known or strongly suspected only from Compsilura concinnata among the parasitoids and pathogens now established, but impacts from the pupal parasitoid Coccygomimus pedalis (Hymenoptera: Ichneumonidae) may also be a concern. No effective action to maintain or restore populations of species affected by exotic parasitoids is possible since the problem extends far beyond any preserve boundaries. It sometimes happens that Compsilura reaches high levels in outbreaking gypsy moth populations. In this case impacts from both species might be reduced with BTK, or even with a quickly lethal non-persistent chemical biocide, which should greatly benefit vulnerable summer caterpillars, although to the detriment of spring species. Gypcheckprobably would not kill quickly enough to prevent maturation of Compsilura. Such use of BTK could backfire and lead to unusually high parasitism of native spring caterpillars since Compsilura females would not be killed by BTK but would be deprived of gypsy moth larvae. Data are not available on this issue. Furthermore sublethal effects on native spring caterpillars could leave them more vulnerable than usual to parasitism (FEIS, 1995, Appendix G). In reality though a manager will probably not know that Compsilura numbers are excessive in time to consider acting.
Species Impact: Public nuisance factors during gypsy moth outbreaks can be severe (see Goebl, in Fosbroke and Hicks, 1987; FEIS, 1995) but are not dealt with here. Defoliation is obviously aesthetically unpleasing and can have direct economic impacts. The extreme number of larvae are sometimes also a legitimate nuisance, even ignoring irrational entomophobia. A few people are allergic to the larval hairs, but neither current control practices nor gypsy moth pose serious human health risks (see FEIS, 1995). Generally biodiversity managers need to consider three possible impacts: tree mortality, impacts of control measures, and sometimes impacts of outbreaks themselves, including defoliation per se on native species. See Threats Section for this topic.
The most obvious long-term ecological impact of outbreaks is tree mortality, which is very variable. Rarely are the affected trees rare species themselves so low or moderate tree mortality might not necessarily concern biodiversity managers. Oaks are most likely to be killed (Nichols, 1980 and many others) followed by conifers, but hickories and others also may be killed. For initial outbreaks 10% to 30% mortality to oaks and other highly susceptible hardwoods is likely if two or more severe defoliations occur, with generally lower mortality in subsequent outbreaks. Mortality is often zero to 10% (Gansner and Herrick, 1979), but in worst case scenarios (e.g. sites selected for the Quimby, 1985 study) can run about 50% of the stand. A useful review is Hicks and Fosbroke (in Fosbroke and Hicks, 1987), but USFS staff can probably suggest more current works. Trees on poor sites such as ridgetops or in poor condition or stressed by low soil moisture are most likely to die. Defoliation insufficient to force oaks to refoliate causes virtually no mortality to hardwoods. Heavy tree mortality may alter future stand composition especially if edaphic conditions are good for less favored species. Healthy hardwoods normally recover from one complete defoliation but hemlocks and some other conifers generally die, although white pines often recover (Stephens, 1984). With the loss of hemlocks to adelgids, white pine is now the most likely conifer to be defoliated in much of the US range of gypsy moth. Pines in extensive pine barrens and pine forests are not at risk of defoliation from the established gypsy moth strain.
Besides tree mortality, other changes such as altered competitive interactions, reduced seed production, changes in herbaceous flora which might affect germination of tree and shrub seeds and others changes can in some situations alter future forest composition, as can deliberate silvicultural manipulations. See Musika (1993) for a useful introduction to the many possible ecological factors besides tree mortality. As he points out composition changes are possible but also vary greatly with site conditions and location. Probably an even more over-riding impact now in many areas such as many National Parks, and places like Connecticut and northern New Jersey where hunting is minimal is extreme herbivory by deer, often leading to drastic reduction or total elimination of tree reproduction. Deer and invasive exotic plants are likely to interact with direct gypsy moth impacts in affecting future forest composition and the relative importance of gypsy moth will vary. Stand composition is a complex issue which is not addressed in detail in this document and review of post FEIS literature on the topic was minimal. In some places poor acid soils prevent replacement of oaks with other species as long as deer and other factors do not severely suppress oak reproduction.
Other impacts of defoliation are numerous but mostly short term (FEIS, 1995, Appendix G), but in certain circumstances localized populations of Lepidoptera and other herbivores could be eradicated if defoliation is severe (see threats section above), especially on west facing xeric ridges where defoliation may occur earlier. In general the conclusion of the FEIS Appendix G that impacts of gypsy moth outbreaks to native Lepidoptera are mostly short term is certainly correct (see also Sample at al., 1996) but the possibility exists for long term impacts.
Soil and litter fauna including salamanders are affected by increased insolation and temperature for several weeks while the canopy refoliates. However, apparently mortality is not generally high since most can move deeper into the humus or find shelter. It is worth remembering that some forests defoliated by gypsy moth are also subject to occasional defoliation by native caterpillars. A temporary rise in water temperature of streams and ponds may be a concern but long term biotic changes are not the norm. Nesting success of birds may or may not be affected positively or negatively by outbreaks. Fee the 1995 FEIS and references in it for more details regarding birds. Gray squirrels are among the most heavily impacted vertebrates due to the loss for several years of hard mast production, especially acorns. Many native and exotic understory plant species are temporarily favored by the increased light during outbreaks (e.g. Musika, 1993), e.g. the globally rare orchid Isotria medeoloides (Brackley, 1985) and Asclepias tuberosa (Schweitzer). Probably the greatest concern for rare and even common native understory plants and their native herbivores if tree mortality occurs is increased invasion of exotic plants or aggressive natives like Smilax rotundifolia with increased light.