Birch leafminer (Fenusa pusilla [Lepeletier]) (= pumila Klug) (Hymenoptera: Tenthredinidae), an invasive Palearctic leaf mining sawfly attacking several species of Betula, has been present in North America for at least 85 years, and was first recorded in Connecticut (Friend 1933). When high proportions of tree leaves are mined, browning makes ornamental birches unsightly, although damage does not normally cause tree death. Investigations on natural sources of mortality associated with the pest in Connecticut (Friend 1933) and Quebec (Cheng & LeRoux 1965, 1969, 1970) showed that parasitism of larvae was insignificant (<5%) compared to the situation in the its native range in central Europe, where parasitism was much higher (38–47%) (Eichhorn & Pschorn-Walcher 1973). This difference prompted the initiation of classical biological control programs in both Canada and the USA, leading to the release of 4 parasitoids: the ichneumonids Lathrolestes nigricollis (Thompson) and Grypocentrus albipes Ruthe, the braconid Phanomeris sp., and the eulophid Chrysocharis nitetis (Walker). The ichneumonids were released in Canada (Quebec and Newfoundland) from 1974–75 (Raske & Jones 1975; Guèvremont & Quednau 1977), and all 4 species were released in NJ, DE, MD, and PA between 1976 and 1982 (Fuester et al. 1984). Later, releases of just the 2 ichneumonids were made in MA (1979–1980), in MA and RI (1989–1991) (Van Driesche et al. 1997), and western Canada in the 1990s (Langor et al. 2000).

Here we present information on the efficacy of these releases in the northeastern United States. A preliminary evaluation (Van Driesche et al. 1997) of the status of the pest in MA and RI conducted from 1990–1995, showed that the pest had declined locally around the principal MA release site in Amherst from 54% of leaves mined in spring to just 3%. Furthermore, by 1995 a difference had developed between Amherst, MA (at various non-release sites within 4 km of the 1980 release site) versus a similar town (Northampton) located 15–20 km from the release point. In the Northampton area, where the parasitoid would have arrived later, 33% of spring birch leaves were still being mined and only 14% of the larvae were parasitized by L. nigricollis. In contrast, in Amherst, closer to the original release site, only 6% of spring leaves were mined and 28% of the larvae were parasitized (Van Driesche et al. 1997). This suggests that at that point in time, there was a zone, centered on the original release area, where the pest was controlled, while further away control was still developing.

Preliminary data published previously, while suggesting the existence by the 1990s of local areas of pest suppression, do not reflect the full extent of area-wide pest control caused by this classical biological control project. Here, we present results of a set of state-level surveys carried out in spring of 2007 by the authors to determine the current abundance of F. pusilla mines on birch species in the northeast USA. Surveys were run in RI and CT (Casagrande), Massachusetts (Van Driesche), New Jersey (Mayer), Pennsylvania and Delaware (Fuester) and Long Island, New York (Gilrein).

MATERIALS AND METHODS

State Surveys in 2007

Surveys were done in 7 states (MA, RI, CT, NY [Long Island only], PA, NJ, DE) (Fig. 1) in May and Jun of 2007 (Table 1). The survey was conducted along roadsides in urban, suburban, or rural residential areas where grey or white (Betula papyrifera Marshall) birch trees were present in landscapes or naturally occurring along roadsides. Following haphazard routes in such areas, birch trees were examined when encountered. From each tree, 100 leaves were plucked without looking at them and then examined to determine the number of leaves bearing distinctive F. pusilla mines. Leaves with leafminer larvae (from these samples and supplemental samples) were taken to the laboratory and dissected to detect eggs or larvae of L. nigricollis. State summaries were computed as the percentage of leaves with F. pusilla mines (summed over all sites) and the percentage of birch leafminer larvae parasitized (summed over all larvae from all sites). One exception to this process was that data from NJ were kept separate by location because strong differences occurred between sites in northern versus southern NJ. For comparison, the state was split into northern (40°NL or above) and southern (below 40°NL) parts.

Fig. 1.

Areas (ellipses) surveyed for birch leafminer (Fenusa pusilla) and its parasitoid Lathrolestes nigricollis in 2007.

TABLE 1.

PERCENTAGE OF GREY OR PAPER BIRCH LEAVES MINED BY FENUSA PUSILLA AND PERCENTAGE OF LEAF MINER LARVAE PARASITIZED BY THE ICHNEUMONID LATHROLESTES NIGRICOLLIS IN SEVEN NORTHEASTERN STATES IN 2007.

RESULTS

Birch leafminer populations have come under complete biological control in all areas surveyed above 40°NL (mid-New Jersey). Mined leaves were extremely difficult to find in MA, CT, RI, PA, and NY (Long Island) (Table 1). In these areas, birch leafminer larvae could not be recovered in large enough numbers to estimate rates of parasitism. In New Jersey, density of the pest has been suppressed at most sites north of 40°NL, but south of this latitude, pest densities remain high (Fig. 2). Parasitism in sites above 40°NL (57.9%, 114/197) was approximately double the 27.5% (25/91) rate found in southern NJ. Infestations of birch leafminer at Newark, DE, were highly variable, with the incidence of mined leaves ranging from 0 to 57% on different trees and averaging 10.4%, intermediate between New Jersey and the other states. Parasitism was low (1.2%), but samples were taken when most of the mines were small. Because Lathrolestes nigricollis females prefer to attack semi-mature and mature larvae (Eichhorn & Pschorn-Walcher 1993), sample timing may have caused parasitism to be underestimated.

Some insight into the development of control over several decades is available from the repeated sampling of one site in RI, where annual samples were taken in most years from 1990 to 2008 (Fig. 3), and at this location, it is clear that the percentage of leaves mined fell from about 87% (avg. for 1990, 1996, 1997, 1998) to about 2.6% (average for 2004, 2005, 2006, 2007, and 2008). During this same period, the rate of larval parasitism by L. nigricollis rose steadily from an average of 5.5% (1990, 1991, 1992, 1995, and 1996) to a high of 38% in 2004. Thereafter, too few mines were present to obtain larvae to examine for parasitism.

Fig. 2.

Percentage of birch leaves mined in 2007 by birch leafminer versus latitude in New Jersey, showing lack of control in southern New Jersey (left) versus much greater pest suppression in northern New Jersey (right).

DISCUSSION

Fenusa pusilla, which remained a high-density pest in the northeastern United States throughout the 20th century, has now come under complete biological control in MA, CT, RI, NY (Long Island), PA, and northern NJ due to the introduced parasitoid L. nigricollis. In southern NJ, the pest remains abundant despite relatively high levels of parasitism. In DE, populations of the pest are variable but lower than in southern NJ. However parasitism rates observed in our DE samples were also low. Why the pest has not been suppressed in southern NJ or DE is unknown. In terms of time, these areas have been colonized for a longer period than the more northerly regions where the pest has declined, so it seems unlikely to be merely a transitional phase moving toward control. Poor control by the parasitoid in southern NJ may relate to poor photoperiodic match between the region from which the parasitoid was originally collected (Austria, at 47–48°NL) and our sample areas in southern NJ (<40°NL), or may be due to southern NJ's higher summer temperatures. Mismatch of photoperiod can limit parasitoid impact if adult emergence is not well timed to the availability of the correct host stage, or if the percentages of the parasitoid population that enter diapause in the first and second generations differ from the diapause rates of the pest in the corresponding generations (Nealis 1985; Van Driesche et al. 1997). In DE, scarcity of suitable birch species also may be important. Only 2 species of birch (Betula lenta L. and Betula nigra L.) occur naturally in Delaware (Taber 1995) and of these, B. nigra is not a preferred host for F. pusilla. Preferred species such as gray birch or European species exist only as planted landscaping trees, and these are often in stressed condition due to climate. Further research is needed in NJ and DE to clarify the importance of these factors.

Fig. 3.

Percentage of grey birch leaves mined by first generation Fenusa pusilla and percentage of larvae parasitized by Lathrolestes nigricollis at Ryan Park in North Kingston, RI, between 1990 and 2008. (Years with no bars prior to 2004 are missing data. After 2005, larvae, and hence parasitism, were undetectable).

The results of our surveys illustrate the importance of continued monitoring of a classical biological control project for a very extended period, here 34 years since the first releases. Evaluations on short time scales (3–6 years), typical of many research projects, may be premature and underestimate ultimate impacts of new biocontrol agents.