Effectiveness of Interspecific Hybridization for Incorporating Birch Leafminer (Fenusa pusilla) Resistance into White-barked Betula

A significant pest affecting commonly planted Betula spp. is the birch leafminer (Fenusa pusilla Lepeletier), an insect that can be present in large populations in the landscape and can greatly reduce the vigor and ornamental value of these trees. Twenty-two interspecific crosses were performed between leafminer resistant and susceptible Betula species in an attempt to create the novel combination of ornamental white bark and significant leafminer resistance. Of the nine successful crosses, two produced resistant offspring. Progeny of the diploid × hexaploid cross B. turkestanica Litvin (2x) × B. alleghaniensis Britt. (6x) displayed a broad range of resistance levels, likely the result of segregating alleles contributed by the hexaploid parent. All crosses involving highly resistant individuals of B. costata Trautv. (2x) yielded leafminer susceptible progeny. These results suggest that the larval antibiosis demonstrated by B. alleghaniensis and B. costata is inherited as a recessive trait, and exhibits a gene dosage effect as evidenced by the B. turkestanica × B. alleghaniensis offspring. While most progeny of the B. populifolia Marsh (2x) × B. maximowicziana Regal (2x) cross were susceptible, a single resistant offspring, which was found to be triploid (3x), displayed a mechanism of resistance similar to that of a hypersensitive response. No strong intersectional barriers to hybridization were observed and all interploidy crosses were successful. The chromosome numbers of B. costata (2n = 2x = 28) and B. turkestanica (2n = 2x = 28) are reported here for the first time. The results of this study indicate that the potential exists for the development of insect resistant, ornamental white-barked birch clones through the implementation of a planned, systematic breeding program.

Breeding for Leafminer Resistance within the Genus Betula

The birch leafminer (Fenusa pusilla) is an important insect pest of white-barked ornamental birches (Betula spp.). Although genetic resistance to birch leafminer exists within this genus, it has not been identified in the desirable white-barked species. Twenty-three interspecific crosses were performed in an attempt to combine ornamental white bark and leafminer resistance. The parental species and resulting hybrid populations were assayed for resistance. All assays utilized adult birch leafminers reared in our laboratory using a method we developed to provide a year-round supply of fresh leafminer adults. Previous studies viewed birch leafminer resistance as the unwillingness or inability of the female leafminer to oviposit eggs into the leaves of resistant Betula species. Our work has shown that female birch leafminers are unable to distinguish between suitable and nonsuitable host species. This work also has discovered two separate mechanisms of inherent resistance to the birch leafminer: leaf chemistry and a hypersensitive response. Two native species, B. alleghaniensis and B. lenta, kill the leafminer larvae shortly after hatching. Analyses suggest that a specific compound or class of compounds is responsible for leafminer mortality in these species and that this resistance is reduced in F1 hybrids that include a susceptible white-barked parent. The second resistance mechanism involves a hypersensitive response to the deposition of the leafminer egg and results in failure of the egg to hatch. Interestingly, this response is seen in one of our breeding lines (B. populifolia × B. maximowicziana), but has not been observed in either parental species. These hybrid progeny are currently in field test plots for further evaluation of leafminer resistance and ornamental characteristics.

ESTABLISHMENT, DISTRIBUTION, AND IMPACT IN SOUTHERN NEW ENGLAND OF LATHROLESTES NIGRICOLLIS (THOMPSON) (HYMENOPTERA: ICHNEUMONIDAE), AN INTRODUCED PARASITOID OF THE BIRCH LEAFMINER, FENUSA PUSILLA (LEPELETIER) (HYMENOPTERA: TENTHREDINIDAE)

Lathrolestes nigricollis (Thompson) (Hymenoptera: Ichneumonidae) was released in New England against Fenusa pusilla (Lepeletier) in 1979–1980 and again in 1989–1990. By 1991, the parasitoid was widely established in southern New England. From 1979 to 1990 the commonness of leafminers at the original release site in the first generation declined from 50–54% of leaves mined to under 3%. Over 4 years at five sites, parasitism of larval birch leafminers increased between leafminer generations from 10.3% (G1) to 27.6% (G2) to 39.2% (G3). In Massachusetts, parasitism decreased (from 28 to 14%) with increasing distance from the original release site, while corresponding damage increased (from 6% of leaves mined to 33%). In areas local to the original release site (<4 km) parasitism has reduced damage from birch leafminer to inconsequential levels. Some sites in other parts of southern New England, particularly roadside sites, still have high levels of mined leaves in spite of the parasitoid’s presence in the general area.

EXOTIC BIRCH-LEAFMINING SAWFLIES (HYMENOPTERA: TENTHREDINIDAE) IN ALBERTA: DISTRIBUTIONS, SEASONAL ACTIVITIES, AND THE POTENTIAL FOR COMPETITION

The exotic birch-leafmining sawflies Fenusa pusilla (Lepeletier), Profenusa thomsoni (Konow), and Heterarthrus nemoratus (Fallen) occurred in Alberta during 1992–1995, but only the first two were abundant. Birch-leafmining sawflies occurred at all sites surveyed in central and southern Alberta, and appeared to be expanding their ranges northward. Adult F. pusilla began emerging in mid-May (approximately 220 DD05), and there were one to three generations per year, depending on location and year. Female F. pusilla were relatively less abundant on young than on mature trees. Profenusa thomsoni began attacking trees in June (between 400 and 550 DD05), and was invariably univoltine. Both species were more abundant and were active earlier on urban trees than at a nearby rural location. The highest catches and most complete representations of seasonal activity were obtained using yellow sticky traps. Larval F. pusilla and P. thomsoni are unlikely to compete directly for leaf resources because their leafmining activities are separated spatially and temporally, but they probably compete intraspecifically.

Use of Foliar-Applied Neem (Azadirachta indica A. Juss.) Seed Extract for the Control of the Birch Leafminer, Fenusa pusilla (Lepeletier) (Hymenoptera:Tenthredinidae)

The effect of 1.0% neenl (Azadirachta indica A. Juss.) seed extract applied as a foliar spray against the birch leafminer (Fenusa pusilla (Lepeletier)) was compared to sprays of water and Metasystox-R™. Sprays were applied against the season's first generation, at oviposition (V-3-86) or early instar (V-10-86). Sprayed foliage was harvested and weighed at late instar, and adults were reared. The extract caused significantly more leafminer mortality than did water, and caused as much mortality as Metasystox-R™. Observations suggested that the extract took longer to kill leafminer larvae than did Metasystox-R™ thereby allowing for more foliar damage. There were no differences in efficacy between times of extract application.

TISSUE PREFERENCE AND DAMAGE BY FENUSA PUSILLA AND MESSA NANA (HYMENOPTERA: TENTHREDINIDAE), LEAF-MINING SAWFLIES ON WHITE BIRCH (BETULA PAPYRIFERA)

Fenusa pusilla (Lepeletier) and Messa nana Klug coexist on white birch near Sudbury, ON. Both species often oviposit simultaneously, but their mines are spatially separated because of differences in ovipositional behavior. Most first-generation F. pusilla oviposit in unfolding leaves arising from currently expanding shoots situated distally on the branch, whereas M. nana oviposits in fully expanded leaves arising from older shoots situated proximally on the branch. Leaf tissues damaged by oviposition and larval feeding were examined. Larvae of F. pusilla consume both palisade and spongy mesophyll, whereas larvae of M. nana consume only palisade. It is suggested that F. pusilla develops faster than M. nana because it feeds on leaves of greater nutritional value and digests more cellular components.

RESISTANCE OF BETULA DAVURICA TO THE BIRCH LEAFMINER, FENUSA PUSILLA (HYMENOPTERA: TENTHREDINIDAE)

To our knowledge, reports of Betula spp. resistance against damage caused by the birch leafminer, Fenusa pusilla (Lepeletier), have not been published. Becker (1938), Friend (1933), and Johnson and Lyon (1976) stated that B. populifolia Marsh., B. papyrifera Marsh., and B. alba L. are the most commonly attacked species. Friend (1933) and Johnson and Lyon (1976) stated that B. lenta L., B. alleghaniensis Britt, and B. nigra L. have never been observed infested or are rarely fed upon.