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Prescribed Fire Effects on Biological Control of Leafy Spurge

Discussion


Prerelease fall or spring burning enhances colonization by A. nigriscutis. Although results of auxiliary samples indicate that standard sweep samples may have failed to detect some small populations in 1995, none was sufficiently established to persist into the second year.

Benefits of burning may be short-lived if habitat is unsuitable for other reasons. Beetles established on only 1 of 15 unburned plots on Walsh-2, Thiesen-1 and -2, Kemmer-1, and Arrowwood-2, (Table 3) suggesting that unburned habitat on these blocks was unsuitable for A. nigriscutis. Five of 7 colonies detected on preburned plots on those blocks in 1995 failed to survive into 1996. In contrast, on Kemmer-2, Storhoff-2, and Arrowwood-1, where beetles established on 8 of 9 unburned plots, all colonies on preburned plots persisted into 1996.

The positive effect of bare ground and the negative effect of litter depth on colonization suggested by our data support the hypothesis (N.R. Spencer, pers. comm.) that A. nigriscutis may be limited in part by factors that impede access to the soil surface. If litter reduction and/or exposure of bare soil are the primary mechanisms facilitating establishment on burned sites, rapid litter buildup and plant regeneration following a burn may cause benefits to be only transitory even in relatively good A. nigriscutis habitat. However, as long as the habitat is otherwise suitable, increased early recruitment from prerelease burning should ultimately enhance control.

One possible objection to prerelease burning concerns potential effects of fire on leafy spurge. Wolters et al.(1994) reported increased leafy spurge stem density in the first growing season following spring (4 May), but not fall (19 Sept.), burns in southwestern North Dakota. Thus, a spring burn followed by an unsuccessful beetle release might leave the manager with an even denser stand of leafy spurge. We observed significant increases in stem density during the first growing season after both spring and fall burns (Table 4). However, the increases did not persist into the second growing season.

It is unlikely that beetles caused the observed decline in stem density on burned plots from 1994 to 1995. Populations were low on most plots in 1995, and stem counts declined uniformly throughout each plot, whereas beetles were concentrated at the plot center. Although Wolters et al.(1994) concluded that fire reduces germination rate of leafy spurge seed, we believe most of the increase in 1994 consisted of a flush of seedlings, which subsequently died due to competition with established plants (Hanson and Rudd 1933, Selleck et al. 1962).

No control of leafy spurge relative to pretreatment stem density was evident with any treatment in the first year after beetles were released, and the unanticipated need to reburn the most successful colonies prevented meaningful assessment of control during the second year. Given the small founding populations used in this study, lack of control during the first year was not unexpected.

However, results from leafy spurge-infested fields at Northern Prairie Wildlife Research Center indicate that prerelease burning can have immediate control benefits when larger numbers of beetles are released. As part of an operational control program (Fellows, Unpublished data), releases of approximately 1,000 A. nigriscutis were made in July 1996, at intervals of about 25 m throughout a habitat unit where the species was known to do well. Approximately half of the unit had been burned in early June 1996. The remainder had not been burned since at least 1967. In late May 1997, we estimated the size of the spurge-free zone at each release site. For 25 sites in the burned habitat, the mean spurge-free radius (SE) was 93 (12) cm, compared to a mean of 36 (9) cm at 10 unburned sites. Mean values at reference sites midway between adjacent release sites were 11 (1) cm in the burned and 15 (1) cm in the unburned zone. Thus beetles on average cleared nearly 7 times more spurge in the burned (2.7 m²) than in the unburned (0.4 m²) habitat during the first year. Additionally, zones of reduced stem density and stunting extended for 3-4 m around most of the sites in the burned, but not in the unburned area.

Because economics and long-term control benefits of preburning fields solely to enhance establishment of A. nigriscutis have not been determined, we cannot currently recommend that prerelease burning be adopted as an operational practice. However, managers should take advantage of scheduled management burns in leafy spurge-infested fields by releasing A. nigriscutis into suitable habitat within the burned area whenever possible.

Our results also demonstrate that established A. nigriscutis colonies in North Dakota can survive burns from early October through mid-May. No difference was found in the population indexes of burned and unburned plots, suggesting that burning had no negative effect on larval survival and eclosure rates. Moreover, based on the response of beetles to preburned release sites, we anticipate that periodic burning at appropriate times will enhance expansion of established colonies and lead to earlier control of leafy spurge.

Spring burning of established colonies must be completed early enough to allow spurge to regrow before beetles emerge. Based on regeneration observed in this study, adequate spurge would probably be available in southeastern North Dakota following burns as late as 25 May in most years. However, assuming regeneration is moisture dependent, a target cutoff date of 15 May is recommended during dry years. Elsewhere, the spring cutoff should be adjusted to match the anticipated local adult emergence pattern and rates of foliar regeneration.

Because egg laying is complete by early September (Rees and Spencer 1991), fields containing A. nigriscutis can likely be burned as early as 1 September without damage to colonies. Even earlier burning, if needed to meet grassland management objectives, may have no negative effect. Brinkman (1997) found peak insemination rates in early July and few adults by early August at an A. nigriscutis insectary near Pollock, South Dakota. Burning as early as 1 August may therefore not substantially reduce egg production. However, because mid-summer burning could affect the nutrient value of spurge or increase soil insolation beyond larval tolerance, early burning should be approached with caution.


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