Los protagonistas

Dale M. Woods and Baldo Villegas

California Department of Food & Agriculture, Biological Control Program, 3288 Meadowview Rd., Sacramento, California 95832

INTRODUCTION

Knapweeds are a fairly large group of plant species comprising a couple of genera in the plant family Asteraceae. All but two species are exotic to North America and most are considered to be invasive. Spotted knapweed (Centaurea stoebe ssp. micranthos more commonly referred to as Centaurea maculosa), diffuse knapweed, (Centaurea diffusa), Russian Knapweed (Acroptilon repens) and yellow starthistle (Centaurea solstitialis) are well known because they are wide-spread, ecosystem dominating plants. Consequently extensive efforts have been devoted to establishing biological controls on these species. Currently, over two dozen different biological controls have been imported and released on either spotted knapweed, diffuse knapweed, Russian knapweed or yellow starthistle. Additionally, several unintentionally introduced biological controls exist in North America on these species. In spite of the extensive effort, progress toward field level control over these weed species has been slow. Two decades after the first (1970s) biological control release on any of these weeds, few weed scientists believed that a significant level of weed control existed. Only in recent years have a number of scientists been documenting area-wide declines in some of these species.

Several additional knapweed species (e.g. tocalote, purple starthistle, squarrose knapweed, and meadow knapweed) have become localized invasive problems and are receiving much more attention. Unfortunately, due to the high cost of foreign exploration and subsequent host testing, there is little chance that these secondary knapweeds will become targets for full-scale new biological control projects.

Our program has been evaluating biological control agents that were previously approved for introduction on one of the primary knapweeds for their ability to establish on and impact various secondary knapweeds. In this paper, I will summarize our work on squarrose knapweed (Centaurea squarrosa) in northeastern California. A combination of three species of insects approved for spotted and diffuse knapweeds appears to be highly successful in virtually eliminating seed production and effectively shortening plant lifespan of squarrose knapweed, and has resulted in dramatic declines in the plant population.

Squarrose Knapweed Biology

Squarrose knapweed, a large tap-rooted short term perennial, is widely scattered in the western United States but is a significant weed only in Utah and in the northeastern corner of California, specifically in Siskiyou, Lassen, Modoc and Shasta counties. It was probably introduced through sheep ranching early in the 20th _{century and} has become quite dominating in certain rangelands and natural areas. Chemical controls are widely used but eradication is not realistic considering the large number and size of infestations. In an effort to reduce control costs, reduce pesticide use and reduce the spread of the weed, our department began considering biological control agents. Bioagent Selection

There are no biological control agents specifically selected for introduction on squarrose knapweed. However during foreign exploration and host testing for biological controls of spotted and diffuse knapweed, some evidence of acceptance of squarrose knapweed by some of the insects under evaluation was accumulated. We initially considered agents that showed some association with squarrose and were available from domestic co- operators for experimental trials. We tried eight biocontrol agents on squarrose knapweed but will discuss only four as the subject of this report. The remaining four, the rust fungus Puccinia jaceae var. diffusa, the green clearwing fly, Terellia virens, the root boring weevil Cyphocleonus achates and the banded gall fly, Urophora affinis did not establish at levels sufficient to evaluate.

Early Releases

The UV knapweed seed head fly, Urophora quadrifasciata Meigen (Diptera: Tephritidae) had migrated to the state before 1993 when we detected it in field sampling. This strong flier probably came from knapweeds in southern Oregon and generally infested the region at a low level (<5% of the seedheads) by 1998 when we began monitoring. We began our active release program at a large infestation near Hawkinsville in Siskiyou County in 1995. The weevil Cyphocleonus achates was initially released followed in 1996 by the broad-nosed seed head weevil, Bangasternus fausti (Reitter) (Coleoptera: Curculionidae) and the lesser knapweed flower weevil, Larinus minutus Gyllenhal (Coleoptera: Curculionidae) in 1997. None of the weevils established very well and the site was eradicated by herbicide applications a few years later. The one lesson that was learned was that the early, very short

seasoned squarrose knapweed provided a poor establishment for weevils collected from the later and longer seasoned spotted and diffuse knapweeds in Oregon.

Second Releases

In spite of the poor results in Siskiyou County, we attempted another small release in 1998 in Lassen County near Pittville using weevils collected earlier in the season. We did not anticipate success so did not devote resources to in-depth studies on plant density or biology. Both B. fausti and L. minutus were released at this site, which later became a primary research site. Additional releases occurred annually at other sites culminating in additional research sites being established in Shasta County in 2002-2004. Releases of the only non seedhead agent, Sphenoptera jugoslavica Obenberger (Coleoptera: Buprestidae) also began in 1998.

Population Buildup

Populations of the two seedhead weevils expanded rapidly in the Pittville area of northeastern California. We did not anticipate that the first two releases (one release of each weevil) at the Pittville sites would establish based on experience in Siskiyou County. However both sites provided excellent habitat for the weevils. When we returned in the fall of 1998 to collect monitoring samples it was clear that the weevils, particularly L. minutus, had been prolific and weevils had matured to adulthood in their new host. Over 20% of the seedheads around the release site had been attacked and supported development at least through early larva. A second set of two sites was established in 1999, four more in 2000 with additional sites annually increasing to a peak in 2003 of 10 sites. End- of-season monitoring was performed at most sites for most years and the results of some of the sites are shown in Fig. 1. Results are displayed for the site that we had the most complete data sets. Each site had a substantial level of initial establishment during the first season. Establishment varied but as much as 20% of the seedheads were attacked in the first season, dependent somewhat on the density of plants and seedheads at release. During the second year, the population ballooned to between 30 and 90% attack. By the end of the third season plants at all sites were about 90% attacked. Attack levels have remained at this level for several years.

End-of-season evaluations for root attack by S. jugoslavica indicates that population buildup has been slower than the weevils but still rapid (not shown). Currently the locations where the beetle was released eight years previous show 90% of the plants with root damage from S. jugoslavica.

Results of Redistribution

Redistribution of the weevils throughout infested areas of the state took place over a seven-year period. During the first five years, weevils were mass collected from spotted and diffuse knapweed in Oregon. Beginning in 2003, weevil collections were made from established California populations and moved to new sites. Releases varied from less than 200 individuals per release in the early years to several thousand per release as weevils became more plentiful. All releases of the beetle S. jugoslavica were from material field collected in Oregon and were never more than 200 individual per release.

Natural Spread

Both seedhead weevils are strong fliers and can take wing when disturbed. In general, however, they walk from stem to stem and drop to the ground when disturbed. Consequently we did not anticipate rapid long distance natural spread. Squarrose knapweed is established in many isolated parts of northeastern California, so we were interested in finding evidence of natural spread that would complement our redistribution efforts. Two years after the initial 1998 release, L. minutus had made a one mile movement to a new site. A second site evaluation showed the weevils to move 3.2 miles in two years to an isolated patch of knapweed but not to patches 5 miles away. Seed Destruction

Squarrose knapweed produces relatively few seeds per seedhead. Under California conditions, healthy insect-free knapweed produces about 1.4 seeds per seedhead. The relatively low number of seeds produced per seedhead compared to other knapweeds is offset by the large number of seedheads that are produced by an individual plant (up to several hundred). These seeds remain enclosed in the seedhead as they mature. At plant maturity, the entire seedhead breaks off and becomes the dispersal unit for the enclosed seeds. The bracts on the seedhead are recurved (turned backwards) creating a hook. This hook is easily attached to animal hair or wool aiding in long distance dispersal. We evaluated seed impacts by directly examining the seedheads at maturity for presence/absence of the insects as well as the number of mature seed. The primary impact of both seedhead weevils is direct seed destruction by larval feeding. Seedheads attacked by either of the two weevils resulted in a near complete destruction of the seeds within the seedhead. Thus attack incidence reflected the amount of seed destruction.

The seed head fly U. quadrifasciata eliminated most of the seeds when it was present (Fig. 2). However, the attack rates were quite low, never achieving over 5%. Seed destruction by either of the two weevils was nearly complete in all cases. Coupled with the high attack rates of the weevils resulted in virtual elimination of all seed production when they are present.

Plant Population Effects

With the dramatic reductions in seed production accompanied by the rapid bioagent population increases, it was logical to attempt documentation of plant population effects. This was attempted by several methods. We monitored squarrose plant numbers, lifespan, and seedhead density and, we estimated plant canopy cover to document a hoped for transition from squarrose knapweed to other vegetation. The first attempt to document declining plant numbers began in 2001 at the site of our 1998 and 1999 releases. Consequently, the data has a unfortunate starting point just as the weevils were reaching maximum density rather than pre-release. Traditional weed biocontrol progresses so slowly that this would not normally be a concern, however the rapid increases of the insects on squarrose knapweed have distorted things. None the less, at both sites started in 1998 we have monitored dramatic declines in squarrose knapweed density from 2001-2005. (Fig. 3)

Plant density evaluations at the two Shasta County sites began concurrent with the initial releases and we hope to validate the observations in Lassen County in upcoming years. Three years after they were initiated we have not observed plant population declines but we have indications that seedling numbers are precipitously down. Presumably, as mature plants age-out and die, they will no longer be replaced with new seedlings and plant populations will decline. Evaluations of plant cover will require multiple years to establish trends and the measures to date are not conclusive. This activity will continue for many years. Impacts of the beetle S. jugoslavica are also not quantifiable with our current data. We are developing additional measures to validate our perceptual evaluations that the large amount of root destruction caused by the beetle is shortening the average life span of adult plants and may contribute to stand declines.