Seasonal Phenology and Impact of Urophora Sirunaseva on Yellow Starthistle Seed Production in California

Biological Control 47 (2008) 172–179

Contents lists available at ScienceDirect

Biological Control

journal homepage: www.elsevier.com/locate/ybcon

Seasonal phenology and impact of Urophora sirunaseva on yellow starthistle seed production in California

Dale M. Woods a,b,*, Michael J. Pitcairn b, Donald B. Joley b,c, Charles E. Turner d, a Department of Plant Sciences, University of Wyoming, 1000 University Avenue, Laramie, WY 82071, USA b California Department of Food and Agriculture, Biological Control Program, 3288 Meadowview Road, Sacramento, CA 95832, USA c California Department of Food and Agriculture, Seed Inspection Laboratory, 3294 Meadowview Road, Sacramento, CA 95832, USA d United States Department of Agriculture, Agricultural Research Service, Exotic and Invasive Weed Research Unit, Albany, CA 94710, USA article info abstract

Article history: The gall fly, Urophora sirunaseva, is one of six insect species released as classical biological control agents Received 28 January 2008 to control the invasive weed, yellow starthistle. Two study sites were established in Northern California Accepted 14 August 2008 to evaluate the per capita impact of galls on seed production. Evaluations during two years at each site Available online 20 August 2008 used small cloth bags placed over developing seedheads to contain developing seeds and insects. Larger seedheads produced more seed but also supported more galls. Within-seedhead gall densities as high as Keywords: 15 were detected. The impact of a single gall was estimated to reduce seed number by between 2.1–2.9 Urophora sirunaseva seeds per gall. A regression model was developed that estimated percentage seed loss as a function of Centaurea solstitialis seedhead size and number of galls. Individual galls were estimated to cause a 5–11% decrease in seed pro- Biological control Gall fly duction compared to ungalled seedheads. The impact of the gall fly was greatest in smaller seedheads at Gall both sites. Gall densities did not reach levels needed to exert significant control of seed production. Seed Seed production production in non-galled seedheads was not affected by the presence of galls in seedheads elsewhere on Gall impact the plant. Gall flies were more likely to attack plants already supporting galls. Two generations of the fly were detected at both sites. The occurrence of overwintering larvae began in mid- to late-July depending on site. By early-August, all new galls held only overwintering larvae. Follow-up samples at one location eight years later showed the gall fly still present but at less than half of the population level observed earlier. Published by Elsevier Inc.

1. Introduction oping larvae stimulate the production of gall tissue. Eggs are usually placed in developing capitula but in some species, such Yellow starthistle (Centaurea solstitialis L., Asteraceae), an inva- as U. cardui in Canada thistle (Lalonde and Shorthouse, 1984), galls sive, exotic winter annual weed in California and other western are produced in stems. In spite of their host specificity and relative states, has been the target of biological control efforts since the ease of establishment, few detailed studies have been published on 1960s (Turner et al., 1995). The guild of insects initially selected the impact that Urophora spp. have on their hosts. In one well- for biological control consisted solely of capitulum-impacting spe- studied system, the knapweed gall flies, U. affinis and U. quadrifas- cies, one of which was the gall forming fly, Urophora sirunaseva ciata, are believed to impact their host on two levels: a direct (Hering) (Diptera: Tephritidae). reduction of total seed number by physical displacement of ach- Tephritid flies have been utilized as biological control agents for enes (hereafter seeds) by the developing galls, and an indirect im- several exotic weeds with varying degrees of success (White et al., pact on seed numbers by disruption of resource partitioning within 1990; Turner, 1996a). In particular, Urophora species have been se- the plant (Harris, 1980). lected as host-specific biological control agents for many of the The yellow starthistle gall fly, U. sirunaseva, was first introduced Asteraceae (Harris, 1980; Freidberg, 1984; Turner, 1996a,b). Adult into North America in 1984. Extensive host specificity testing Urophora females oviposit within the host plant where the devel- (Clement and Sobhian, 1991; Sobhian, 1993; Turner, 1994) demon- strated a high-level of host specificity in this insect and a federal permit was granted. For the first year of releases, gall flies collected * Corresponding author. Address: California Department of Food and Agriculture, in northern Greece were released in California while others col- Biological Control Program, 3288 Meadowview Road, Sacramento, CA 95832, USA. lected in Turkey were released in Idaho. Additional collections Fax: +1 916 262 2059. E-mail address: [email protected] (D.M. Woods). from Greece, in 1985, 1990 and 1991, were released in California, Deceased. Idaho, Oregon, and Washington (Turner et al., 1995). The fly was

1049-9644/$ - see front matter Published by Elsevier Inc. doi:10.1016/j.biocontrol.2008.08.011 D.M. Woods et al. / Biological Control 47 (2008) 172–179 173 established at several locations by 1989 and an extensive distribu- produced few seedheads, and two were located near the base tion campaign was initiated in California (Villegas, 1996). Cur- where plants were larger and produced many seedheads. The four rently the fly is widely distributed throughout the range of transects at Winters were placed parallel across the hillside with yellow starthistle in California (Pitcairn et al., 2008). the lowest transect near the base of the hillside and the other tran- Adult female flies oviposit in closed flower buds of starthistle sects placed every 2 m up the hillside. Metal rods (n = 40) were and developing larvae stimulate the formation of lignified, uniloc- placed along each transect in a stratified random method (each ular galls, which are partially submersed in the receptacle. Previ- rod was randomly located with a random-number table within ous laboratory studies showed the fly to be bivoltine and to each half-meter section) for a combined total of 160 rods per site. overwinter as mature larvae within galls (Zwolfer,} 1969; Turner, The nearest flowering plant to each rod was selected as a study 1994; Turner et al., 1994). The initial presumption was that the plant. As each seedhead on the selected plant reached maturity flies, free from their natural parasites, would increase to high-pop- and the post-pollination florets began to oxidize, a small cotton ulation levels and substantially reduce seed production of their bag was tied over the seedhead to confine developing seeds and host plant in the western United States. any emerging insects. Sites were visited weekly to monitor plants Here we report on a field study to examine the impact of and all post-pollination seedheads were bagged. To provide a bet- U. sirunaseva on yellow starthistle seed production. The impact of ter estimate of the attack rate, additional seedheads on nearby an exotic species results from its geographic distribution, its popu- plants were bagged to increase the weekly sample size to 100 lation density, and its per capita amount of damage (Parker et al., when the weekly total of bagged seedheads dropped below 100 1999). In this study, we estimate the suppressive effect of individ- seedheads. ual galls on seed production and examine the within-season The yellow starthistle plants growing at the Winters site were variation in the activity of the fly relative to the production of particularly small. Large plants (having more than three seed- seedheads by its host plant. The study was performed at two dis- heads) were under-represented in the regular transects. Therefore, tinctly different locations where the gall fly was established in in 1995, at the season end, an additional set (n = 80) of non-bagged California. At the time, the bud weevil, Bangasternus villosus, an- plants, each with at least four seedheads, (‘large’ plants) were se- other exotic biological control agent, was present at low levels at lected from every fourth rod and harvested. Seed production could both sites, but no other biological control agents were present. not be evaluated from these plants so during the second year (1996), an additional four transects were established and ten large plants per transect (n = 40) were selected. These were monitored 2. Materials and methods and bagged along with the regular plants throughout the season.

2.1. Study sites 2.3. Estimating seed loss due to U. sirunaseva

Two ungrazed field sites with populations of yellow starthistle Each study plant was harvested upon death (August–October), were selected as study sites. The ‘Ukiah’ site (39° 4.400N, 123° brought to the laboratory and each seedhead on every plant was 12.300W) was a steep, starthistle-infested grassy hillside adjacent evaluated. External diameters of the intact seedheads were mea- to a grape vineyard near the town of Ukiah in Mendocino County. sured and recorded in 1-mm size classes. Seedheads were then Most rain falls in California during the winter, when yellow star- carefully dissected for evidence of insect damage and the number thistle grows through the seedling and rosette stages. The summer and status of galls (empty or with larva), and the number of pap- flowering season of yellow starthistle in Ukiah is hot (maximum pus-bearing and non-pappus-bearing seeds were counted. All ma- daily temperatures average 30–33 °C) with an average cumulative ture seeds were weighed, then germinated in controlled summer rainfall of only 12 mm. In 1990, this site received one of environmental chambers at 20 °C on wet blotter paper with an 8/ six releases of U. sirunaseva obtained from foreign material with 16 h light/dark cycle to determine viability. Seeds not germinating 372 adult flies reared from galls imported from Greece. Our studies by seven days were cut open and those filled with healthy embryo at this site occurred in 1993 and 1994. tissue were also considered viable. Seed production was estimated The ‘Winters’ site (38° 28.870N, 122° 2.350W) was a dry hillside as the sum of germinated seed and ungerminated viable seed. All overlooking the Sacramento Valley near the town of Winters in seedheads and plant parts were oven dried at 60 °C and weighed Solano County. It was distinctly warmer and drier than the Ukiah separately. A total of 4029 seedheads were bagged and evaluated site with daily summer high-temperatures averaging 34–36 °C during the two-year study (1993–94) at Ukiah and 1783 during and an average cumulative summer rainfall of only 5 mm. Gall flies the two years (1995–96) at Winters. released at this site were locally collected as galled capitula (here- The impact of galls on seed production was examined in three after seedheads) from established populations near Newcastle in ways. First, the direct impact of individual galls was calculated Placer County, approximately 90 km distant. The galled seedheads using multiple linear regression with seedhead size and number were placed in screen-topped containers and adult flies were al- of galls as independent variables and the number of seed per seed- lowed to emerge on-site. Based on laboratory rearing of subsam- head as the dependent variable. Each year was analyzed separately. ples, over 1300 flies were released during 1993 and 1994 at this Secondly, the potential maximum direct impact of galls was eval- site. Our studies took place in 1995 and 1996. uated by similar analysis using only maximumly-producing seed- Plant and seedhead densities were estimated at Ukiah in fall heads. In this case, for each seedhead size and gall number 1994 and at Winters in fall 1995 by counting all the seedheads combination, the number of seeds produced by three seedheads on all plants rooted in 40, quarter square meter frames along tran- with the highest number of seeds was regressed against seedhead sects at each site. size and gall number. These results estimated the loss in maximum seed production compared to the loss of seed averaged across all 2.2. Study plants seedheads. Lastly, direct impact of galls on seed production was estimated Four 20-m transects were established at each site. The transects as the proportional loss of seed compared to ungalled seedheads. were placed to represent the primary variation in yellow starthistle Mean seed number was calculated for all combinations of the seed- populations at each site. At Ukiah, two transects were located on head size and gall number. Proportional seed loss was estimated by the upper ridge of the hillside where plants were small and dividing each combination mean by the mean number of seeds for 174 D.M. Woods et al. / Biological Control 47 (2008) 172–179 that size seedhead with no galls. For example, for all seedheads shallow soil that dried out rapidly in the summer, in contrast to 4 mm in diameter, the mean number of seeds produced in seed- the deeper soil and cooler temperatures at Ukiah. Plant density heads with one-gall, two-galls, etc., were each divided by the aver- at Ukiah was considerably lower and, consequently, plants were age number of seeds for 4 mm seedheads without galls. The much larger than those at the Winters site, both in weight and proportion of seed destroyed, S, within a seedhead due to the pres- number of seedheads (Table 1). The flowering season at the Win- ence of galls was described using the equation: ters site was roughly half the duration of the Ukiah site (Table 1). Larger plants at each site produced larger seedheads than smaller S ¼ a H þ b G þ intercept plants (at Ukiah, y = 0.49lnx + 6.92), r2 = 0.316; at Winters, where H is the diameter of the seedhead (mm) and G is the number y = 0.89lnx + 6.55, r2 = 0.63). The seed weevil, B. villosus, damaged of galls. To increase sample size, data from both years was com- less than 1% of seedheads over both years at Ukiah and approxi- bined for each site. mately 1% and 5% of the seedheads at Winters during 1995 and 1996. 2.4. Impacts beyond the attacked seedhead 3.2. Distribution of galls among plants and seedheads In order to evaluate the impact of galls to other parts of the plant beyond the attacked seedhead, we compared ungalled seed- Gall fly populations, estimated by the percent of seedheads heads on ungalled plants to ungalled seedheads on galled plants. with galls, were much higher at Ukiah than at Winters (Table Plants were placed into two groups: plants with at least one gall 2). Gall flies did not attack all seedheads or all plants. When and those without galls. Within each year and bagging date, we present, most galls occurred singly or in small groups of 2–5 compared seed production by ungalled seedheads from ungalled galls per seedhead. These clusters represented 86% and 73% of plants to seed production by ungalled seedheads in the same size the galled seedheads during the two years at Ukiah and 94% class that developed on galled plants. Mean seed production was and 96% of the galled seedheads at Winters. The frequency dis- compared only where three or more seedheads were produced tribution of galls in galled seedheads for Ukiah in 1993 and on both galled and ungalled plants within each sampling date. Winters in 1996 is shown in Fig. 1 which typified both years at each site. Gall density in terms of galls per galled seedhead 2.5. Seasonal activity of the gall fly and the maximum number of galls per seedhead was much higher at Ukiah (Table 2). Large plants with many heads were Two analyses were made on the seasonal activity of the gall fly. In more likely to be attacked by U. sirunaseva than the smaller order to evaluate the bivoltine nature of the gall fly, bagged samples plants (Fig. 2). For example, 72% of the large plants collected for the second year at each site (1994 and 1996) were processed in 1995 from Winters had at least one galled seedhead com- within six months of collection. The developmental stage of the flies pared to 19% on the smaller plants. (larva or adult) and their health (dead or alive) were noted for each individual gall. Empty galls represented flies that emerged during 3.3. Direct impact of galls on seed production the summer generation, while those remaining in the galls as mature larvae represented the overwintering generation. Seed production by yellow starthistle was strongly correlated The plant preference of the gall flies was examined by sorting with seedhead size (at Ukiah y = 5.39x 12.86, r2 = 0.862, plants into four groups based on number of seedheads. We com- P < 0.001 in 1993; y = 7.61x 29.75, r2 = 0.970, P < 0.001 in pared those groups in which the first three seedheads were galled 1994; at Winters, y = 4.94x 12.27, r2 = 0.991, P < 0.001 in to those whose first three seedheads were not galled. Groups in- 1995; y = 6.22x 33.12, r2 = 0.967, P < 0.001 in 1996. Linear cluded: small plants having 4–10 seedheads; medium plants hav- regressions showed that each mm increase in size accounted ing 11–19 seedheads; large plants having 20–29 seedheads; and for an additional 4.9–7.6 seeds in non-galled seedheads, depend- huge plants having over 30 seedheads. Plants with mixed results ing on the site and year. Seedhead size varied from 3–12 mm in for the first three seedheads were excluded from this analysis. diameter with a median of 7 mm. Most (95%) seedheads were in The attack rate on subsequently produced seedheads of each plant the 5–9 mm size classes. Some seedheads without galls produced was evaluated for each plant group. The Winters site had very few no seed while the largest seedheads produced up to 88 seeds. large plants so only the data for Ukiah was analyzed. Plants from The seedheads that produced no seeds were excluded from fur- both years at Ukiah were combined in order to accumulate a large ther analysis. enough sample size in each category. Larger seedheads were more likely to have galls than the smaller seedheads (Fig. 3A), particularly at Ukiah where there were 2.6. Long-term follow up both more gall flies and a higher number of large seedheads. Large seedheads did not however, always support a higher num- The Ukiah site was again sampled in 2000 to assess any long- ber of galls per galled seedhead (Fig. 3B). The exception was term changes in the population density of the gall fly. Twenty Ukiah in 1994 and, to a lesser degree, in 1993. The difference plants were collected at the end of the summer, (closest plant to may be due to the higher density of gall flies relative to the avail- every eight rod of pre-existing transects) and processed as above ability of seedheads and, thus, higher oviposition pressure at to evaluate attack by any of the yellow starthistle biological control Ukiah. Thus, larger seedheads, which have the potential to pro- agents. Seed production was not evaluated. The Winters site had duce more seed per seedhead, could support a larger number of been repeatedly grazed and was not sampled. galls per seedhead. The direct impact of individual galls on the average number of seeds per seedhead is presented by seed type in Table 3. The 3. Results presence of each gall caused a reduction of between 2.1 and 2.9 seeds per seedhead depending upon the year. The impact of each 3.1. Weed and insect populations gall under maximum seed production conditions increased to between 4.0 and 5.4 seeds in an individual seedhead. Galls appear Environmental conditions differed substantially between the to have a slightly greater impact on pappus-bearing seed than two study sites. The Winters site was an exposed hillside with non-pappus seed, both on average and in the largest seedheads. D.M. Woods et al. / Biological Control 47 (2008) 172–179 175

Table 1 Characteristics of yellow starthistle plants at two sites over two years in California

Ukiah Winters 1993 1994 1995 largea 1995 regular 1996 regular 1996 largeb Population density (plants/m2) 8.6 77.7 Mean plant dry wt. g (S.E.) 4.4 (0.5) 3.3 (0.7) 0.4 (0.03) 0.5 (0.07) 2.8 (0.4) Mean # seedheads/plant (S.E.) 12.1 (1.5) 9.9 (1.9) 5.8 (0.5) 1.4 (0.1) 2.5 (0.4) 10.4 (1.3) Flowering season (weeks) 13 11 5 5 7

a Second set of large plants (>3 seedheads) harvested at end of summer but not bagged. b From extra transects of large (>3 seedheads) plants.

Table 2 Gall distribution in yellow starthistle plants monitored over two years at two sites in California

Ukiah Winters 1993 1994 1995 largea 1995 regular 1996 regular 1996 largeb % Plants w/galls 61.9 46.5 72.5 18.9 13.0 46.3 % Seedheads w/galls 46.4 55.7 33.6 25.2 10.8 19.9 Mean galls per sampled seedhead (S.E.) 1.5 (0.1) 2.4 (0.1) 0.9 (0.04) 0.5 (0.1) 0.3 (0.1) 0.5 (0.1) Mean galls per galled seedhead (S.E) 3.1 (0.1) 4.1(0.1) 2.7 (0.2) 2.1 (0.2) 1.8 (0.2) 2.3 (0.1) Maximum # galls per seedhead 14 15 9 12 6 10

a Second set of large plants (>3 seedheads) harvested at end of summer but not bagged. b From extra transects of large (>3 seedheads) plants.

Fig. 1. Frequency distribution of galls in galled seedheads at Ukiah 1993 (solid bars) Fig. 2. Relationship of plant size, estimated by number of seedheads, to attack by and Winters 1996 (shaded bars). Urophora sirunaseva. Plants were grouped into categories based on the number of seedheads per plant. The Ukiah data is based on bagged plants along the transects. The Winters data includes the regular transects and the large plants to increase representation in the large categories. Losses of pappus-bearing seed accounted for between 78% and 92% of the total seed loss depending on the year, while pappus- bearing seed made up 70.6% of the total seed produced in non- 3.4. Impacts beyond attacked heads galled seedheads during the four years of our study. Percent seed destruction was estimated as the proportional Seed production in ungalled seedheads produced on galled decrease in average seed number per seedhead compared to plants at Ukiah was slightly higher (102% S.D. = 34), but not signif- ungalled seedheads. The linear model explained 59% and 51% icantly different from seed production in similarly sized ungalled of the variance in seed destruction for Ukiah and Winters, seedheads produced on gall free plants. At Winter, seed production respectively (Table 4). More complex non-linear models did not was higher still (118% S.D. = 35) of that in ungalled plants. signiﬁcantly increase the amount of explained variance. The regression equations estimated an 11% (Winters) and 5% (Ukiah) 3.5. Seasonal activity of U. sirunaseva decrease in seed production with the addition of each gall to a seedhead. According to the ﬁtted equations, 100% seed destruc- The production of new post-pollination seedheads by yellow tion was not expected to occur at Ukiah, even at the highest starthistle began in late-June (Winters) and early-July (Ukiah) densities of galls observed per seedhead (Fig. 4). At Winters, and continued through August and into September (Winters) and 100% seed destruction was estimated for seedheads with more through October (Ukiah) (Fig. 5). For three years (1994–1996), than seven galls in smaller seedheads and more than 10 galls seedhead production showed a single peak in mid season. In in the larger seedheads. 1993 at Ukiah, seedhead production occurred in two peaks, one 176 D.M. Woods et al. / Biological Control 47 (2008) 172–179

Table 4 Relationship between percent seed destruction, seedhead size class, and gall number for yellow starthistle at Ukiah (1993–1994 data combined) and Winters (1995–1996 data combined)

Location Slope Seedhead size No. of galls Intercept r2 P

Ukiah 0.0236 0.0548 0.1552 0.51 <.01 Winters 0.0421 0.1068 0.3505 0.59 <.01

See text for function equation.

Fig. 4. Regression surface describing the proportion of seed destroyed at Ukiah in 1993 and 1994 as a function the number of Urophora sirunaseva galls per seedhead and seedhead size (see text for function equation; ﬁlled circles represent original data).

tion rates were observed among the early seedheads (Fig. 6) then declined approximately 2–3 weeks later during both years at Ukiah and in 1995 in Winters. A second, more protracted peak of activity Fig. 3. Distribution (A), and density (B), of galls of Urophora sirunaseva within occurred in August. High-oviposition rates were absent among the yellow starthistle seedheads of various sizes. Associated regression equations for early seedheads at Winters in 1996 where only one distinct peak of (A) y = 9.96x 25.18, r2 = 0.938, P < 0.001 for 1993; y = 10.1x 12.99, r2 = 0.953 activity was observed. The largest peak of gall formation at the 2 P < 0.001 or 1994; y = 3.67x + 1.85, r = 0.719, P = 0.008 for 1995; y = 2.87x 3.15, Ukiah site was the second generation. In contrast, the largest peak r2 = 0.509, P = 0.047 for 1996. Associated regression equations for (b): y = 0.17x + 1.65, r2 = 0.336, P = 0.102 for 1993; y = 0.67x + 0.36, r2 = 0.917, at the Winters site is in the early part of the year with a very small P < 0.001 for 1994; y = 0.02x + 2.03, r2 = 0.021, P = 0.221 for 1995; y = 0.12x + 1.80, peak in mid summer. This single peak of U. sirunaseva gall forma- r2 = 0.105, P = 0.594 for 1996. tion is closely timed to yellow starthistle development which also had a short season at this site (Fig. 6). Rapid processing of the samples shortly after harvest in 1994 Table 3 and 1996 allowed an evaluation of development of the overwinter- Seed reduction in yellow starthistle seedheads associated with each gall of Urophora ing stage of the gall fly. The presence of empty galls indicated sum- sirunaseva mer emergence while individuals remaining as larvae represented 1993 1994 1995 1996 the overwintering generation. Galls produced in early-July resulted All seedheads almost exclusively (90%) in summer emergence of gall flies (Fig. 7). Total seed 2.2 2.1 2.1 2.9 The proportion shifted from summer emergence to an overwinter- Pappus-bearing only 1.8 1.9 1.7 2.7 ing stage during July, with 50% of galls producing overwintering Non-pappus bearing only 0.4 0.3 0.5 0.2 larvae in mid-July at Winters and late-July at Ukiah. By mid-Au- Three largest seed producers gust, all new galls produced only overwintering larvae. Total seed 4.3 4.0 5.3 5.4 Plants whose first three seedheads were attacked by U. siruras- Pappus-bearing only 3.4 3.4 4.1 5.0 eva were far more likely to attract gall flies in future seedheads Non-pappus bearing only 0.8 0.6 1.2 0.4 than plants not galled in the first three seedheads (Fig. 8). Subse- Seed production for each seedhead was regressed against the number of galls found quently produced seedheads on each plant tended to follow the in the seedhead. pattern of the first three heads. On average, subsequently-pro- Values are slopes from regression equations calculated for each year. duced seedheads were seven times more likely to remain non- galled if the first three heads were non-galled. early-August and the other in mid-September. Adult flies were already present in the field at both sites when the first plants began 3.6. Observations at Ukiah six years later to flower and were also present when flowering ceased at the end of summer. Oviposition occurred throughout the summer as fresh The gall fly was present at the Ukiah site six years after the im- galls could be detected for each sample date (Fig. 5). High-oviposi- pact study was completed. Galls were found in 65% of the Ukiah D.M. Woods et al. / Biological Control 47 (2008) 172–179 177

Fig. 5. Yellow starthistle seedhead production and gall development at Ukiah 1993 (A), Ukiah 1994 (B), Winters 1995 (C), and Winters 1996 (D). The total numberof seedheads and galled seedheads developing on all transect plants were divided by the number of days since the last sampling interval to calculate the number of new seedheads (closed squares) and galled seedheads (open circles) produced on the sampled plants per day during the sampling interval.

Fig. 6. Seasonal distribution of yellow starthistle galls at two sites in California. A total of 160 plants were bagged each week in 1993–1995 and 200 in 1996. The ﬁnal Fig. 7. Developmental stage of Urophora sirunaseva in ﬂowerheads of yellow weeks of bagging at Winters included three or fewer seedheads so the percent starthistle at two sites in California. Galls either had overwintering larva at time of attack is not shown. bagging or were empty (adults emerged). Heads with dead larva were excluded. 178 D.M. Woods et al. / Biological Control 47 (2008) 172–179

(Turner et al., 1994). There was also a substantial increase in gall density: 1.9 galls per galled seedhead in 1992 (Turner, 1994) which increased to 3.1 and 4.6 in 1993 and 1994, respectively. Overall, 68% of the galled seedheads at the Ukiah site evaluated in our study had more than one gall. This is a substantial increase over the 56% found by Turner (1994) for his best site in 1992. Gall densities have not maintained statewide as we have only detected in-head gall densities of greater than eight on one other occasion out of 46,555 seedheads dissected statewide through 2004 (per- sonal observations). Even if the gall fly had maintained populations at the higher Ukiah levels, U. sirunaseva does not appear to have the potential to have a major impact on seed production by yellow starthistle in California as individual galls have a limited impact. Quantifying the impact of galls in yellow starthistle provided some challenges due to the increased seed production in larger seedheads. By com- paring gall effects within seedhead size classes, we estimated the impact of a single gall as 2.1–2.9 seeds per gall. The impact may be somewhat larger (4.0–5.4 seeds per gall) when comparisons Fig. 8. Percentage of seedheads attacked on an individual plant after the first three are made of the effect that single galls can have on maximally pro- seedheads were either all galled (solid bars) or all not galled (diagonal shading). ducing heads. Still, much higher population densities of U. sirunaseva are needed to significantly impact total seed production at the plants collected in 2000, but in only 21% of the seedheads. This was population level. less than half of the attack rate observed in 1994. The false peacock Reduction of seed by U. sirunaseva is fairly similar to other Uro- fly, Chaetorellia succinea, a common insect of yellow starthistle phora species in other systems. Harris (1980) found that each accidentally introduced in the early 1990s (Balciunas and Villegas, U. affinis reduced production by 2.4 seeds per seedhead in spotted 1999) had invaded the site and occurred in 55% of the heads. No knapweed and each U. quadrifasciata reduced production by 1.9 other biological control agents (including B. orientalis) were seeds per seedhead. Mays and Kok (1996) calculated similar values recovered. and noted that substantial reduction in seed production did not occur until there were at least four galls in a spotted knapweed 4. Discussion seedhead. Yellow starthistle also seems to have a threshold necessary for During host testing, Turner (1994) felt that, in the absence of it impact, however, the substantially larger seedhead size and per- own natural enemies, U. sirunaseva would become a good biologi- head seed production of yellow starthistle diminished the relative cal control agent of yellow starthistle once insect populations in- impact of single galls even further. Smaller seedheads were more creased to their full potential. The gall fly has proven to be fairly affected by the presence of 1–3 galls than were the larger seed- easy to establish and to distribute throughout California. A massive heads in terms of seed production. For seedheads larger than distribution program (Villegas, 1996) introduced the gall fly 7 mm, a single gall or two appeared to have little impact on total throughout the state and it established well in most locations seed production. Yet a single gall in a size 5 mm seedhead reduced (Pitcairn et al., 2008). The Ukiah site was established in 1990 and seed production by over 25%. However, for all head sizes when gall by 1992, 44% of seedheads were infested (Turner et al., 1994). density reached four or more per seedhead, seed production was Our results show that the gall fly population rose to 46% in 1993 dramatically impacted. This was particularly acute at Winters and to 56% in 1994 but declined by October 2000 (10 years after where no seedhead with more than six galls had over three seeds the first release) when galls were found in only 21% of the (15% of normal for unattacked heads). Seedheads size 6 mm and seedheads. smaller seldom supported more than six galls. Larger seedheads We cannot explain why U. sirunaseva has not increased to levels can handle increasingly more galls without an apparent loss of which would control the weed, but competition with other intro- seed numbers, perhaps due to a greater capacity to accept physical duced biological control agents is possible. The successful estab- distortion of the receptacle. Unfortunately, highly concentrated lishment of at least three other insect species at most sites in galls were not common and seedheads with more than four galls California creates competition for a limited resource (Pitcairn occurred less than a fourth of the time for any given year. et al., 2003, 2008). The recent immigration of C. succinea is partic- The larger seedheads of yellow starthistle will require extremely ularly important. In 2000, over 50% of the seedheads at the Ukiah high-levels of attack by U. sirunaseva to cause significant reduction in site were infested with C. succinea, replacing U. sirunaseva as the seed production on the population level. With an average reduction most common biological control insect at the site. In a 1985 field of 2.1 seeds per gall, an average of 14 galls per head would be neces- study, Clement and Sobhian (1991) found that U. sirunaseva at- sary to achieve a 90% seed reduction at Ukiah and 11 galls per head at tacked only 2.6% of the capitula in a garden plot in Greece. With Winters. In contrast, our field sample average of 1.9 and 0.5 galls per ten species of insects attacking yellow starthistle in that study, head and would account for only 7.6% and 2.6% reduction of the seed U. sirunaseva faced substantial competition. Apparently, as more produced at the Ukiah and Winters sites, respectively. insects fill this niche in California, we might expect U. sirunaseva The phenology of the yellow starthistle flowering season was not to decline to a minor role. The gall fly does not appear to have pro- always synchronized with activity by U. sirunaseva as in the knap- gressed distinctly better at other sites in California. A state-wide weed system in the northern parts of the continent. Spotted knap- survey in 2001/2002 found U. sirunaseva galls at 61% of the 421 weed in much of North America has a reasonably short flowering sites surveyed and only eight sites had greater than 25% of the period and activity by U. affinis is well timed to exploit this. In the seedheads with galls (Pitcairn et al., 2003, 2008). drier portions of the Central Valley of California, yellow starthistle In our study, the maximum galls per seedhead (15) was higher behaves much as it does at Winters, with a short flowering season than previously reported (nine in Greece and 12 in California) but also limiting U. sirunaseva to a single generation per year. How- D.M. Woods et al. / Biological Control 47 (2008) 172–179 179 ever the deeper soils infested by yellow starthistle through other Lalonde, R.G., Shorthouse, J.D., 1984. Developmental morphology of the gall of parts of the state provide adequate moisture for an extended season Urophora cardui (Diptera: Tephritidae) in the stems of Canada thistle (Cirsium arvense). Canadian Journal of Botany 62, 1372–1384. that supports new bud formation over several months as it did at Mays, M.T., Kok, L.T., 1996. Establishment and dispersal of Urophora affinis (Diptera: Ukiah. In spite of the gall fly remaining active over the entire summer Tephritidae) and Metzneria paucipunctella (Lepidoptera: Gelechiidae) in flowering period in our study, attack levels did vary over the season, Southwestern Virginia. Biological Control 6, 299–305. Parker, I.M., Simberloff, D., Lonsdale, W.M., Goodell, K., Wonham, M., Kareiva, P.M., dropping between generations. Williamson, M.H., Von Holle, B., Moyle, P.B., Byers, J.E., Goldwasser, L., 1999. Urophora sirunaseva has a distinctly different mode of impact Impact: toward a framework for understanding the ecological effects of (seed displacement) than the other yellow starthistle seedhead invaders. Biological Invasions 1, 3–19. Pitcairn, M.J., Villegas, B., Woods, D., Wilbur, G., Duffy, A., El-Bawdri, M., 2003. biological control agents which feed directly on developing seeds. Statewide survey of yellow starthistle biological control agents. In: Woods, D.M. Shorthouse (1989), in his study of the hard galled U. affinis, de- (Ed.), 2002 Biological Control Program Annual Summary. California Department scribed morphological changes in knapweed seedheads believed of Food and Agriculture, Plant Health and Pest Prevention Services, Sacramento, California. to explain seed reductions. The galls of U. sirunaseva are somewhat Pitcairn, M.J., Villegas, B., Woods, D.M., Yacoub, R., Joley, D.B., 2008. Evaluating similar in shape to those of U. affinis so we might expect that losses implementation success for seven seed head insects on Centaurea solstitialis in could be similarly ascribed, including: ovipositional injury, California, USA. In: Julien, M. (Ed.), In: Proceedings of the 12th International destroying an occasional floret, physical displacement of seeds, Symposium on the Biological Control of Weeds, La Grande Motte, France. pp. 610–616. and the redirection of resources to gall feeding nutritive support Shorthouse, J.D., 1989. Modification of flowerheads of diffuse knapweed by the gall- cells in the seedhead at the expense of seed production. The woody inducers Urophora affinis and Urophora quadrifasciata (Diptera: Tephritidae). In: galls of U. sirunaseva seem to physically occupy the space of 2–3 Delfosse, E.S. (Ed.), Proceedings of the 7th International Symposium on the Biological Control of Weeds. Rome, Italy. pp. 221–228. seeds and might also be expected to reduce seed production over Sobhian, R., 1993. Life history and host specificity of Urophora sirunaseva (Hering) an entire plant by draining plant resources. The metabolic sink ef- (Diptera: Tephritidae), an agent for biological control of yellow starthistle, with fect reported in knapweeds restricts the development of additional remarks on the host plant. Journal of Applied Entomology 116, 381–390. Turner, C.E., 1994. Host specificity and oviposition of Urophora sirunaseva (Hering) flower buds, which abort, killing developing larvae and therefore (Diptera: Tephritidae), a natural enemy of yellow starthistle. Proceedings limiting the ultimate gall population that can be supported by Entomological Society of Washington 96, 31–36. the plant (Harris, 1980). Yellow starthistle has a more extended Turner, C.E., 1996a. Tephritidae in the biological control of weeds. In: McPheron, B.A., Steck, G.J. (Eds.), Fruit Fly Pests: A World Assessment of their Biology and flowering season that makes such a comparison difficult. Flower Management. St. Lucie Press, Delray Beach, Florida, pp. 157–164. bud initiation in yellow starthistle may occur long after the galls Turner, C.E., 1996b. Tephritid flies in the biological control of yellow starthistle. In: in other seedheads have matured and no longer metabolically McPheron, B.A., Steck, G.J. (Eds.), Fruit Fly Pests: A World Assessment of their Biology and Management. St. Lucie Press, Delray Beach, Florida, pp. 171–176. affect the plant. Effects on seed production beyond the galled Turner, C.E., Johnson, J.B., McCaffrey, J.P., 1995. Yellow starthistle. In: Nechols, J.R. seedhead were not detected with our data and, if present, are (Exec. Ed.), Biological Control in the Western United States. University of likely to be small on such an environmentally plastic invasive California, Division of Agriculture and Natural Resources, Publication 3361, pp. plant. 270–275. Turner, C.E., Sobhian, R., Joley, D.B., Coombs, E.M., Piper, G.L., 1994. Establishment of Urophora sirunaseva (Hering) (Diptera: Tephritidae) for biological control of yellow starthistle in the western United States. Pan-Pacific Entomologist 70, References 206–211. Villegas, B., 1996. Distribution of the seedhead gall fly, Urophora sirunaseva,in Balciunas, J., Villegas, B., 1999. Two new seed head flies attack yellow starthistle California for the biological control of yellow starthistle. In: Bezark, L.G. (Ed.), (Diptera: Tephritidae). California Agriculture 53 (2), 8–11. 1995 Biological Control Program Annual Summary, California Department of Clement, S.L., Sobhian, R., 1991. Host-use patterns of capitulum-feeding insects of Food and Agriculture, Division of Plant Industry, Sacramento, California. yellow starthistle: results from a garden plot in Greece. Environmental White, I.M., Groppe, K., Sobhian, R., 1990. Tephritids of knapweeds, starthistles and Entomology 20, 724–730. safflower: results of a host choice experiment and the taxonomy of Terellia Freidberg, A., 1984. Gall Tephritidae (Diptera). In: Ananthakrishnan, T.N. (Ed.), luteola (Wiedemann) (Diptera: Tephritidae). Bulletin of Entomological Research Biology of Gall Insects. Oxford & IBH Pub. Co., New Delhi, India, pp. 129–167. 80, 107–111. Harris, P., 1980. Effects of Urophora affinis Frfld. and U. quadrifasciata (Meig.) Zwolfer,} H. 1969. Urophora siruna-seva (Hg) (Dipt: Trypetidae), a potential insect for (Diptera: Tephrititdae) on Centaurea diffusa Lam and C. maculosa Lam. the biological control of Centaurea solstitialis L. in California. Commonwealth (Compositae). Zeitschrift fur angewandte Entomologie 90, 190–201. Institute of Biological Control, Technical Bulletin Number 11, p. 154.