Large-scale releases of Trichogramma ostriniae to suppress Ostrinia nubilalis in commercially grown processing and fresh market sweet corn

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1 J. Appl. Entomol. 131(6), (27) doi:.1111/j x Journal compilation Ó 27 Blackwell Verlag, Berlin Large-scale releases of Trichogramma ostriniae to suppress Ostrinia nubilalis in commercially grown processing and fresh market sweet corn J. Gardner 1, M. P. Hoffmann 1, S. A. Cheever 1, A. J. Seaman 2, P. Westgate 3 and R. V. Hazzard 3 1 Department of Entomology, Cornell University, Ithaca, NY, USA; 2 New York State Integrated Pest Management Program, New York Agricultural Experiment Station, Geneva, NY, USA; 3 Department of Entomology, University of Massachusetts, Amherst, MA, USA Ms. received: December 13, 26; accepted: March 3, 27 Abstract: We evaluated the egg parasitoid Trichogramma ostriniae (Pang et Chen) (Hym.: Trichogrammatidae) for control of European corn borer [Lep.:Crambidae: Ostrinia nubilalis (Hu bner)] in fields of commercial processing sweet corn in New York in 22 and 23. We made inoculative releases of 7 T. ostriniae/ha when corn was at midwhorl and regional O. nubilalis activity had begun. Each release field was paired with a similar non-release control. The incidence of egg mass parasitism, number of stalk tunnels, incidence of ear damage and effect on insecticide spray decisions were evaluated. Parasitism of cumulative number of O. nubilalis egg masses was 1.3 ± 7.2% and 43. ±.7% (mean ± SEM) in release fields in 22 and 23 respectively. The incidence of ear damage by O. nubilalis was not affected by T. ostriniae in either year of the study. There was no difference in stalk damage between release fields and control fields in either 22 or 23. However, individual comparisons between paired release and control fields showed reduced stalk tunnelling in of 19 fields, but ear damage was reduced in only two of 19 fields. Neither the number of actual insecticide sprays nor the number of decisions to spray based on sequential sampling were affected significantly by releasing T. ostriniae. In a separate but related study, where releases were conducted in commercial fresh market sweet corn and used higher rates of release, T. ostriniae had no effect on the number of insecticidal sprays. In an additional experiment using small plots of sweet corn, ear damage was similar whether plots were treated with T. Ostriniae or with insecticide. Additional research should focus on improving the timing, frequency and rate of releases. Key words: Zea mays, biological control, egg parasitoid, inoculative release 1 Introduction European corn borer, Ostrinia nubilalis is a pest of corn (Zea mays) in the USA and other parts of the world. Yield losses in field corn have been documented (e.g. Lynch et al. 198; Lynch 198; Bode and Calvin 199) and it is estimated that as few as one larva per plant can cause economic injury (Mason et al. 1996; Myers and Wedberg 1999; Bigler et al. 199). After its accidental introduction into North America, efforts have been made to introduce natural enemies of O. nubilalis. A number of parasitoids have been imported from Europe and Asia and a few have become established (Anonymous 199; Mason et al. 1996), a tachinid fly, Lydella thompsoni Herting (Dipt.: Tachinidae) and two wasps, Macrocentrus grandii Goidanich (Hym.: Braconidae) and Eriborus terebran Gravenhorst (Hym.: Ichneumonidae) (Anonymous 199; Mason et al. 1994). These parasitoids and several predators help suppress O. nubilalis populations, but generally do not consistently keep infestations below acceptable economic injury levels (Burkness et al. 24), especially in sweet corn. Augmentative releases of an effective biological control agent for O. nubilalis in corn could help to reduce populations below damaging levels. To date, a number of trichogrammatids have been considered for this purpose (Smith 1996). Trichogramma brassicae Bezdenko, Trichogramma evanescens Westwood, Trichogramma platneri Nagarkatti, Trichogramma minutum Riley, and Trichogramma nubilale Ertle and Davis and others have been scrutinized, but success has been limited. Losey and Calvin (1991) reported that T. minutum and T. platneri did not parasitize O. nubilalis egg masses and that Trichogramma pretiosum and Trichogramma georgia showed only minimal activity. T. brassicae is used successfully in Europe (e.g. Bigler et al. 22) and has been studied in the USA and Canada (Yu and Byers 1994; Mertz et al. 199), but a scarcity of reports from North America indicates that it has not gained acceptance here. Of North American

2 WCR special issue submission 433 species, T. nubilale has demonstrated good potential (Ertle and Davis 197; Curl and Burbutis 1978; Andow and Prokrym 199; Prokrym and Andow 1991; Losey and Calvin 199). Losey and Calvin (1991) reported that in laboratory trials 8% of females of T. nubilale parasitized at least a portion of an egg mass, and in the field they parasitized nearly 8% of naturally occurring egg masses and 3% of artificially placed egg masses. In other research with T. nubilale, Andow et al. (199) reported from 2% to 6% parasitism of O. nubilalis egg masses in sweet corn by T. nubilale. Unfortunately, this parasitoid is reported to be either too erratic (Andow et al. 199), too expensive, or too selective for commercial production (Losey and Calvin 199). In contrast to the species aforementioned, earlier studies identified Trichogramma ostriniae as a strong candidate for augmentative biological control of O. nubilalis in corn (Wang et al. 1984; Chiu and Chen 1986; Zhang 1986; Hassan and Guo 1991; Pavlík 1993). In its native range, it is found in a rather large geographic and climatic area extending from Hunan in southern China (approximately 2 N) to Jilin in the north-east (approximately 42 N) (Pang 1986; Zhang 1986; Zong et al. 1986), indicating broad adaptability. In the first field release trials in the north-eastern USA, researchers recorded >97% parasitism of naturally occurring O. nubilalis eggs in sweet corn when experimental releases were made at a density of approximately T. ostriniae per hectare (Mason et al. 1996). More recent research has proven this parasitoid to be highly effective at controlling O. nubilalis in sweet corn (Seaman et al. 1997; Wang and Ferro 1998; Wang et al. 1999; Wright et al. 21; Hoffmann et al. 22; Kuhar et al. 22; Wright et al. 22). Releases in sweet corn in New York showed parasitism of sentinel O. nubilalis eggs to average approximately 6% (Hoffmann et al. 1992) and parasitism of naturally-occurring O. nubilalis eggs ranges from 7% to 8% (Seaman et al. 1997). Wright et al. (22) reported that inoculative releases in small plots of sweet corn reduced damage by approximately % and life table studies from small plots indicate that T. ostriniae increased total O. nubilalis mortality from 61% to 92% in sweet corn and from 8% to 93% in field corn (Kuhar et al. 22). At very high O. nubilalis egg density, T. ostriniae exhibited a positive density-dependent functional response in the field (Wang and Ferro 1998). T. ostriniae also appears to be more efficient than T. nubilale at finding O. nubilalis egg masses and provides higher levels of parasitism within the field (Wang et al. 1999). Other research has shown additional favourable traits of T. ostriniae. For instance, it can successfully parasitize O. nubilalis eggs throughout most of their embryonic development (Hoffmann et al. 199), it is easily reared commercially (Hoffmann et al. 21), can be stored for a period of time (Pitcher et al. 22), and does not lose fitness even after many generations of rearing on factitious hosts such as Ephestia or Sitotroga spp. (Hoffmann et al. 21). It has also demonstrated proficient dispersal and parasitism in sweet corn (Wright et al. 21). To date, we have not observed any conclusive evidence of T. ostriniae overwintering and establishing in New York. Thus, in the absence of any evidence suggesting utility as a classical biological control agent, seasonal re-release is indicated. The objective of the study reported herein was to evaluate the efficacy of seasonal inoculative releases of T. ostriniae for controlling O. nubilalis in commercially managed fields of processing and fresh market sweet corn. Previous studies conducted by ourselves and colleagues were generally in small research plots and small fields of fresh market sweet corn that received no insecticidal treatments. The research reported herein was conducted primarily on commercial sweet corn that received insecticidal sprays, thus we could test integrating T. ostriniae with commercial practices typical of our region. 2 Materials and Methods 2.1 Efficacy of Trichogramma ostriniae in processing sweet corn Trials were conducted in 22 and 23 north of Penn Yan, NY, USA. The T. ostriniae used in the study were originally obtained from northern China before rearing at the Biological Control Laboratory, USDA, Mission, TX. Since 1993 we have maintained the colony at Cornell University, using Sitotroga cerealella (Olivier) (Lep.: Gelechiidae) and Ephestia kuenhiella Zeller (Lep.: Pyralidae) as the maintenance hosts. In early spring of each year the colony is reared for four generations on O. nubilalis before re-establishment on E. kuehniella. In each year of the study a single inoculative release of the parasitoid was made into each of 11 commercial processing sweet corn fields. In 22, all fields were nonorganic, but in 23 three of the release fields were organically managed and received insecticidal sprays of an organically approved formulation of spinosad (Entrust Ò ; Dow AgroSciences, Indianapolis, IN). In each year of the study, T. ostriniae was released at a nominal rate of 7 females per hectare using parasitized E. kuehniella eggs in which the parasitoid was allowed to develop until approximately 1 day before anticipated emergence. Each of those fields receiving the parasitoid was paired with a nearby control field of the same cultivar and a similar planting date but at least 3 m distant to minimize contamination by released T. ostriniae. Organic release fields were paired with organic control fields. Fields receiving the parasitoid ranged from 2. to 16.2 ha and were typically planted in May and June. Parasitoids were introduced into individual fields after O. nubilalis activity had been detected by a regional pheromone trap network and when the corn was approximately 4 cm height. Parasitoids were introduced by uniformly distributing release cartons consisting nominally of T. ostriniae females in parasitized eggs glued in disposable 3 ml conical drinking fountain cups with one part water : one part glue (Glue-All TM, Elmers Products Inc., Columbus, OH). The cups were stapled closed and several pin holes were made along the folded edge to allow egress of emerged wasps. The cups were fastened to corn stalks in the field with a strip of plastic flagging tape. Release containers were distributed throughout the fields in uniform grids (approximately 6 m 6 m) so that two

3 434 J. Gardner et al. release containers were hung on a single plant in each.4 ha based on the findings of Wright et al. (21), who inferred that the T. ostriniae distribution would be relatively uniform throughout the field when using that spacing. Ten to 14 days after placement, single release cones from two randomly chosen loci were inspected for parasitoid emergence. In 22, emergence ranged from 63% to 97%. In 23, fungal growth within several release cartons prevented counting of emergence holes, but emergence exceeded 97% in cones that had not molded beyond salvageable (table 1). The fungal growth appeared to be saprobic, but we have not thoroughly evaluated its effect on emergence. To evaluate the level of parasitism, at weekly intervals following the inoculative release, we examined corn plants for O. nubilalis egg masses. In 22, 2 2 plants/field were examined. In 23, the number varied because we used a sequential sampling plan developed for fresh market sweet corn (described in next section). If few egg masses were located while sequentially sampling, searching was continued until approximately 2 plants had been examined. All egg masses that were found were taken back to the laboratory to allow any parasitized eggs that were still white to blacken, an obvious indicator of parasitism. Differences in number of egg masses between release and control fields were tested with analysis of variance. At harvest, samples of plants from each field were assessed for frequency of damaged stalks and ears. Differences in stalk and ear damage were assessed using paired t-tests for each year of the study. Differences in stalk and ear damage between individually paired release and control fields were evaluated using chi-squared tests with P-values adjusted for multiple tests using the Benjamini false discovery rate method (Schlotzhauer and Littell 1987; SAS Institute 24), with the assumption that individual plants were independent despite the possibility of an aggregated distribution of O. nubilalis. Two fields of organic sweet corn receiving releases of T. ostriniae in 23 were omitted from comparative analyses, because their paired organic control fields had abnormally high egg parasitism, that was determined to be caused by species other than T. ostriniae. 2.2 Effect of Trichogramma ostriniae on actual sprays and spray decisions in processing and fresh market sweet corn All of the processing sweet corn in the study was commercially managed and the actual insecticide spray decisions were made by a private consulting firm, with sprays applied aerially by a certified operator. However, to impose more scientific rigour on the spray decision process, in 23 we used a sequential sampling plan devised for fresh market sweet corn (Hoffmann et al. 1996; Cornell Cooperative Extension 23) to determine whether observed egg mortality resulting from T. ostriniae releases would potentially influence the number of insecticide treatment decisions. Thus, scouting by the private consulting firm resulted in actual insecticide sprays, but sampling by the researchers did not. In the sequential sampling plan that we used, groups of five plants are inspected for infestation by the Lepidoptera corn worm complex of O. nubilalis, Spodoptera frugiperda (J. E. Smith) (the fall armyworm), and Helicoverpa zea (Boddie) (the corn earworm). The sampling plan keeps a running tally of number of plants infested with larvae or eggs and when a critical level is exceeded, it indicates the need for control (insecticide treatment). Unparasitized egg masses are included in the running tally, but parasitized (black) egg masses are not. The control decision is based on % infestation before silking and % infestation after commencement of silking, and the spray criterion is more conservative than for processing corn because the fresh market sector tolerates less damage, typically no more than % damaged ears. Complementary additional studies were also conducted to evaluate the effect of T. ostriniae on the number of actual insecticide sprays in conventionally managed commercial fresh market sweet corn in New York and Massachusetts. In New York, there were 13 fields paired with controls in 22 and 12 release fields paired with controls in 23. In Massachusetts, there were five release fields each receiving T. ostriniae in both 22 and 23, and each was paired with an untreated control. Fields varied in size but were typically <2 ha and parasitoids were released as described above, except that in Massachusetts, two releases were made, 1 week apart so that females per hectare were released. In New York, insecticide treatments were applied based on the sequential sampling plan (Hoffmann et al. 1996), but in Massachusetts, scouting consisted of sampling groups of plants with the decision to treat when infestations exceeded % of plants sampled. Differences in the number of insecticide applications were analysed with chi-squared tests. 2.3 Efficacy of Trichogramma ostriniae compared with insecticidal control of Ostrinia nubilalis Lastly, because insecticide sprays (range of 4/field) in the commercially managed processing sweet corn that was described above may have obscured the effect of T. ostriniae on O. nubilalis survival, we also conducted trials comparing the effect of an insecticide spray alone against the effect of T. ostriniae release alone. Our goal was to separate the effects of insecticide from those of T. ostriniae. Plots measuring 23 m 3 m (.7 ha) were planted Aurora, NY on 27 June, 23 with sweet corn ÔSeneca SpringÕ (Seminis, Oxnard, CA). The experiment was a completely randomized design with 17 plots. Nine plots received T. ostriniae at 7 /ha on 2 July, with the release containers positioned in the centres of plots. There were eight non-release plots that served as insecticide sprayed controls, and they were spatially separated by approximately m from release plots to minimize contamination by T. ostriniae. Weekly beginning 22 July, all plots were sequentially sampled as described previously, to determine if a spray treatment was warranted, and parasitized egg masses were excluded from the sequential plan s running tally. On 13 August, six of the release plots and four of the non-release plots were over threshold and all non-release control plots were sprayed with the pyrethroid, k-cyhalothrin, at labelled rates (Warrior Ò 29 ml/ha, Syngenta Crop Protection, Greensboro, NC) using a high-clearance boom sprayer equipped with drop-legs to insure coverage. Weekly decision and parasitism sampling was continued until harvest. If few egg masses were found during the sequential sampling, additional plants were also searched to locate sufficient egg masses to evaluate causes of egg mortality. Sample size thus ranged from approximately to 2 plants per sample event, depending on O. nubilalis egg mass density. When O. nubilalis egg masses were found, they were digitally photographed until their fate had been determined. Digitized images were used to count the number of eggs in each egg mass, and the fate of each egg was categorized as successfully emerged, predated or parasitized. The number of successfully emerged eggs was considered to be the number of neonatal larvae. At harvest, random plants per plot were evaluated for ear and stalk damage and number of O. nubilalis larvae. Analysis of variance was used to determine differences between T. ostriniae plots and insecticide sprayed plots for total eggs, neonatal larvae, larvae infesting stalks and ears, and ear damage.

4 WCR special issue submission 43 Table 1. Cumulative Ostrinia nubilalis egg masses, percentage of cumulative egg masses parasitized, plant damage, insecticide sprays and insecticide treatment decisions in processing sweet corn with and without Trichogramma ostriniae Year Field Release size (ha) Plant date Release date Release % emerge Cumulative % Egg masses egg masses 1 parasitized Stalks damaged Ears damaged Number sprays Over threshold 2 Release Control Release Control Release Control Release Control June July June 18 July June 18 July June 18 July June 2 July * June 2 July * June 2 July June 1 August * June 1 August * 63 12* June 1 August * May 3 July * May 3 July * May 9 July * May 16 July * June 16 July * June 16 July June 23 July * 6 3* June 1 August June 1 August * 23 * Standardized to plant sample size. 2 Fields were over threshold when exceeding critical values for infestation as determined by sequential sampling (Hoffmann et al. 1996). 3 Organically managed field with spinosad applied to control O. nubilalis. *Significantly lower value (P.) after Benjamini false discovery rate adjustment for multiple tests.

5 436 J. Gardner et al. 3 Results 3.1 Efficacy of Trichogramma ostriniae in processing corn Data for individual release fields are presented in fig. 1, illustrating variability in the density of O. nubilalis egg masses and the level of parasitism between years and fields. O. nubilalis oviposition (cumulative number of egg masses per field, adjusted to a plant sample size per sample event) in 23 was substantially greater than in 22 (29.9 ± 73. vs ± 3.9 respectively, mean ± SEM; F ¼.89; d.f. 1, 39; P ¼.2), but was not significantly different between release and control fields in either year of the study (F ¼.; d.f. 1, 16; P ¼.9 and F ¼.3; d.f. 1, 19; P ¼.9 respectively) (fig. 2). In 22, the weather from May through September was generally hot and dry (average 19.1 C; precipitation 34.1 cm in 6 rainfall events) and O. nubilalis oviposition began earlier in the year (fig. 2). We observed an increase in parasitism commensurate with O. nubilalis increase and occurring within 14 days after initiation of releases (fig. 2a). In contrast, the weather in 23 from May through September was relatively cool and wet (average 18.1 C; precipitation 42.9 cm in 91 rainfall events) and O. nubilalis oviposition increased later in the season (fig 2b). As a consequence, T. ostriniae releases occurred during a lull in oviposition and did not begin to increase until about 34 days after releases were initiated (fig. 2b). On a percentage basis, parasitism lagged behind but increased with increasing O. nubilalis oviposition (fig. 3). In 22, the percentage of egg masses parasitized increased nearly linearly from % to 1% (fig. 3a). In 23, released parasitoids rapidly began parasitizing host eggs, but parasitism declined before rebounding nearly linearly to achieve 43% at the end of the season (fig. 3b). Parasitism was largely confined to release fields, but was also observed in control fields later in the season (fig. 2). Parasitism in control fields was 8% of 17 egg masses in 22 and 26% of 273 egg masses in 23. Subsamples of parasitized O. nubilalis eggs from control fields in 22 indicated the presence of T. ostriniae, T. minutum (Riley), and probably Trichogramma itsybitsi (Pinto and Stouthamer), T. pretiosum (Riley) and T. brassicae (Bezdenko). In the two pairs of EGG MASSES / PLANTS Field 2 Field 3 Field 4 Field Field 6 Field 7 Field 8 Field 9 Field Jul 28-Jul 8-Aug 19-Aug 3-Aug Total Parasitized Field 1 Field 2 Field 3 Field 4 Field 6 Field 9 Field 7 Field Field 29-Jun 14-Jul 29-Jul 13-Aug 28-Aug Fig. 1. Total number and number parasitized Ostrinia nubilalis egg masses from individual processing sweet corn fields in central New York receiving a single inoculative release of 7 Trichogramma ostriniae/ha. Field 1 from 22 is omitted because no O. nubilalis egg masses were found. Field 8 from 23 is omitted because its control had abnormally high parasitism of O. nubilalis egg masses

6 WCR special issue submission (a) 22 Release 12 (c) 22 Control 9 6 Total Parasitized 9 6 Egg masses/ plants 3 14-Jul 29-Jul 13-Aug 28-Aug 12-Sep (b)- 23 Release 29-Jun 19-Jul 8-Aug 28-Aug 17-Sep 3 14-Jul 29-Jul 13-Aug 28-Aug 12-Sep (d) 23 Control 29-Jun 19-Jul 8-Aug 28-Aug 17-Sep Fig. 2. Number (mean ± SEM) of parasitized Ostrinia nubilalis egg masses by sample date, in release and control fields of processing sweet corn in central New York using data from all fields, combined within treatment Percentage or number Number O. nubilalis egg masses Number O. nublialis in traps Percentage parasitism 29-Jun 14-Jul (a) 22 (b) Jul 13-Aug 28-Aug 12-Sep Fig. 3. Percentage (mean ± SEM) parasitism of cumulative number of Ostrinia nubilalis egg masses in central New York processing sweet corn, superimposed with pheromone trap moth counts and mean number of egg masses found per field organically managed fields from 23 that were excluded from analyses, parasitism in the two control fields was 9% and 3%, while in their paired release fields it was 18% and 41%, respectively, indicative a high level of parasitism by a species other than T. ostriniae. The predominant species from the organic control fields was T. pretiosum. Because of the high level of parasitism in these organic control fields, an anomaly rarely seen in our previous field trials, data from the two controls and their paired release fields were excluded from subsequent statistical analyses. There was no difference in stalk damage between release fields and control fields in either 22 or 23 (t ¼ )1.8, P ¼.973; t ¼ )1.9, P ¼.38 respectively). Chi-squared comparisons between paired fields showed that five of release fields benefited from T. ostriniae reducing stalk damage in 22. Those fields were limited to the later planted fields (table 1). In 23, comparisons showed that five of nine fields benefited from releases, but two of nine release fields had a higher incidence of stalk damage (table 1). The incidence of ear damage by O. nubilalis was not affected by T. ostriniae in either year of the study. Comparisons of paired fields generally also showed no robust reduction in ear damage attributable to T. ostriniae (table 1). 3.2 Effect of Trichogramma ostriniae on actual sprays and spray decisions in processing and fresh market sweet corn Processing sweet corn fields receiving T. ostriniae in 22 had a total of 28 actual insecticide applications compared with 34 actual applications in control fields. In 23, release fields received the same number of actual sprays as control fields (table 1). In neither case was there a significant difference in insecticide treatment applications between release and control fields. Likewise, sequential sampling did not indicate a significant difference in the potential number of spray decisions (v 2 ¼ 2.61, d.f. ¼ 1 and P ¼.6). Sampling was conducted weekly for a total of 138 sampling events, with control fields over threshold on 28 occasions compared with 19 for fields treated with T. ostriniae (table 1). In commercial fresh market sweet corn, control fields were sprayed on 61 occasions compared with 46 for fields treated with T. ostriniae. Again, the difference was not significant (v 2 ¼ 2., d.f. ¼ 1andP¼.147). 3.3 Efficacy of Trichogramma ostriniae relative to insecticidal control of Ostrinia nubilalis Results from the experiment that directly compared the effect T. ostriniae to an insecticidal spray are presented in fig. 4. Pooled across sampling events, the

7 438 J. Gardner et al. Number per plant Total eggs total number of eggs oviposited per plant was similar, regardless of treatment (F ¼.9, d.f. ¼ 1, and P ¼.7734), but T. ostriniae reduced the number of neonatal larvae per plant (F ¼ 12.21, d.f. ¼ 1, and P ¼.33). Although the number of larvae surviving in stalks at the time of harvest was higher with T. ostriniae than with insecticide (F ¼ 9.2, d.f. ¼ 1, and P ¼.7), there was no commensurate increase in proportion of ears damaged per plot (F ¼ 1.28, d.f. ¼ 1, 16 and P ¼.273), nor in the number of larvae surviving in the ears (F ¼.94, d.f. ¼ 1, and P ¼.348). 4 Discussion * Neonatal larvae Stalk larvae * Insecticide Trichogramma Ear larvae Fig. 4. Comparisons of Ostrinia nubilalis eggs, neonatal larvae, stalk larvae and ear larvae (mean ± SEM) in central New York sweet corn treated with either a single release of 7 Trichogramma ostriniae/ha, or sprayed with k-cyhalothrin. *Indicates significant difference between treatment Although encountering substantial variability in weather and O. nubilalis egg mass density, T. ostriniae parasitized a substantial portion of the O. nubilalis egg masses that we found. Because parasitized egg masses typically exhibit % O. nubilalis mortality, T. ostriniae was likely responsible for approximately 1% and 43% cumulative egg mortality for 22 and 23 respectively. The level of parasitism was comparable with levels encountered in smaller-scale studies of inoculative releases of T. ostriniae in sweet corn (Wright et al. 21; Hoffmann et al. 22; Kuhar et al. 22), and was comparable or better than reported for inundative releases of T. brassicae at per hectare over releases (Mertz et al. 199), or using T. nubilale at females per hectare over 2 3 releases (Andow et al. 199). It comes as no surprise that weather and host abundance would influence the efficacy of T. ostriniae. Wang et al. (24) documented that T. ostriniae had a shorter generational time when reared at 33 C than at 17 C, observed differences T. ostriniae functional response at different temperatures (Wang and Ferro 1998), and found that temperature affected the level of parasitism by T. ostriniae (Wang et al. 1997). In our 22 study, the maximum daily temperatures were higher and precipitation was less than in the second year of the study. Four fields had parasitoid releases by 18 July, 22 and O. nubilalis oviposition increases were detected by 1 August. All remaining releases in 22 occurred during a period of high O. nubilalis oviposition and releases were well timed with host availability (fig. 3a). In 23, the weather was cooler with more precipitation than in 22, and corn growth was delayed. Thus, parasitoid releases coincided with a time when O. nubilalis oviposition was static or in decline, shortly to be followed by a rapid resurgence (see fig. 3b). In the earlier planted fields, a residual low density of corn borer eggs appears to have provided sufficient resources for T. ostriniae to persist and exploit an upsurge in egg masses produced in August. When the parasitoids were introduced into laterplanted fields, their release better coincided with the increase in O. nubilalis egg masses, and they were generally more effective. Although timing of releases and level of parasitism was perhaps better in 22 than in 23, in both years, stalk damage and ear damage were not reduced, but individual chi-squared comparisons of paired release and control fields suggest the possibility of reduced damage. We provide the caveat that the results should be interpreted cautiously, because samples were drawn from locations ( stalks each) within the field, and were analysed with the liberal assumption of independence among stalks. Regardless, a reduction in plant larval number may not necessarily result in reduced ear damage. For example, in our experiment where insecticidal sprays were directly compared with T. ostriniae, stalk damage and larval number were lower with insecticide than with T. ostriniae, but there was no difference in ear damage. Neither the number of actual sprays nor potential spray decisions were significantly reduced by T. ostriniae, but there was a numerical trend toward fewer sprays. In the experiment directly comparing T. ostriniae to a threshold-based insecticidal spray, the number of stalk larvae per plant was greater in the T. ostriniae treatment, but the number of neonatal larvae was less than in the insecticide treatment. Ultimately, the number of larvae found in the ears was equivalent between treatments, and the parasitoid was equivalent to a single well-timed spray that kept the population below threshold thereafter. Thus, we infer that the effect of T. ostriniae on ear larvae and ear damage by preventing eclosion of O. nubilalis, was functionally equivalent to the effect of an insecticide application on later larval stages. It would appear that although the number of neonatal larvae was reduced by T. ostriniae, there was a sufficient number remaining to elevate the number of stalk larvae relative to the sprayed control. Ostensibly, the insecticide spray was effective in preventing larval establishment within the stalk, to an extent that was greater than larval mortality caused by predation and other causes in the T. ostriniae plots. From its inception, the purpose of an inoculative release of T. ostriniae in sweet has been to suppress O. nubilalis infestations rather than provide a guarantee of commercially acceptable control. The benefit of the releases is to be measured through monitoring infestations, and the observed reduction in infestation then translated into reduced insecticidal input. The results reported herein provide additional

8 WCR special issue submission 439 evidence that T. ostriniae is an excellent candidate for augmentative biological control of O. nubilalis in sweet corn, in some instances providing adequate control alone, and in other instances helping to reduce insecticidal inputs. Additional refinements in timing and rates of release could enhance the benefit of releases. For instance, recent work by Spangler et al. (23) shows that the number of ear larvae are correlated with a critical period of O. nubilalis oviposition, thus, our future efforts will focus on targeting and bracketing that critical period through more frequent releases with higher numbers of parasitoids. In addition, to more adequately evaluate the effectiveness of insecticide treatments and T. ostriniae release, experiments that compare insecticide treatments alone, T. ostriniae release alone, the combination of the two, and neither insecticide nor T. ostriniae, are being conducted. Acknowledgements This research was supported by the CSREES-USDA Regional IPM Program Grant # Any opinions, findings, conclusions or recommendations expressed in this publications are those of the author(s) and do not reflect the view of the US Department of Agriculture. Additional financial support came from Hatch funding and from USDA Regional Project NC-2. We thank Birdseye Foods, Inc., its growers and Richard Wildman of Agricultural Consulting Service, Inc. Rochester, NY for generous access to field sites. We also thank Prof. John Pinto, University of California, Riverside for assistance with identifying Trichogramma specimens. References Andow DA, Prokrym DR, 199. Plant structural complexity and host-finding by a parasitoid. Oecologia 82, Andow DA, Klacan GC, Bach D, Leahy TC, 199. Limitation of Trichogramma nubilale (Hymenoptera: Trichogrammatidae) as an inundative biological control of Ostrinia nubilalis (Lepidoptera: Crambidae). Environ. Entomol. 24, Anonymous, 199. 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