Introduction

The navel orangeworm Amyelois transitella (Walker) is a pest of California almonds, pistachios, walnuts, and figs (Bentley et al. 2000, 2003; Zalom et al. 2002; Coviello et al. 2003). In California tree nuts, recommendations for management of this pest have focused primarily on cultural practices (Zalom et al. 1984) and, if populations still threaten economic loss, on application of chemical insecticides as soon as the nuts are susceptible and ovipositing females are present (Bentley et al. 2000, 2003; Zalom et al. 2002). Malathion is registered for control of nitidulid beetles in figs and could conceivably reduce navel orangeworm loss (Coviello et al. 2003), but growers avoid using it because of problems with post-harvest intervals and worker re-entry, and have instead used post-harvest fumigation to mitigate insect damage to the fruit (Burks personal observation).

The development of efficient systems of monitoring and mating disruption for navel orangeworm management has lagged behind that of other lepidopteran pests of horticultural crops, due in part to technical limitations. Only the principal component of the female sex pheromone has been identified (Coffelt et al. 1979). This component, (Z,Z)-11,13-hexadecadianal, is not sufficient to efficiently bring males to a point source and is particularly vulnerable to degradation in the field. Monitoring of this pest is therefore dependent on oviposition traps, which are out-competed by the presence of a susceptible host in the orchard (Rice 1976; van Steenwyck and Barnett 1985). An aerosol timed-release system, in which the pheromone is stored in a liquid organic solvent prior to being released at timed intervals, has been one method of avoiding problems with degradation (Shorey and Gerber 1996). While Minks and Cardé (1988) suggested that ideally the optimal blend should be characterized in the laboratory prior to large-scale field trials, they indicated that, in two of the 11 species reviewed, the natural blend was not the best disruptant. There have been subsequent examples in which researchers have concluded that use of more simple blends (Evenden et al. 1999) or a single pheromone component (van Deventer and Blommers 1992; Ryne et al. 2001) was the most pragmatic and cost-effective approach for mating disruption. Previous studies of mating disruption for control of navel orangeworm in almonds have shown that release of (Z,Z)-11,13-hexadecadienal can reduce male capture in female-mated flight traps, mating of unmated females, and reduce crop damage (Curtis et al. 1985; Shorey and Gerber 1996). Studies with this and other lepidopteran pests have suggested that aerosol timed release systems placed around the perimeter of plots to be protected could be equally efficacious in prevention of location of females by males, and could save labor costs compared to such devices placed evenly through the protected plot (Shorey and Gerber 1996; Shorey et al. 1996).

In the current study, we examine the impact of this timed-release system in Calimyrna figs. Calimyrnas, like other commercial fig varieties in California, mature and dry over an extended time and are generally harvested several times at 5–10 day intervals in August and September (Ferguson et al. 1990). Unlike the other varieties, Calimyrna figs have only one significant crop maturing in August and September rather than an additional spring crop that would be harvested as dried fruit around the end of June. The single Calimyrna crop typically does not loose its latex and soften until late July (Smilanick 1979). In this study, we examined the ability of timed release aerosol dispensers placed around 16-ha plots of Calimyrna figs, with treatment beginning in either April or July, to completely prevent capture of males in female-baited flight traps throughout this plot. We also use our trapping data to compare seasonal abundance of navel orangeworm with an established degree-day model.

Materials and Methods

Results

While the mating disruption technique used significantly reduced the number of moths captured in traps at both years and in both sites (Tables 1 and 2), it was more effective in 2002 than in 2001 and in site 1 than in site 2. In 2001 there were significant differences between treatment and control trap means in 21 of the 22 weeks at site 1, but only in 11 of the 22 weeks in site 2. In 2002 there was no significant difference between mean trap counts in the treatment and control plots in most of the 15 weeks prior to the beginning of the mating disruption treatment on 1 July. In the cases in which there were significant differences, the mean count in the control plot (i.e., the treatment plot from the previous year) was greater than that in the treatment plot in three cases and less in two. Following the beginning of pheromone application in the week of 26 June 2002, there was a significant difference between the mean trap capture in the last 9 of the 11 weeks of mating disruption treatment in both sites. In the instances in 2002 when mating disruption was applied and there was no significant difference between captures in treatment and control plots, the number of males captured in the control plots was also not significantly different from zero (i.e., 10 July at both sites and 26 June and 3 July at site 2).

No significant correlations between location and trap capture were demonstrated using the ANOVA or parametric techniques such as Pearson's correlation, but Fisher's Exact test demonstrated that, in the absence of mating disruption, moths were usually more likely to be found in the 12 peripheral traps of the 4 × 4 grids than in the 4 central traps. Of the 111 weekly trap counts from plots not under mating disruption (the controls in both years and treatments from 13 March to 19 June of 2002) that met the criteria for calculation of Fisher's Exact Test (i.e., a total of more than 5 moths in both the 4 central and in the 12 peripheral traps), the central traps contained fewer moths than 25% of the total count in 102 cases (54 of which differed significantly) and more moths in 9 cases (1 of which differed significantly) (Fig. 2A). Of the 28 cases of plots under mating disruption in 2001 and 2002 from which the weekly trap counts met the criteria for calculation of Fisher's Exact Test, the central traps contained more than 25% of the moths captured in 14 cases and 25% or less in 14 cases, and none of these differed significantly from 25% (Fig. 2B).

Unmated females in mesh bags in the flight traps lived at least 4 days under mid-summer field conditions (Fig. 3). The proportion of the eventual trap capture was ∼60% after the first night, 87% after the third night, and 97% after the fifth night. The total number of navel orangeworms caught in the two traps was 41 and 30 (9–14 July 2002), 43 and 34 (15–21 July), 78 and 60 (22–28 July), 67 and 64 (31 July–6 August), and 85 and 83 (7–13 August). The maximum daily and minimum nightly temperatures during these periods were (40.2, 36.7, 35.6, 36.3, 38.4 °C) and (17.8, 16.2, 13.6, 12.3, 12.2 °C), respectively. The average daily temperature range during these periods was 18.8 ± 0.4 °C (n = 34). The mean daily minimum relative humidity (RH) was 27 ± 1% (n = 34), and the mean nightly maximum was 84 ± 1% RH.

Degree-day calculations were made for 2002 using 24 March as a biofix (Fig. 4). An accumulation of 410 DD was attained on 9 June, and 607 DD was attained on 30 June. Using 30 June as the start of a theoretical second flight for 2002, 410 DD were accumulated on 3 August and 607 DD were accumulated on 23 August. By comparison, mean moths per flight trap in the absence of mating disruption fell from ∼10 to ∼2 through the month of May, from ∼2 to ∼0 in June, and remained low until ∼24 July, after which steady increases in trap capture were observed.

Discussion

The data presented here for the navel orangeworm provide the most rigorous verification to date of the hypothesis of Shorey and co-workers (Shorey and Gerber 1996, Shorey et al. 1996), that peripherally-arranged aerosol dispensers can very effectively disrupt lepidopteran sexual communication throughout large plots. Previously, timed release of (Z,Z)-11,13-hexadecadienal using peripherally-placed aerosol dispensers was shown to significantly reduce trap capture of navel orangeworm in 16-ha plots in almonds, pistachios, and walnuts (Shorey and Gerber 1996) but, the tests, involving large (16-ha) plots, were conducted for only 1 week, and the time of year was not specified. The present data demonstrate that time of year and seasonal changes in abundance are important in such tests.

We attempted to examine spatial distribution of moths within the 16-ha plots because of potential concern about the peripherally-placed dispensers adequately affecting all parts of the plot. While we found statistically greater captures in the peripheral parts of the untreated block but not of the treated block, the more important finding was that there is no evidence that males were consistently able to find females in the center of the protected plots farthest from the peripheral dispensers. The present data show that, under favorable circumstances, we can completely prevent capture of moths in traps throughout a 16-ha treatment plots in spite of captures of 10–20 moths per trap in comparison plots. The greater efficacy in 2002 compared to 2001 may be due to a more compatible solvent and/or less exposure to solar radiation and heat (i.e., in June). It also seems likely that the sparser canopy at site B compared to A, and the greater solar radiation and heat likely experienced by the dispenser units at that site, were factors in the effectiveness of the system. The observations of similar trap counts and no systematic significant differences in trap counts between the control and treatment plots from 13 March to 19 June 2002 (i.e., before pheromone treatments) demonstrates similar abundance in the plots used in this study, and shows that there was no detectable residual effect of pheromone applied the previous year on the ability of navel orangeworm males to locate calling females in the traps.

The data presented here indicate that navel orangeworm abundance is low in figs during the second flight, in June and July. The model used for the degree-day predictions illustrated in Fig. 4 was developed using development data obtained from navel orangeworm on old-crop and fresh almonds (Sanderson et al. 1989), and it must be recognized that navel orangeworm larvae may develop at different rates on old-crop and fresh figs. Regardless of possible differences in development rates between crops, the empirical data in this study, which show low abundance during much of June and July, is important because mating disruption generally works better under conditions of low initial population density (Cardé and Minks 1995). We characterized survival and calling at our location under summer conditions in order to be certain that the low activity that we observed in June and July was not an artifact of the effect of heat stress on survival and calling of the live females in our monitoring traps.

A previous study of the effect of fruit maturity on susceptibility to infestation by nitidulid beetles (Smilanick 1979) showed that Calimyrna figs are not susceptible to infestation by nitidulids prior to softening, loss of latex, and enlargement of the ostiole (stage 2 of Smilanick 1979), which first occurs in late July or early August. In extensive sampling of ripening Calimyrna figs taken from trees at a different location over a 10 week period in 2000, we found no navel orangeworm in 1219 stage 1 or 2 examined fruit, and none prior to 22 August. These findings suggest that delaying the beginning of mating disruption from the beginning of the first flight until shortly before the appearance of susceptible figs (i.e., as we did in 2002) would provide the same or better protection while reducing material costs. This is similar to the practice of Curtis et al. (1985), who started mating disruption around 1 July in order to begin shortly before hull split in Nonpareil almonds. Other data show that the population dynamics for navel orangeworm is similar in figs and almonds (present data cf. Rice 1976; Sanderson et al. 1989).

We assessed crop damage in the current study (Burks and Brandl 2004). This was complicated by the facts that insect damage is generally internal to the fruit and often not apparent from the outside, and that the insect pest has often left the fruit by the time its damage is identified. Sixteen 50-fig samples were taken from figs on the ground at each of the 16 flight trap points for each of 3 harvests in 2001 and 2002. In 2001 the figs were examined by the official market order inspectors who determine only whether a fig is insect-infected. In 2002 the procedure was repeated but the infected figs were turned back to us, and we determined the proportion of figs damaged by navel orangeworm, nitidulids, and other insect pests. While we usually found less total insect damage in treated then the control plots, the differences generally weren't statistically significant and, more importantly, we found that navel orangeworm was responsible for 10–20% of the insect damage at these 2 sites in 2002, and for ∼30% of the insect damage in samples taken from the same locations in 2003. In both cases nitidulid beetles were responsible for the majority of the insect damage. These findings demonstrate the importance improved identification of insect pests responsible for fig damage, and indicate that, if species-specific pest control tactics must be used, those directed against nitidulid beetles should be emphasized over those directed against the navel orangeworm. The findings from this study are nonetheless relevant to almonds, in which the navel orangeworm is a serious pest.

Disclaimer

This is a report of research results only. The use of trade, firm, or corporation names in this report is for the convenience of the reader, and does not constitute or imply endorsement or approval by the United States Department of Agriculture or the Agricultural Research Service of any product or service to the exclusion of others that may be suitable.