EFFECT OF RELATIVE HUMIDITY CONDITIONS ON. RESPONSIVENESS OF EUROPEAN CORN BORER. (Ostrinia nubilalis) MALES TO FEMALE SEX.
Journal of Chemical Ecology, Vol. 19, No. 1, 1993
EFFECT OF RELATIVE HUMIDITY CONDITIONS ON RESPONSIVENESS OF EUROPEAN CORN BORER (Ostrinia nubilalis) MALES TO FEMALE SEX PHEROMONE IN A WIND TUNNEL
L U C I E R O Y E R and J E R E M Y N. McNEIL* D~partement de biologie Universit~ Laval Sainte-Foy, Quebec, Canada GIK 7P4
(Received January 17, 1992; accepted August 27, 1992) Abstract--The responsiveness pf 3-day-old European com borer males to three concentrations of 97 : 3 Z : E 11-tetradecenyl acetate, the female sex pheromone, was studied over a range of relative humidities (43-100%) in a wind tunnel. The proportion of males taking flight and reaching the source decreased, while the proportion exhibiting in-flight arrestment of upwind progress increased under high humidity conditions at all three concentrations of pheromone tested. The relationships between relative humidity and these behaviors were best described by polynomial equations. Key Words--European corn borer, Ostrinia nubilalis, Lepidoptera, Pyralidae, male responsiveness, relative humidity, wind tunnel.
INTRODUCTION
Finding a mate is a key event in sexual reproduction, being the first step in assuring a genetic representation in future populations. In some insect species, mate seeking is facilitated by the aggregation o f females at specific zones within the habitat, termed conventional encounter sites (Parker, 1978a; Thonthill and Alcock, 1983), and/or by the emission o f a volatile sex pheromone by females (Thorrdaill and Alcock, 1983). European corn borer (ECB), Ostrinia nubilalis (Hbn), (Lepidoptera, Pyralidae) adults aggregate in tall dense vegetation outside fields o f host plants (Caffrey and Worthley, 1927; Showers et al., 1976) where *To whom correspondence should be addressed. 61 0098-0331/93/01(104)061$07.00/0 9 1993 Plenum Publishing Corporation
62
ROYERANDMcNEm
females call (overt behavior associated with the emission of the sex pheromone) and matings occur (Showers et al., 1976; Sappington and Showers, 1983). However, in arid southeastern Alberta, where weeds are unavailable outside cultivated areas, ECB moths are found at the center of irrigated cornfields, the most humid zone of the habitat (Lee, 1988). These observations, together with the adverse effect of low humidity on the calling behavior (Webster and Card6, 1982; Royer and McNeil, 1991) and on the length of the premating period of ECB females (Royer and McNeil, 1991), suggest that high humidity is an important ecological factor responsible for the selection of the observed conventional encounter site of this species. As no detailed information is available on the effect of relative humidity on the receptivity of males to the female sex pheromone in this or any other lepidopteran, we examined the responsiveness of 3-day-old ECB males to pheromone lures under wind tunnel conditions. METHODS AND MATERIALS The responsiveness of 3-day-old univoltine ECB males to sex pheromone sources loaded with 30 (W. Roelofs, Geneva, New York), 100, or 200/xg of 97:3 Z : E 11-tetradecenyl acetate (Raylo Chemicals Ltd), was independently tested in a 0.25 m/sec laminar air current (determined using a TA3000T thermal anemometer, Airflow Developments Ltd) from the fifth through the seventh hour of the scotophase. One hour before the beginning of the test, naive males, maintained at 25 + 0.5~ and 45 ___ 5% relative humidity under a 16:8 hr light-dark photoperiod where they had access to a source of water to drink, were transferred to the environment chamber housing the wind tunnel (Fitzpatrick et al., 1988) at 20 + 1 ~ When different concentrations were tested on the same day, clean air was allowed to circulate in the tunnel during a period of at least 3 rain between trials. We recorded the following behaviors: taking flight, in-flight arrestment of upwind progress (Willis and Baker, 1988) following the initiation of oriented flight toward the source, and reaching the lure. Males not responding within 60 sec but that took flight when physically disturbed were classified as unresponsive. Relative humidity conditions (measured using an Abbeon certified hygrometer, model AB 167B) varied from 43% to 100% over the experimental period of several months, but remained constant during trials on any given day. A total of 99, 182, and 316 ECB males were tested to pheromone sources loaded with 30, 100, and 200 ~tg, respectively. Best curvilinear regressions, weighted for the sample size of each point (Snedecor and Cochran, 1978), were fitted to describe the relationship between humidity conditions and different parameters of the flight response of males to the sex pheromone. Each point corresponds to the proportion of males exhibiting a given behavior in any given replicate, when at least five individuals were tested.
HUMIDITY AND PHEROMONE RESPONSE OF CORN BORER
63
RESULTS
Male flight capacity remained unchanged over the range of humidity conditions tested, for all males not responding to the female sex pheromone promptly took flight when physically disturbed. However, humidity conditions explained at least 41% of the variation observed in different parameters of the ECB males' responsiveness to the female sex pheromone (Figures 1-3). At all pheromone
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FIG. 1. Relation between humidity conditions and the proportion of ECB males taking flight to 30, 100, and 200/zg of the sex pheromone at 20~
64
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F16.2. Relation between humidity conditions and the proportion of males reaching sex pheromone sources loaded with 30, 100, and 200/xg, at 20~
concentrations tested, the percent of males taking flight (Figure 1) and the proportion of those taking flight that reach the source (Figure 2) decreased as relative humidity increased. The reduction in the number of receptive males reaching the lure was due to the increased proportion of males exhibiting in-
HUMIDITY AND PHEROMONE
65
RESPONSE OF CORN BORER
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66
ROYER AND McNEIL
flight arrestment of upwind progress with increasing relative humidity (Figure 3).
DISCUSSION
Direct comparisons of the results obtained with the three different pheromone concentrations are not possible for two reasons. We were not always able to test a sufficient number of individuals at each concentration during the same trial as the availability of 3-day-old virgin males fluctuated. Furthermore, as the lures came from different sources, they could vary slightly with respect to the isomeric ratio or chemical purity. However, the relationships between relative humidity and the three parameters measured were similar for all concentrations of the female sex pheromone, clearly indicating that relative humidity affects the male receptivity in the ECB. At all pheromone concentrations tested, there was a decrease in male receptivity with an increase in humidity (Figure 1). Furthermore, at high humidities fewer receptive males reached the source (Figure 2), due to an increase of inflight arrestment of upwind progress (Figure 3). In-flight arrestment is correlated with either complete adaptation of antennal neurons or attenuation of their firing rate (Baker et al., 1988; Baker and Haynes, 1989) to high pheromone concentration (Willis and Baker, 1988). Thus, the increase in arrestment with increasing humidity at a given concentration of pheromone suggests that high relative humidity enhances stimulus perception. The same modification of stimulus perception by humidity could also explain why males, capable of flight, did not always respond when exposed to pheromone. The antennae of several Lepidoptera respond to humid air (Grant, 1975), and cleloconica sensilla, known to perceive humidity in other species (Altner et al., 1977), are found adjacent to trichodea sensilla on the antennae of O. nubilalis (Cornford et al., 1973). Thus, the observed responses could result from an interaction of pheromonal and humidity stimuli at the level of either peripheral receptors or the central nervous system, whereby the quantity of pheromone required to stimulate the male is less at higher than at lower relative humidities. An effect at the peripheral level would be analogous to the finding in Trichoplusia ni, where receptor response to the major component of the sex pheromone increased as the concentration of a minor component, not directly perceived by the receptor tested, increased (O'Connell, 1986). Loughner and Brindley (1971) reported that decreasing temperature conditions in the scotophase were important for onset of sexual activities by the ECB. However, in subsequent analyses under a range of field conditions DeRozari et al. (1977) found that the availability of free water, in the form of dew, was the most important cue. Obviously, as DeRozari et al. pointed out,
HUMIDITY AND PHEROMONE RESPONSE OF CORN BORER
67
the drop in temperature would contribute directly to the formation of dew. The decline in temperature and dew formation would also be accompanied by an increase in relative humidity, which can affect the mating success in certain lepidopteran species (Kanno and Sato, 1980; Mbata, 1986). Humidity conditions are important for reproduction in the ECB (Loughner and Brindley, 1971; Royer and McNeil, 1991), and the decline in mating success at low relative humidity could be due to reduced adult flight activity (Broersma et al., 1976) or female calling behavior (Webster and Card6, 1982; Royer and McNeil, 1991). It could also result from a change in male receptivity, as the number of individuals reaching the 30-/zg source declined from 70.4 (N = 27) to 42.1% (N = 19) when tested at 40 (35-44) and 20 (15-24) % relative humidity (unpublished data). Males not reaching the source appeared to loose the pheromone plume rather than exhibit the arrestment behavior observed at high humidities. Thus, it is not surprising that receptive Ostrinia nubilalis females aggregate in humid microhabitats (Showers et al., 1976; Sappington and Showers, 1983; Lee, 1988). However, the decline in male receptivity at the higher humidity conditions seems counterintuitive, given that the lifetime reproductive success of a male depends on the number of mates acquired (Royer and McNeil, 1992), which, in part, is determined by his ability to find receptive females (Parker, 1978b; Bell, 1990) that, as mentioned above, aggregate at high humidity sites. A plausible explanation for this apparent contradiction is possible if the emission rate from a calling female is significantly less than the lures used in these experiments. Thus, even if there is an increased sensitivity to the pheromone at high humidity, males would be able to respond to and locate calling females due to the low concentrations of pheromone present. The possibility that the emission rate of females is low does not seem unreasonable, as ECB female pheromone glands only contain approximately 2.5 ng (Glover et al., 1987). However, additional data on the actual release rates from calling females and the response of males to such concentrations under different environmental conditions are required to validate this explanation. Roelofs (1978), when examining the effect of concentration on the male response to different pheromone blends, suggested that environmental factors could also modify male responsiveness. This has since been demonstrated experimentally for temperature (Linn et al., 1988) and, if our hypothesis concerning the interaction between pheromone and relative humidity stimuli is correct, blend perception may change with relative humidity conditions. The modification of blend perception by environmental conditions could explain why, in field trials, some males of one ECB pheromonal strain went to the wrong blend (Klun and Huettel, 1988), even though male responsiveness is genetically determined (Roelofs et al., 1987). It is clear that if we are to use pheromone traps effectively in management programs we must understand the effects of different environmental factors on the pheromone-mediated mating behavior of both sexes
68
ROYER AND McNEIL
( M c N e i l , 1991) a n d , for species t h a t u s e specific e n c o u n t e r sites, p a r t i c u l a r a t t e n t i o n s h o u l d b e g i v e n to t h e e c o l o g i c a l factors in t h e s e m i c r o h a b i t a t s . T h i s k n o w l e d g e c o u l d h e l p e x p l a i n f l u c t u a t i o n s in p h e r o m o n e trap efficiency, b o t h w i t h i n a n d b e t w e e n years. Acknowledgments--We thank Jacques Gobeil and Alain Labrecque for their technical assistance, and M.A. Willis for reviewing a previous version of this manuscript. This study was supported by a "La fondation de l'Universit6 Laval" postgraduate scholarship to L.R. and by grants from Conseil de Recherches en Prche et Agro-alimentaire du Qurbec (CORPAQ) and Fonds pour la Formation de Chercheurs et l'Aide ~ la Recherche (FCAR) to J.N.M.
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