Does the Small White Butterfly (Pieris rapae L.) Aggregate Eggs on Plants with Greater Gas Exchange Activity?

Transcription

1 Journal of Insect Behavior, Vol. 14, No. 4, 2001 Does the Small White Butterfly (Pieris rapae L.) Aggregate Eggs on Plants with Greater Gas Exchange Activity? A. Mark Langan, 1,2 C. Philip Wheater, 1 and Peter J. Dunleavy 1 Accepted January 10, 2001; revised February 22, 2001 Few studies have investigated insect egg-laying preferences in relation to photosynthesis or transpiration of their host plants. It has been suggested that intravarietal preferences of the small white butterfly (Pieris rapae L.: Pieridae) include larger plants with characteristically higher transpiration rates. Interestingly this species, like many other Lepidoptera, may detect biogenic CO 2 gradients associated with photosynthesis. We studied egg-laying preferences in working farm environments examining relationships among host choice, plant gas exchange activity, and plant size. Females discriminated between plants in monocultures on the basis of height. A balance of pre- and post alighting preferences resulted in plants of medium size receiving eggs. Post alighting preferences led to plants, but not alighted leaves, with higher rates of photosynthesis supporting eggs. These findings do not support a mechanistic basis for the use of gas exchange activity during host selection but, for the first time, indicate the greater physiological activity of crop plants that ultimately received the eggs of a pest insect. KEY WORDS: host plant selection; Pieris rapae; photosynthesis; transpiration; brassica crops. 1 Department of Environmental and Geographical Sciences, Manchester Metropolitan University, Manchester M1 5GD, UK. 2 To whom correspondence should be addressed. Fax: m.langan@mmu. ac.uk /01/ $19.50/0 C 2001 Plenum Publishing Corporation

2 460 Langan, Wheater, and Dunleavy INTRODUCTION Despite considerable evidence that herbivory has an impact on plant gas exchange activity (for review see Welter, 1989), very few attempts have been made to investigate the photosynthetic or transpirational activity of plants visited by insects during egg-laying. Gas exchange activity associates strongly with many other important plant characteristics (see Durzan, 1968) which can influence host choice by phytophagous insects. These include nutrient content (Myers, 1985; Oleksyn et al., 1998), plant health (Habeshaw, 1984), and productivity/size (Lambers and Poorter, 1992). Usually, younger leaves have the greatest gas exchange potential (Lurie et al., 1979) and, due to high levels of nitrogen, are most nutritious for many developing lepidopteran larvae (Scriber and Slansky, 1981). Intuitively, there seems to be good reason for some insect species to select plants, or tissues of plants, with superior rates of gas exchange activity. Knowledge of such preferences, resulting from either direct or indirect insect responses to plants, may be particularly useful for studies of insect plant interactions in which the yield and nutrient content of potential host plants are important. Host plant selection by Pieris rapae (the small white butterfly) is extensively documented (e.g., Courtney, 1986; Hern et al., 1996), probably resulting from the ubiquitous nature of this pest species. Egg-laying P. rapae discriminate between potential host plants using visual cues before alighting (Jones, 1977; Ives, 1978; Myers, 1985) and using leaf surface chemicals detected by chemoceptors on the tarsi after alighting (Renwick and Radke, 1988). Manipulated cage and field experiments have demonstrated that P. rapae females distinguish between cabbage varieties and plants of the same variety. Intravarietal preferences are for plants that are larger (Ives, 1978), greener [associated with a higher nitrogen and water content (Myers, 1985)], and located at crop edges (Jones, 1977) and for leaves with acceptable balances of attractants and deterrents (Chew and Renwick, 1995). These studies exposed egg-laying females to plants that contrasted in certain characteristics, thus discrimination between crop plants on host preferences in working farm environments (i.e., large-scale cabbage monocultures of uniform age) requires further investigation. It has been suggested that studies of host selection will benefit from approaches that consider plant physiological activity (Hern et al., 1996). P. rapae is one of only a few phytophagous invertebrates that have been included in studies that measured gas exchange activity of host plants during oviposition. Myers (1985) found that P. rapae females deposited more eggs on recently watered cabbage plants that had characteristically higher transpiration rates. In contrast, another lepidopteran pest (Ostrinia nubilalis; the corn borer moth) allocated eggs to leaves which contained preferred concentrations of

3 Egg-Laying Behavior and Plant Gas Exchange Activity 461 specific mineral nutrients, and not to plants with higher photosynthetic rates (Phelan et al., 1996). In one lepidopteran, a mechanism has been discovered for detection of photosynthetic activity: during egg-laying the nocturnal moth (Cactoblastis cactorum) uses its labial palp organ (LPO) to detect CO 2 gradients associated with the photosynthesis of a CAM plant (Stange et al., 1995). In common with many Lepidoptera, P. rapae possess an LPO with high concentrations of sensillae which may be capable of detecting CO 2 gradients (Lee et al., 1985). However, there is little known about the generality of CO 2 detection among the Lepidoptera and how it may relate to host plant choice (Stange, 1996). Recent innovations in the development of portable infrared gas analyzers provide a tool for ecologists to make nondestructive measurements of gas exchange activity of specific areas of leaves in the field. When high intensities of light are used to saturate leaf surfaces, this technique provides an indication of the potential of leaf areas for exchange of CO 2 (photosynthesis) and H 2 O (transpiration). This study investigates how photosynthesis, transpiration, size, and injury level of cabbage plants are associated with host plant preferences in cabbage monocultures by P. rapae. MATERIALS AND METHODS Oviposition behavior of P. rapae was recorded between 1100 and 1600 BST from 27 July to 18 August 1995 in a late cabbage crop (var. Slawdena) grown at Barton Moss Farm, Irlam, Manchester (2 25 N, W). The first observations were made at an early stage of crop growth, 24 days after transplantation (DAT). Females exhibiting egg-laying behavior were pursued by a single observer and all positions of contact with plants (considered to be investigation by the female) were marked with a numbered peg (following Jones, 1977) and a pole to facilitate relocation. Within 30 min the width (minor axis) of the largest leaves and the presence of butterfly eggs or larvae of marked plants were recorded. The minor axis of the largest leaves of harvested var. Slawdena plants correlated well with their total leaf area (r S = 0.767, P = 0.027, n = 8) but not their height (r S = 0.132, P = 0.863, n = 8). Rates of photosynthesis and transpiration of plants contacted during egg-laying flights were measured using a portable infrared gas analyzer (LCA3; Analytical Development Company, Hoddesdon, UK) used in the differential mode. Air was dried as it entered the analyzer to reduce error in CO 2 measurements. The cuvette (PLC-B; containing an area of 625 mm 2 ) was positioned over the leaf area surrounding the egg or position of contact. A second measurement of gas exchange activity was taken, as representative of the plant, from the apex of the largest leaf that exhibited no evidence of senescence. Using nondestructive hand searches, the numbers of P. rapae

4 462 Langan, Wheater, and Dunleavy eggs and larvae on each plant were recorded. For comparison with crop plants that were not visited during egg-laying, 10 randomly located plants in close proximity (<20 m) were inspected to measure the same characteristics as examined during oviposition observations. Many variables measured during the observation of egg-laying runs failed to conform to the assumptions of parametric tests, i.e., were skewed and/or failed to conform to Levene s test for homogeneity of variances. In addition, sample sizes were unbalanced. Consequently, rank means tests (Meddis, 1984) were used for comparisons between plants that were rejected after alighting and those that received eggs. To account for variation between butterflies (see Papaj and Rausher, 1983), data were blocked by individual. Chi-square tests were used to determine whether egg-laying females avoided plants already supporting conspecific eggs or larvae, a scenario which would reduce the likelihood that plants were selected more than once. Chi-square tests were also used to provide an insight into the sequential nature of oviposition runs. To decipher whether previous oviposition events influenced egg-laying decisions (i.e., whether there were differences from a rejected plant to an accepted plant, and vice versa), 2 2 contingency tables were constructed examining plants that were previously accepted and later either accepted or rejected against factor values (e.g., photosynthetic rate) that were higher or lower than at the previous event. To determine whether eggs were aggregated on plants, the distribution of P. rapae eggs on plants during peak densities of the second generation was compared against a random (Poisson) distribution on the basis of equal plant susceptibility. A second approach was implemented to compare plants based on the presence of P. rapae eggs. Surveys of 96 randomly located crop plants were carried out during peak densities of the second and third generations of P. rapae. The timing of surveys coincided with early (27 DAT; during the period of behavioral observations) and late (72 DAT; when observations were complete but P. rapae adults were still present) stages of crop growth. To characterize plants, measurements of height, leaf size, and gas exchange activity were recorded as before. Additional plant factors were recorded: damage rankings of 0 (none) to 12 (total) using leaf damage charts as a guideline (Endersby et al., 1991); the number of leaves; and, due to the welldocumented response of P. rapae to edge effects (for references see Hern et al., 1996), the distance of the plant from the crop edge. For comparison of plants with and without eggs, principal components analysis (PCA) was used to extract a smaller number of independent variables from a large number of measurements, and stepwise discriminant analysis (SDA) was used to extract factors that explain group membership (i.e., plants that did or did not support P. rapae eggs).

5 Egg-Laying Behavior and Plant Gas Exchange Activity 463 RESULTS During the second generation of P. rapae adults, 15 egg-laying runs were observed, each comprising 4 to 12 visits to plants. Alighted plants were either accepted (egg laid) or rejected (no egg laid) and not weighted in relation to the number of eggs received. Most eggs were laid singly (n = 58), although on six occasions eggs were laid in pairs and once three eggs were laid consecutively on the same leaf. This produced a total of 101 events (65 accepted and 36 rejected plants) and a mean of 6.7 plants visited per run. During four egg-laying runs, females laid eggs on all the plants upon which they alighted (n = 28 eggs in total). For comparisons between alighted plants that were accepted and those that were rejected, egg-laying runs that consisted only of accepted plants were omitted. Compared to a random selection of surrounding plants in the crop, plants visited by egg-laying females were taller but did not differ in gas exchange activity or leaf size (Table I). The presence of conspecifics did not deter oviposition since, after alighting, females did not discriminate between plants already supporting conspecific eggs (χ1 2 = 0.049, P = 0.820) and those supporting larvae (χ1 2 = 0.089, P = 0.764). After tarsal contact with leaves, females laid more eggs on shorter plants but not on those with larger leaves Table I. Comparison Using Rank Means Tests (in All Cases df = 1) of the Gas Exchange Activity and Size of Plants Investigated During Egg-Laying Flights a Median (Quartile 1, Quartile 3) Plant factor Investigated Random H Investigated plants Photosynthetic rate (µmol m 2 s 1 ) 16.7 (11.8, 21.1) 16.1 (11.4, 22.6) Transpiration rate (mmol m 2 s 1 ) 3.2 (2.6, 3.9) 3.4 (2.2, 4.0) Plant height (mm) 325 (305, 349) 285 (268, 311) Largest leaf size (mm) 272 (254, 301) 263 (245, 296) Accepted Rejected Accepted plants Leaf photosynthetic rate (µmol m 2 s 1 ) 14.6 (11.3, 16.7) 14.2 (10.9, 16.7) Leaf transpiration rate (mmol m 2 s 1 ) 2.1 (1.3, 3.2) 2.2 (1.2, 3.0) Plant photosynthetic rate (µmol m 2 s 1 ) 17.9 (15.7, 19.8) 16.2 (12.2, 18.2) Plant transpiration rate (mmol m 2 s 1 ) 3.10 (2.0, 4.1) 3.25 (2.2, 4.0) Plant height (mm) 310 (291, 341) 345 (307, 372) Largest leaf size (mm) 275 (258, 296) 290 (265, 314) a Visits to plants (n = 101) were compared with a random selection of crop plants (n = 96). After alighting, plants were either accepted (n = 45) or rejected (n = 36) and measurements of gas exchange activity were taken from both the leaf area contacted (leaf) and the largest leaf as representative of the plant (plant). Significant differences between rank means (P < 0.05).

6 464 Langan, Wheater, and Dunleavy (Table 1). Rates of photosynthesis and transpiration of rejected leaf areas (corresponding to the location of tarsal contact) did not differ significantly from those of leaf areas that received eggs. Comparisons using gas exchange measurements taken from the largest leaves of plants suggested that, after investigation by a female, plants that received eggs had higher rates of photosynthesis but not of transpiration (Table I). There was no evidence that the sequence in which plants were rejected/accepted was associated with preferences for the measured plant characteristics (photosynthesis, χ1 2 = 0.061, P = 0.805; transpiration, χ1 2 = 0.006, P = 0.937; plant height, χ 1 2 = 1.231, P = 0.267; leaf size, χ1 2 = 0.006, P > 0.999). The distribution of eggs (n = 79) on 96 crop plants at the time of behavioral observations (27 DAT) did not deviate significantly from random (i.e., from a Poisson distribution; χ4 2 = 3.14, P > 0.05). Attempts to characterize plants that supported P. rapae eggs during both early and late stages of crop growth were not successful. Axes generated by PCA (Table II) were not easily interpreted for either survey date and no axes were extracted by DA to discriminate between plants with and plants without eggs. Table II. Discriminating Principal Components and Their Constituent Variables for Crop Plants (n = 96) at Early (27 DAT) and Late (72 DAT) Stages of Plant Growth a Rotated factor PCA axis Variable loading 27 DAT PCA 1 Transpiration rate Crop edge Photosynthetic rate Plant damage PCA 2 Number of leaves Largest leaf size PCA 3 Plant height Leaf damage DAT PCA 1 Plant height Transpiration rate Largest leaf size PCA 2 Leaf damage Plant damage Crop edge PCA 3 Number of leaves Photosynthetic rate a No axis qualified for stepwise discriminant analysis in an attempt to distinguish plants that supported P. rapae eggs.

7 Egg-Laying Behavior and Plant Gas Exchange Activity 465 DISCUSSION In accordance with studies that provided egg-laying P. rapae with a range of plant types (Jones, 1977; Ives, 1978), females observed in this study discriminated between potential host plants in a cabbage monoculture. Overall, visits were made to taller plants which did not differ in leaf size compared to surrounding plants (Table I). Since the size of the largest leaves correlated well with the total leaf area, this suggests that greater leaf areas did not induce landings. Previously, P. rapae have been shown to approach and oviposit on larger plants, with egg numbers being best predicted by total leaf areas, and not the height of plants (Ives, 1978). Vision is undoubtedly important in host plant location of many phytophagous insects (Miller and Strickler, 1984), and it has been suggested that P. rapae rely heavily on plant color to induce landings but do not respond to leaf size or shape (Renwick and Radke, 1988). Using this assumption, it was suggested that preferences for larger plants (see Ives, 1978) could be explained by changes in color that occur with plant age (Renwick and Radke, 1988). Although color was not measured, results from the current study suggest that plants of the same age were distinguished before alighting by their height but not by other measurements of size or gas exchange activity. The susceptibility of a plant to discovery from herbivores is influenced by its apparency (Feeny, 1976), and in dense crops, taller plants may be more conspicuous, as they protrude from the crop canopy layer. Many butterflies are attracted to more obvious plants during egg-laying (Courtney, 1986), such as Pieris virginiensis, which target upright host plants in the diverse background vegetation of woodlands (Cappucino and Kareiva, 1985). The balance created by preferences (see Table I) for opposing traits of taller plants (pre alighting) and shorter plants (post alighting) provides some insight into why plant height was not useful for deciphering characteristics of plants that supported P. rapae eggs using multivariate techniques (Table II). We assumed that females did not make the decision to lay before alighting (see Courtney, 1986) or respond directly to the height of plants after alighting. Moreover, the sequence in which plants were accepted/rejected did not influence preferences in the current study. Therefore, it is likely that the leaf surface chemical profiles of the leaves of shorter plants alighted upon were preferred by females (Chew and Renwick, 1995). Jones (1977) found that pre- and post contact discrimination by P. rapae correlates poorly, since landings were more frequent on larger hosts, but younger tissues were preferred after alighting. This scenario agrees with the findings of the current study, as it resulted in plants of medium age and size receiving more eggs (Ives, 1978). The inability of DA to characterize crop plants with P. rapae

8 466 Langan, Wheater, and Dunleavy eggs based on their gas exchange activity or injury level may reflect the complexity of factors that determine host choice, particularly in environments with superoptimal resources and high similarity between plants. This study did not support Jones (1987), who found that U.K. populations of P. rapae aggregated eggs on certain plants. This may suggest that differences between plants were not sufficient to elicit repeated oviposition. However, it is possible that aggregations of P. rapae eggs may not be detected using the Poisson distribution when, as in the current study, mean densities of eggs are lower than a single egg per plant (see Harcourt, 1961). It must be considered that the flight patterns/directions of egg-laying females could be essentially random (see Root and Kareiva, 1984). Indeed, females very rarely returned to the same plant during egg-laying [also called zero returns (Jones, 1977)]. In the current study, the mean resettling frequency for individuals was 8% (±3%), whereas rates of 60% for British populations have been documented (Jones, 1987). The magnitude of these differences may reflect the availability/density of potential hosts or variation in flight conditions. Evidence that gas exchange activity was related to P. rapae host plant selection was equivocal. Examination of the specific areas of leaves on which butterflies alighted, and subsequently rejected, did not differ in photosynthetic or transpirational activity compared to those leaf areas that received eggs (Table I). Hence, it is unlikely that gas exchange activity at the leaf surface directly influenced the decision to lay. This was unsurprising since females alighted leaves of different ages during egg-laying and young leaf tissues are more active physiologically (Lurie et al., 1979). This behavior would have exposed females to a range of leaf surface chemistry profiles, which are widely accepted as the cue for egg-laying. Interestingly, like gas exchange activity, leaf surface chemistry fluctuates diurnally as well as with leaf age (Rosa et al., 1994). Measurements from leaves of a similar age suggested that, after investigation, plants that received eggs had higher photosynthetic rates, although this was not the case for plants with higher transpiration rates (Table I). In this study, plants were not watered to increase transpiration, unlike the plants investigated by Myers (1985), which resulted in increased rates of oviposition. Preferences for higher gas exchange activity at the level of the plant, but not the individual leaf, suggests that deposition of eggs ultimately results from preferences for other traits which may interact with photosynthesis. Allocation of offspring to plants with higher photosynthetic rates at the time of oviposition suggests that plants with superior rates of productivity receive more offspring. Preferences for plants (of the same age) with higher productivity provides an interesting parallel with earlier findings that egglaying P. rapae prefer larger, younger plants (Ives, 1978). However, the relationship between gas exchange and productivity is complex and dry matter

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