- Email: [email protected]
Potential ofAspongopus viduatusF. (Heteroptera: Pentatomidae) as a Biocontrol Agent for Squirting Cucumber,Ecballium elaterium(L.) A. Rich. (Cucurbitaceae)
BIOLOGICAL CONTROL ARTICLE NO.
7, 48–52 (1996)
0063
Potential of Aspongopus viduatus F. (Heteroptera: Pentatomidae) as a Biocontrol Agent for Squirting Cucumber, Ecballium elaterium (L.) A. Rich. (Cucurbitaceae) DAVID BEN-YAKIR,* DROR GAL,*,† MIKI CHEN,*
AND
DAVID ROSEN†
*Department of Entomology, The Volcani Center, Institute of Plant Protection, ARO, P.O. Box 6, Bet Dagan 50250, Israel; and †Department of Entomology, Faculty of Agriculture, The Hebrew University of Jerusalem, P.O. Box 12, Rehovot 76100, Israel Received August 16, 1994; accepted January 9, 1996
a perennial weed of Mediterranean origin (Zohary and Dothan, 1978; Everett, 1981). It is common in the Mediterranean Basin and often becomes an agricultural pest (Kleifeld, 1986). It is also a reservoir for plant viruses that infect cucurbitaceous crops (Rana and Mondelli, 1985; Antignus et al., 1990). Selective synthetic herbicides are not available to control this weed in some major broad-leaf vegetable crops (Anonymous, 1992). Moreover, the number of herbicides available for its control continuously decreases, reflecting the current policy of reducing pesticidal pollution in Israel. A similar policy is being implemented in other countries as well. Buckingham (1994) urged the development of technology for augmentative releases of natural enemies as an alternative method for weed control. Augmentation of a native noctuid has been adopted as a control method against water lettuce in Thailand (Napompeth, 1982). In the United States, a native tortricid has been studied extensively as an inundative biological control agent for nutsedge (reviewed by Wapshere et al., 1989). Because no biological control agent for squirting cucumber was listed by Julien (1992), we initiated a search for such agents in Israel. During 1990, we found squirting cucumber plants near Rehovot infested with a large number of Aspongopus viduatus F. (Heteroptera: Pentatomidae). These infested plants dried out and died within a few days. This bug is distributed from the northern Ethiopian region to the Middle East and has been reported to be a pest of watermelon in Egypt (Priesner and Alfieri, 1953). In Israel, this bug was found by Linnavuori (1960) in Jericho on squash (Cucurbita pepo L.), cucumber (Cucumis sativus L.), and watermelon (Citrullus lanatus (Thunb.) Matsum. & Nakai). To the best of our knowledge, it has never been reported as a pest of any crop by Israeli growers. Any insect considered for inundative biological weed control must first be proven safe to nontarget plants, should provide effective control within a short time, and should be easy to rear in large numbers (Wapshere
Squirting cucumber, Ecballium elaterium, is a common weed in the Mediterranean Basin. It often becomes an agricultural pest and may also serve as a reservoir for viruses that infect cucurbitaceous crops. We found squirting cucumber plants near Rehovot, Israel, infested with a large number of Aspongopus viduatus bugs, and the infested plants dried out and died within a few days. In this study, we evaluated the potential of A. viduatus as an inundative biocontrol agent for squirting cucumber by examining whether it is safe to crop plants, whether it can provide effective control within a short time, and how easy it is to rear it in large numbers. In no-choice tests, the bug was unable to become established on any of 21 different crop plants, except watermelon. When given a choice between watermelon and squirting cucumber, 80% of the bugs preferred the latter, regardless of their previous host plant or rearing conditions. There was a direct relationship between bug load and weed morbidity. Twenty adult bugs killed a squirting cucumber plant within 1 week, and the plants did not recover after the bugs had been removed. When A. viduatus was reared in the laboratory at 24 6 2°C on squirting cucumber, it took 5–6 weeks to develop from egg to adult. Each pair of adult bugs gave rise to about 470 adult progeny over a period of 5 months. Thus, prospects for mass rearing of A. viduatus and effective control of squirting cucumber by augmentative releases appear to be good. However, the risk from this bug to watermelon must be determined under field conditions before it can be used safely as an augmentative biocontrol agent. r 1996 Academic Press, Inc. KEY WORDS: Aspongopus viduatus; Ecballium elaterium; biocontrol; weeds; augmentative releases.
INTRODUCTION
Squirting cucumber, Ecballium elaterium (L.) A. Rich. (Cucurbitaceae; the only species in that genus), is 1049-9644/96 $18.00 Copyright r 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
48
Aspongopus viduatus, POTENTIAL FOR BIOCONTROL
et al., 1989). In this study, we evaluated the potential of A. viduatus as an inundative biocontrol agent for squirting cucumber by examing how well it would fit these requirements. MATERIALS AND METHODS
Insects Unless stated otherwise, adult bugs were collected from squirting cucumber plants in a field near Rehovot shortly before they were used in the various tests. The nymphs used were the progeny of the field-collected adults, which were reared indoors on potted squirting cucumber plants. Indoor rearing and experimental conditions were 24 6 2°C, 70–80% RH, and 15-h photophase. All outdoor experiments were conducted at The Volcani Center, Bet Dagan. Host Preference: No-Choice Test This study was conducted indoors during June, 1993. Twenty-one plant species, all of which are important agricultural crops in Israel, were used in this study (Table 1). Squirting cucumber plants and empty pots served as positive and negative controls, respectively.
TABLE 1 Host Plants Evaluated for Suitability to Aspongopus viduatus Family Compositae
Cruciferae Cucurbitaceae
Gramineae
Leguminosae
Liliaceae Malvaceae Solanaceae
Umbelliferae
Scientific name Carthamus tinctorius L. Cynara scolymus L. Helianthus annuus L. Brassica oleracea L. var. capitata L. Cucumis melo L. Cucumis sativus L. Cucurbita moschata Duchesne Cucurbita pepo L. Citrullus lanatus (Thunb.) Matsum. & Nakai Avena sativa L. Triticum aestivum L. Zea mays L. Arachis hypogaea L. Cicer arietinum L. Phaseolus vulgaris L. Allium cepa L. Gossypium herbaceum L. Capsicum annuum L. Lycopersicon esculentum Miller Solanum melongena L. Daucus carota L.
Common name (cultivar) Safflower (S-541) Artichoke (Purple) Sunflower (3.1.3) Cabbage Melon (Ein Dor) Cucumber (Delila) Pumpkin (Tripoli 20) Squash (Erlika)
Watermelon (Crimson) Oat (Soviet) Wheat (Nirit) Corn (Jubilee) Peanut (Shulamit) Chickpea (Spanish) Bean (Yokon) Onion (Eclipse) Cotton (Eden) Pepper (Orly) Tomato (Daniella) Eggplant (Classic) Carrot (Favor)
49
All plants were grown from seed, except for artichoke and squirting cucumber, which were grown from seedlings. Plants were grown individually in 10-liter pots containing sandy soil, fertilized with 4 g/liter of Osmocote (Sierra Chemical Co., Milpitas, CA), and kept outdoors in a screen house. Young plants (prereproductive stage) were used as hosts, because that is the life stage of squirting cucumber we aim to control. The cucurbitaceous plants were used when they started to bloom and had developed ca. 20 leaves. Plants of other families were used when they reached a height of 20 cm and had developed ca. 15 leaves. After the bugs were placed on a plant, the pot was covered with a white plastic 40-mesh screen to prevent their escape. There were five replicates (i.e., a replicate being a single potted plant) per treatment, which were arranged at random. When a plant started to show signs of stress (e.g., yellowing) it was replaced with a fresh plant of the same kind. New squirting cucumber plants are usually colonized by a few reproductively active adults, and their first offspring nymphs appear as early as 8 days postcolonization (Ben-Yakir et al., unpublished observations). We used an infestation that simulated this colonization sequence. Each plant was first infested with one male and one female. To rule out possible effects of the previous host plant on the bug’s reproduction, eggs produced during the first 10 days of infestation were removed. Eight days after the first infestation, 20 newly hatched nymphs were added to each plant. Observations were carried out for a total of 20 days. Infested plants were examined every 48 h and the following parameters were recorded: (a) adult survival; (b) survival and molting success of first-instar nymphs; and (c) egg production (from the 10th day of infestation on). Host Preference: Choice Test The relative attractiveness to the bugs of watermelon and squirting cucumber plants was compared under outdoor conditions during July 1993. A pair of potted plants, one of each species, was placed in the ground so that the soil at the top of the pot was even with the surrounding ground level. Five pairs of adult bugs were released at the center of a 20-cm open space that was left between the plants. Following the release, the plants were covered with a screen cage (30 mesh, 100 3 50 3 40 cm). The location of the bugs was determined 48 h after their release. Only bugs that were found on or within 2 cm of a plant were recorded as responding. We used bugs from three sources: (i) fieldcollected on squirting cucumber; (ii) reared indoors on squirting cucumber; and (iii) reared indoors on watermelon for one generation. This was done in order to determine whether previous host and rearing conditions may affect the bugs’ preferences. Each test, with bugs of a different source, was run four times (repli-
50
BEN-YAKIR ET AL.
cates) and after each run the position of the pair of plants was rotated 90° to the right in order to avoid directional bias. Rate of Bug Infestation Required to Kill a Squirting Cucumber Plant This study was conducted outdoors and was repeated twice during August and September 1993. Plants were infested with 0, 10, 20, or 40 adult bugs (equal numbers of males and females) per plant, with four single-plant replicates per treatment, and they were examined every 24 hr. The state of morbidity of each plant (score) was determined using a five-step scale: I, no injury; II, some leaves yellow; III, all leaves yellow; IV, ca. 70% of leaves completely dry; and V, entire plant desiccated. All bugs were removed from the plants 7 days after infestation, and the plants were kept for an additional month in order to determine the extent, if any, of their recovery. Scores recorded on the fifth day after infestation were used for the statistical analysis. Differences between the results obtained during the two runs were analyzed using Wilcoxon’s signed rank test (Wilcoxon, 1947). Differences between all treatments were analyzed using Friedman’s test (Friedman, 1937). Scores obtained on each run were ranked and then subjected to a linear regression analysis in order to determine the relations between the infestation rate and the state of morbidity. Development of Bugs in the Laboratory This study was conducted indoors, during the fall and winter of 1992–1993, simulating the production of bugs for release in spring. Eight couples of newly molted adults were each placed on a potted squirting cucumber plant. These plants were examined every 2 days and when eggs were found, they were removed, counted, and observed until hatching. When each fe-
male died, the total number of eggs she had laid was recorded. The duration of egg development and percentage of hatch were studied by observing five lots of 20 eggs each, which had been randomly collected from the various females. All eggs within a lot were laid on the same day. The duration of development of first-instar nymphs was studied separately from the rest of nymphal development, because their survivorship was lower and more variable than that of successive instars. Seven plants were infested with 20 newly hatched nymphs per plant, and the bugs were observed until they completed their first molt. Another five plants were infested with 10 second-instar nymphs per plant, and the bugs were observed until they reached maturity. Developmental times and survivorship were determined. Plants that withered during these studies were replaced with new ones. RESULTS
Host Preference: No-Choice Test Twenty days after placement, all the bugs on watermelon and squirting cucumber survived (Fig. 1). In contrast, none of the bugs on noncucurbit plants survived, with the exception of bean, on which 30% of the bugs did survive (data not shown). On other cucurbit plants, 40–70% of the bugs survived (Fig. 1). An exception to the latter was cucumber, on which all bugs died within 2 days after placement, while they survived up to 10 days without access to any plant (Fig. 1). Newly hatched nymphs survived and molted successfully only on squirting cucumber and watermelon. Between the 10th and 20th days after placement, the bugs did not produce eggs on any of the plant species, except squirting cucumber and watermelon. During that period, the mean numbers of eggs (6SD) collected on watermelon
FIG. 1. Survival of Aspongopus viduatus adults on cucurbit plants in a no-choice test (two bugs per pot, five pots per plant species).
51
Aspongopus viduatus, POTENTIAL FOR BIOCONTROL
TABLE 2 Influence of Previous Host Plant and Rearing Conditions on Plant Preference by A. viduatus Source of bugs Plant
Habitat
Mean number of respondersa 6 SD
Squirting cucumber Watermelon
Field Indoors Indoors
10 961 10
a
% Preferring squirting cucumber 6 SD 79 6 3 70 6 10 78 6 2
Ten bugs were used in each run; there were four runs per test.
and on squirting cucumber plants were 37 6 24 and 53 6 15, respectively, and all these eggs hatched. Host Preference: Choice Test In all tests, more than 90% of the bugs responded. Regardless of their previous host plant or rearing conditions, 70–80% of the responding bugs preferred squirting cucumber over watermelon (Table 2). Rate of Bug Infestation Required to Kill a Squirting Cucumber Plant Plants that were free of bugs showed no signs of morbidity. Although the deterioration of squirting cucumber appeared to occur faster in August (Fig. 2a) than in September (Fig. 2b), there was no significant difference between run dates (T 5 0.48; P . 0.05). There were significant differences between treatments (F 5 26.5; df 5 5,18; P , 0.01). A highly significant linear correlation (r 5 0.94; P , 0.0001) was found between the infestation rate (expressed on a logarithmic scale) and the state of morbidity. During August, all plants infested with 20 bugs died (state V) within 1 week, and these plants did not recover within a month after the bugs had been removed. Although plants that had been infested with 10 bugs only showed yellowing of leaves (states II–III) after 7 days of exposure, their state continued to deteriorate in the absence of bugs, and only 25% of them recovered. During September, of the plants that were infested with either 10 or 20 bugs, 75 and 25% recovered, respectively.
FIG. 2. Changes in state of morbidity of squirting cucumber plants following infestation with A. viduatus during August (a) and September (b) 1993 (each point represents the means of four plants; I, no injury; II, some leaves yellow; III, all leaves yellow; IV, ca. 70% of leaves completely dry; and V, entire plant desiccated).
(Priesner and Alfieri, 1953) and India (Anonymous, 1973; Mishra and Sharma, 1989). Although A. viduatus has not been known as a pest of any cucurbit crop plants in Israel, in this study it established itself successfully on watermelon plants. However, when given a choice between watermelon and squirting cucumber, 80% of the bugs preferred the latter. While no-choice tests may help in evaluating the suitability of a plant as a food source for an insect, they do not provide any information about other mechanisms that
Development of Bugs in the Laboratory Females lived a mean of 150 days (N 5 8, SD 5 37) and produced eggs throughout their life. A mean of 741 eggs were produced per female (N 5 8, SD 5 197). Developmental times and survivorship of the bugs are presented in Table 3. Development from egg to adult took ca. 41 days and overall survivorship was ca. 64%. DISCUSSION
Pentatomid bugs of the genus Aspongopus have been reported as pests of cucurbit crop plants in Egypt
TABLE 3 Developmental
Timea
(Mean 6 SD) and Survivorship of A. viduatus at 24°C
Life stage
No. of bugs
No. of replicates
Developmental time
% Survivors
Egg N-1 N-2 to Adult
20 20 10
5 7 5
9.0 6 0.5 6.8 6 0.5 24.9 6 2.1
100 85 6 15 76 6 23
a Number of days in which 50% of the bugs completed their development.
52
BEN-YAKIR ET AL.
may affect host selection (see Harris and Zwolfer, 1968). In the choice tests, the bugs may have relied on chemical cues (kairomones) to prefer squirting cucumber over watermelon. One such cue may be cucurbitacin, a highly toxic and bitter chemical present in cucurbit plants. Cucurbitacin is known to act as an attractant to some insects (Metcalf et al., 1980) and as a repellent to others (Bar-Nun and Mayer, 1989). While squirting cucumber contains a high concentration of this chemical (Lavie and Willner, 1958), cultivated cucurbit crops have been selected for a low concentration of cucurbitacin (Chambliss and Jones, 1966). In India, cucumber plants were found to be the preferred host of Aspongopus janus F. (Mishra and Sharma, 1989), a close relative of A. viduatus. In our study, A. viduatus died within 2 days after it was placed on cucumber, much faster than in the absence of any host plant. Evidently, the bugs attempted to feed on the cucumber and were affected by a toxic substance. Infestation with 20 adult bugs per plant killed squirting cucumber within 1 week, with almost no recovery of the weed. This rapid destruction, and the fact that many plants which had been only partly affected died without recovery, indicate that, besides actual feeding, salivary toxins of the bug may also be involved. When A. viduatus was reared indoors at 24 6 2°C on squirting cucumber, it took 5–6 weeks to develop from egg to adult. Each pair of adult bugs gave rise to about 470 new adults over a period of 5 months. This high reproductive capacity and the simple and inexpensive rearing procedure indicate that mass rearing of this bug is likely to be economically viable. Moreover, as the adult bugs are large, long-lived, and slow-moving, they may be easily collected at a release site after 1 week to be reused at other sites. Effective control of squirting cucumber is expected to reduce the availability of reservoir plants for viruses that infect cucurbitaceous crops. Whether this will actually lower the incidence of a particular viral disease needs to be studied. The fact that A. viduatus may accept watermelon as a host would, of course, preclude its importation into other countries as a classical biocontrol agent. Within its area of endemicity, although it exhibited a clear preference for squirting cucumber, the risk from this bug to watermelon must be determined under field conditions. If the bug is proven safe, prospects for its mass rearing and effective control of squirting cucumber by augmentative releases appear to be good. In any case, it may be safely used for releases in areas that are not adjacent to fields of watermelon. ACKNOWLEDGMENTS The authors are grateful to Ms. T. Feler, Department of Zoology, Tel-Aviv University, for identification of the bug. We also thank Dr. Y. Kleifeld, Newe Ya’ar Research Center, ARO, Professor M. Mayer, The
Hebrew University, and Drs. Z. Mendel and D. Nestel, The Volcani Center, ARO, for their generous assistance and advice. This study was partly supported by the Israel Ministry of Agriculture through the Chief Scientist’s Fund, as Project 131-0851. This paper is a contribution from the Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel, No. 1425-E, 1994 series.
REFERENCES Anonymous. 1973. Insects not known to occur in the United States [No. 194 of series]. Red pumpkin bug (Aspongopus (Coridius) janus (Fabricius)). Coop. Econ. Insect Rep. 23, 197–198. Anonymous. 1992. ‘‘Database of Pesticides Permitted to Use in Israel.’’ Department of Plant Protection and Inspection, Israel Ministry of Agriculture. Antignus, Y., Pearlsman, M., Ben-Yoseph, R., and Cohen, S. 1990. Occurrence of a variant of cucumber green mottle mosaic virus in Israel. Phytoparasitica 18, 50–56. Bar-Nun, N., and Mayer, A. M. 1989. Cucurbitacins—Repressors of induction of laccase formation. Phytochemistry 28, 1369–1371. Buckingham, G. R. 1994. Biological control of aquatic weeds. In ‘‘Pest Management in the Subtropics. Biological Control—A Florida Perspective’’ (D. Rosen, F. D. Bennett, and J. L. Capinera, Eds.), pp. 413–480. Intercept, Andover, UK. Chambliss, O. L., and Jones, C. M. 1966. Cucurbitacins: Specific insect attractants in Cucurbitaceae. Science 153, 1392–1393. Everett, T. H. 1981. ‘‘The New York Botanical Garden Illustrated Encyclopedia of Horticulture,’’ p. 1159. Garland, New York. Friedman, M. 1937. The use of ranks to avoid the assumption of normality implicit in the analysis of variance. J. Am. Stat. Assoc. 32, 675–701. Harris, V. E., and Zwolfer, H. 1968. Screening of phytophagous insects for biocontrol of weeds. Can. Entomol. 100, 295–303. Julien, M. H. 1992. ‘‘Biological Control of Weeds: A World Catalogue of Agents and Their Target Weeds,’’ 3rd ed. Commonwealth Agric. Bureaux, Farnham Royal, England. Kleifeld, Y. 1986. Cotton monoculture—The weeds love it. Hassadeh 66, 1088–1091. [in Hebrew] Lavie, D., and Willner, D. 1958. The constituents of Ecballium elaterium L. III. Elatericine A and B. J. Am. Chem. Soc. 80, 710–714. Linnavuori, R. 1960. Hemiptera of Israel. Ann. Zool. Soc. ‘Vanamo,’ Helsinki 22, 1–71. Metcalf, R. L., Metcalf, R. A., and Rhodes, A. M. 1980. Cucurbitacins as kairomones for diabroticine beetles. Proc. Natl. Acad. Sci. USA 77, 3769–3772. Mishra, R. K., and Sharma, R. N. 1989. Host preference by adult Aspongopus janus F. Indian J. Entomol. 51, 480–481. Napompeth, B. 1982. Biological control research and development in Thailand. Proc. Int. Conf. Plant Prot. Tropics, 1982, 301–323. Priesner, H., and Alfieri, A. 1953. A review of the Hemiptera and Heteroptera known to us from Egypt. Bull. Soc. Fouad 1er Entomol. 37, 1–19. Rana, G., and Mondelli, D. 1985. Solanum nigrum L. and Ecballium elaterium Rich., hosts of virus pathogenic to cultivated plants in Apulia. Informator-Fitopatol. 35, 43–46. Wapshere, A. J., Delfosse, E. S., and Cullen, J. M. 1989. Recent developments in biological control of weeds. Crop Prot. 8, 227–250. Wilcoxon, F. 1947. Probability tables for individual comparisons by ranking methods. Biom. Bull. 3, 119–122. Zohary, M., and Dothan, F. N. 1978. ‘‘Flora Palaestina. Part three (Text).’’ Jerusalem Academic Press, Jerusalem.
7, 48–52 (1996)
0063
Potential of Aspongopus viduatus F. (Heteroptera: Pentatomidae) as a Biocontrol Agent for Squirting Cucumber, Ecballium elaterium (L.) A. Rich. (Cucurbitaceae) DAVID BEN-YAKIR,* DROR GAL,*,† MIKI CHEN,*
AND
DAVID ROSEN†
*Department of Entomology, The Volcani Center, Institute of Plant Protection, ARO, P.O. Box 6, Bet Dagan 50250, Israel; and †Department of Entomology, Faculty of Agriculture, The Hebrew University of Jerusalem, P.O. Box 12, Rehovot 76100, Israel Received August 16, 1994; accepted January 9, 1996
a perennial weed of Mediterranean origin (Zohary and Dothan, 1978; Everett, 1981). It is common in the Mediterranean Basin and often becomes an agricultural pest (Kleifeld, 1986). It is also a reservoir for plant viruses that infect cucurbitaceous crops (Rana and Mondelli, 1985; Antignus et al., 1990). Selective synthetic herbicides are not available to control this weed in some major broad-leaf vegetable crops (Anonymous, 1992). Moreover, the number of herbicides available for its control continuously decreases, reflecting the current policy of reducing pesticidal pollution in Israel. A similar policy is being implemented in other countries as well. Buckingham (1994) urged the development of technology for augmentative releases of natural enemies as an alternative method for weed control. Augmentation of a native noctuid has been adopted as a control method against water lettuce in Thailand (Napompeth, 1982). In the United States, a native tortricid has been studied extensively as an inundative biological control agent for nutsedge (reviewed by Wapshere et al., 1989). Because no biological control agent for squirting cucumber was listed by Julien (1992), we initiated a search for such agents in Israel. During 1990, we found squirting cucumber plants near Rehovot infested with a large number of Aspongopus viduatus F. (Heteroptera: Pentatomidae). These infested plants dried out and died within a few days. This bug is distributed from the northern Ethiopian region to the Middle East and has been reported to be a pest of watermelon in Egypt (Priesner and Alfieri, 1953). In Israel, this bug was found by Linnavuori (1960) in Jericho on squash (Cucurbita pepo L.), cucumber (Cucumis sativus L.), and watermelon (Citrullus lanatus (Thunb.) Matsum. & Nakai). To the best of our knowledge, it has never been reported as a pest of any crop by Israeli growers. Any insect considered for inundative biological weed control must first be proven safe to nontarget plants, should provide effective control within a short time, and should be easy to rear in large numbers (Wapshere
Squirting cucumber, Ecballium elaterium, is a common weed in the Mediterranean Basin. It often becomes an agricultural pest and may also serve as a reservoir for viruses that infect cucurbitaceous crops. We found squirting cucumber plants near Rehovot, Israel, infested with a large number of Aspongopus viduatus bugs, and the infested plants dried out and died within a few days. In this study, we evaluated the potential of A. viduatus as an inundative biocontrol agent for squirting cucumber by examining whether it is safe to crop plants, whether it can provide effective control within a short time, and how easy it is to rear it in large numbers. In no-choice tests, the bug was unable to become established on any of 21 different crop plants, except watermelon. When given a choice between watermelon and squirting cucumber, 80% of the bugs preferred the latter, regardless of their previous host plant or rearing conditions. There was a direct relationship between bug load and weed morbidity. Twenty adult bugs killed a squirting cucumber plant within 1 week, and the plants did not recover after the bugs had been removed. When A. viduatus was reared in the laboratory at 24 6 2°C on squirting cucumber, it took 5–6 weeks to develop from egg to adult. Each pair of adult bugs gave rise to about 470 adult progeny over a period of 5 months. Thus, prospects for mass rearing of A. viduatus and effective control of squirting cucumber by augmentative releases appear to be good. However, the risk from this bug to watermelon must be determined under field conditions before it can be used safely as an augmentative biocontrol agent. r 1996 Academic Press, Inc. KEY WORDS: Aspongopus viduatus; Ecballium elaterium; biocontrol; weeds; augmentative releases.
INTRODUCTION
Squirting cucumber, Ecballium elaterium (L.) A. Rich. (Cucurbitaceae; the only species in that genus), is 1049-9644/96 $18.00 Copyright r 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
48
Aspongopus viduatus, POTENTIAL FOR BIOCONTROL
et al., 1989). In this study, we evaluated the potential of A. viduatus as an inundative biocontrol agent for squirting cucumber by examing how well it would fit these requirements. MATERIALS AND METHODS
Insects Unless stated otherwise, adult bugs were collected from squirting cucumber plants in a field near Rehovot shortly before they were used in the various tests. The nymphs used were the progeny of the field-collected adults, which were reared indoors on potted squirting cucumber plants. Indoor rearing and experimental conditions were 24 6 2°C, 70–80% RH, and 15-h photophase. All outdoor experiments were conducted at The Volcani Center, Bet Dagan. Host Preference: No-Choice Test This study was conducted indoors during June, 1993. Twenty-one plant species, all of which are important agricultural crops in Israel, were used in this study (Table 1). Squirting cucumber plants and empty pots served as positive and negative controls, respectively.
TABLE 1 Host Plants Evaluated for Suitability to Aspongopus viduatus Family Compositae
Cruciferae Cucurbitaceae
Gramineae
Leguminosae
Liliaceae Malvaceae Solanaceae
Umbelliferae
Scientific name Carthamus tinctorius L. Cynara scolymus L. Helianthus annuus L. Brassica oleracea L. var. capitata L. Cucumis melo L. Cucumis sativus L. Cucurbita moschata Duchesne Cucurbita pepo L. Citrullus lanatus (Thunb.) Matsum. & Nakai Avena sativa L. Triticum aestivum L. Zea mays L. Arachis hypogaea L. Cicer arietinum L. Phaseolus vulgaris L. Allium cepa L. Gossypium herbaceum L. Capsicum annuum L. Lycopersicon esculentum Miller Solanum melongena L. Daucus carota L.
Common name (cultivar) Safflower (S-541) Artichoke (Purple) Sunflower (3.1.3) Cabbage Melon (Ein Dor) Cucumber (Delila) Pumpkin (Tripoli 20) Squash (Erlika)
Watermelon (Crimson) Oat (Soviet) Wheat (Nirit) Corn (Jubilee) Peanut (Shulamit) Chickpea (Spanish) Bean (Yokon) Onion (Eclipse) Cotton (Eden) Pepper (Orly) Tomato (Daniella) Eggplant (Classic) Carrot (Favor)
49
All plants were grown from seed, except for artichoke and squirting cucumber, which were grown from seedlings. Plants were grown individually in 10-liter pots containing sandy soil, fertilized with 4 g/liter of Osmocote (Sierra Chemical Co., Milpitas, CA), and kept outdoors in a screen house. Young plants (prereproductive stage) were used as hosts, because that is the life stage of squirting cucumber we aim to control. The cucurbitaceous plants were used when they started to bloom and had developed ca. 20 leaves. Plants of other families were used when they reached a height of 20 cm and had developed ca. 15 leaves. After the bugs were placed on a plant, the pot was covered with a white plastic 40-mesh screen to prevent their escape. There were five replicates (i.e., a replicate being a single potted plant) per treatment, which were arranged at random. When a plant started to show signs of stress (e.g., yellowing) it was replaced with a fresh plant of the same kind. New squirting cucumber plants are usually colonized by a few reproductively active adults, and their first offspring nymphs appear as early as 8 days postcolonization (Ben-Yakir et al., unpublished observations). We used an infestation that simulated this colonization sequence. Each plant was first infested with one male and one female. To rule out possible effects of the previous host plant on the bug’s reproduction, eggs produced during the first 10 days of infestation were removed. Eight days after the first infestation, 20 newly hatched nymphs were added to each plant. Observations were carried out for a total of 20 days. Infested plants were examined every 48 h and the following parameters were recorded: (a) adult survival; (b) survival and molting success of first-instar nymphs; and (c) egg production (from the 10th day of infestation on). Host Preference: Choice Test The relative attractiveness to the bugs of watermelon and squirting cucumber plants was compared under outdoor conditions during July 1993. A pair of potted plants, one of each species, was placed in the ground so that the soil at the top of the pot was even with the surrounding ground level. Five pairs of adult bugs were released at the center of a 20-cm open space that was left between the plants. Following the release, the plants were covered with a screen cage (30 mesh, 100 3 50 3 40 cm). The location of the bugs was determined 48 h after their release. Only bugs that were found on or within 2 cm of a plant were recorded as responding. We used bugs from three sources: (i) fieldcollected on squirting cucumber; (ii) reared indoors on squirting cucumber; and (iii) reared indoors on watermelon for one generation. This was done in order to determine whether previous host and rearing conditions may affect the bugs’ preferences. Each test, with bugs of a different source, was run four times (repli-
50
BEN-YAKIR ET AL.
cates) and after each run the position of the pair of plants was rotated 90° to the right in order to avoid directional bias. Rate of Bug Infestation Required to Kill a Squirting Cucumber Plant This study was conducted outdoors and was repeated twice during August and September 1993. Plants were infested with 0, 10, 20, or 40 adult bugs (equal numbers of males and females) per plant, with four single-plant replicates per treatment, and they were examined every 24 hr. The state of morbidity of each plant (score) was determined using a five-step scale: I, no injury; II, some leaves yellow; III, all leaves yellow; IV, ca. 70% of leaves completely dry; and V, entire plant desiccated. All bugs were removed from the plants 7 days after infestation, and the plants were kept for an additional month in order to determine the extent, if any, of their recovery. Scores recorded on the fifth day after infestation were used for the statistical analysis. Differences between the results obtained during the two runs were analyzed using Wilcoxon’s signed rank test (Wilcoxon, 1947). Differences between all treatments were analyzed using Friedman’s test (Friedman, 1937). Scores obtained on each run were ranked and then subjected to a linear regression analysis in order to determine the relations between the infestation rate and the state of morbidity. Development of Bugs in the Laboratory This study was conducted indoors, during the fall and winter of 1992–1993, simulating the production of bugs for release in spring. Eight couples of newly molted adults were each placed on a potted squirting cucumber plant. These plants were examined every 2 days and when eggs were found, they were removed, counted, and observed until hatching. When each fe-
male died, the total number of eggs she had laid was recorded. The duration of egg development and percentage of hatch were studied by observing five lots of 20 eggs each, which had been randomly collected from the various females. All eggs within a lot were laid on the same day. The duration of development of first-instar nymphs was studied separately from the rest of nymphal development, because their survivorship was lower and more variable than that of successive instars. Seven plants were infested with 20 newly hatched nymphs per plant, and the bugs were observed until they completed their first molt. Another five plants were infested with 10 second-instar nymphs per plant, and the bugs were observed until they reached maturity. Developmental times and survivorship were determined. Plants that withered during these studies were replaced with new ones. RESULTS
Host Preference: No-Choice Test Twenty days after placement, all the bugs on watermelon and squirting cucumber survived (Fig. 1). In contrast, none of the bugs on noncucurbit plants survived, with the exception of bean, on which 30% of the bugs did survive (data not shown). On other cucurbit plants, 40–70% of the bugs survived (Fig. 1). An exception to the latter was cucumber, on which all bugs died within 2 days after placement, while they survived up to 10 days without access to any plant (Fig. 1). Newly hatched nymphs survived and molted successfully only on squirting cucumber and watermelon. Between the 10th and 20th days after placement, the bugs did not produce eggs on any of the plant species, except squirting cucumber and watermelon. During that period, the mean numbers of eggs (6SD) collected on watermelon
FIG. 1. Survival of Aspongopus viduatus adults on cucurbit plants in a no-choice test (two bugs per pot, five pots per plant species).
51
Aspongopus viduatus, POTENTIAL FOR BIOCONTROL
TABLE 2 Influence of Previous Host Plant and Rearing Conditions on Plant Preference by A. viduatus Source of bugs Plant
Habitat
Mean number of respondersa 6 SD
Squirting cucumber Watermelon
Field Indoors Indoors
10 961 10
a
% Preferring squirting cucumber 6 SD 79 6 3 70 6 10 78 6 2
Ten bugs were used in each run; there were four runs per test.
and on squirting cucumber plants were 37 6 24 and 53 6 15, respectively, and all these eggs hatched. Host Preference: Choice Test In all tests, more than 90% of the bugs responded. Regardless of their previous host plant or rearing conditions, 70–80% of the responding bugs preferred squirting cucumber over watermelon (Table 2). Rate of Bug Infestation Required to Kill a Squirting Cucumber Plant Plants that were free of bugs showed no signs of morbidity. Although the deterioration of squirting cucumber appeared to occur faster in August (Fig. 2a) than in September (Fig. 2b), there was no significant difference between run dates (T 5 0.48; P . 0.05). There were significant differences between treatments (F 5 26.5; df 5 5,18; P , 0.01). A highly significant linear correlation (r 5 0.94; P , 0.0001) was found between the infestation rate (expressed on a logarithmic scale) and the state of morbidity. During August, all plants infested with 20 bugs died (state V) within 1 week, and these plants did not recover within a month after the bugs had been removed. Although plants that had been infested with 10 bugs only showed yellowing of leaves (states II–III) after 7 days of exposure, their state continued to deteriorate in the absence of bugs, and only 25% of them recovered. During September, of the plants that were infested with either 10 or 20 bugs, 75 and 25% recovered, respectively.
FIG. 2. Changes in state of morbidity of squirting cucumber plants following infestation with A. viduatus during August (a) and September (b) 1993 (each point represents the means of four plants; I, no injury; II, some leaves yellow; III, all leaves yellow; IV, ca. 70% of leaves completely dry; and V, entire plant desiccated).
(Priesner and Alfieri, 1953) and India (Anonymous, 1973; Mishra and Sharma, 1989). Although A. viduatus has not been known as a pest of any cucurbit crop plants in Israel, in this study it established itself successfully on watermelon plants. However, when given a choice between watermelon and squirting cucumber, 80% of the bugs preferred the latter. While no-choice tests may help in evaluating the suitability of a plant as a food source for an insect, they do not provide any information about other mechanisms that
Development of Bugs in the Laboratory Females lived a mean of 150 days (N 5 8, SD 5 37) and produced eggs throughout their life. A mean of 741 eggs were produced per female (N 5 8, SD 5 197). Developmental times and survivorship of the bugs are presented in Table 3. Development from egg to adult took ca. 41 days and overall survivorship was ca. 64%. DISCUSSION
Pentatomid bugs of the genus Aspongopus have been reported as pests of cucurbit crop plants in Egypt
TABLE 3 Developmental
Timea
(Mean 6 SD) and Survivorship of A. viduatus at 24°C
Life stage
No. of bugs
No. of replicates
Developmental time
% Survivors
Egg N-1 N-2 to Adult
20 20 10
5 7 5
9.0 6 0.5 6.8 6 0.5 24.9 6 2.1
100 85 6 15 76 6 23
a Number of days in which 50% of the bugs completed their development.
52
BEN-YAKIR ET AL.
may affect host selection (see Harris and Zwolfer, 1968). In the choice tests, the bugs may have relied on chemical cues (kairomones) to prefer squirting cucumber over watermelon. One such cue may be cucurbitacin, a highly toxic and bitter chemical present in cucurbit plants. Cucurbitacin is known to act as an attractant to some insects (Metcalf et al., 1980) and as a repellent to others (Bar-Nun and Mayer, 1989). While squirting cucumber contains a high concentration of this chemical (Lavie and Willner, 1958), cultivated cucurbit crops have been selected for a low concentration of cucurbitacin (Chambliss and Jones, 1966). In India, cucumber plants were found to be the preferred host of Aspongopus janus F. (Mishra and Sharma, 1989), a close relative of A. viduatus. In our study, A. viduatus died within 2 days after it was placed on cucumber, much faster than in the absence of any host plant. Evidently, the bugs attempted to feed on the cucumber and were affected by a toxic substance. Infestation with 20 adult bugs per plant killed squirting cucumber within 1 week, with almost no recovery of the weed. This rapid destruction, and the fact that many plants which had been only partly affected died without recovery, indicate that, besides actual feeding, salivary toxins of the bug may also be involved. When A. viduatus was reared indoors at 24 6 2°C on squirting cucumber, it took 5–6 weeks to develop from egg to adult. Each pair of adult bugs gave rise to about 470 new adults over a period of 5 months. This high reproductive capacity and the simple and inexpensive rearing procedure indicate that mass rearing of this bug is likely to be economically viable. Moreover, as the adult bugs are large, long-lived, and slow-moving, they may be easily collected at a release site after 1 week to be reused at other sites. Effective control of squirting cucumber is expected to reduce the availability of reservoir plants for viruses that infect cucurbitaceous crops. Whether this will actually lower the incidence of a particular viral disease needs to be studied. The fact that A. viduatus may accept watermelon as a host would, of course, preclude its importation into other countries as a classical biocontrol agent. Within its area of endemicity, although it exhibited a clear preference for squirting cucumber, the risk from this bug to watermelon must be determined under field conditions. If the bug is proven safe, prospects for its mass rearing and effective control of squirting cucumber by augmentative releases appear to be good. In any case, it may be safely used for releases in areas that are not adjacent to fields of watermelon. ACKNOWLEDGMENTS The authors are grateful to Ms. T. Feler, Department of Zoology, Tel-Aviv University, for identification of the bug. We also thank Dr. Y. Kleifeld, Newe Ya’ar Research Center, ARO, Professor M. Mayer, The
Hebrew University, and Drs. Z. Mendel and D. Nestel, The Volcani Center, ARO, for their generous assistance and advice. This study was partly supported by the Israel Ministry of Agriculture through the Chief Scientist’s Fund, as Project 131-0851. This paper is a contribution from the Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel, No. 1425-E, 1994 series.
REFERENCES Anonymous. 1973. Insects not known to occur in the United States [No. 194 of series]. Red pumpkin bug (Aspongopus (Coridius) janus (Fabricius)). Coop. Econ. Insect Rep. 23, 197–198. Anonymous. 1992. ‘‘Database of Pesticides Permitted to Use in Israel.’’ Department of Plant Protection and Inspection, Israel Ministry of Agriculture. Antignus, Y., Pearlsman, M., Ben-Yoseph, R., and Cohen, S. 1990. Occurrence of a variant of cucumber green mottle mosaic virus in Israel. Phytoparasitica 18, 50–56. Bar-Nun, N., and Mayer, A. M. 1989. Cucurbitacins—Repressors of induction of laccase formation. Phytochemistry 28, 1369–1371. Buckingham, G. R. 1994. Biological control of aquatic weeds. In ‘‘Pest Management in the Subtropics. Biological Control—A Florida Perspective’’ (D. Rosen, F. D. Bennett, and J. L. Capinera, Eds.), pp. 413–480. Intercept, Andover, UK. Chambliss, O. L., and Jones, C. M. 1966. Cucurbitacins: Specific insect attractants in Cucurbitaceae. Science 153, 1392–1393. Everett, T. H. 1981. ‘‘The New York Botanical Garden Illustrated Encyclopedia of Horticulture,’’ p. 1159. Garland, New York. Friedman, M. 1937. The use of ranks to avoid the assumption of normality implicit in the analysis of variance. J. Am. Stat. Assoc. 32, 675–701. Harris, V. E., and Zwolfer, H. 1968. Screening of phytophagous insects for biocontrol of weeds. Can. Entomol. 100, 295–303. Julien, M. H. 1992. ‘‘Biological Control of Weeds: A World Catalogue of Agents and Their Target Weeds,’’ 3rd ed. Commonwealth Agric. Bureaux, Farnham Royal, England. Kleifeld, Y. 1986. Cotton monoculture—The weeds love it. Hassadeh 66, 1088–1091. [in Hebrew] Lavie, D., and Willner, D. 1958. The constituents of Ecballium elaterium L. III. Elatericine A and B. J. Am. Chem. Soc. 80, 710–714. Linnavuori, R. 1960. Hemiptera of Israel. Ann. Zool. Soc. ‘Vanamo,’ Helsinki 22, 1–71. Metcalf, R. L., Metcalf, R. A., and Rhodes, A. M. 1980. Cucurbitacins as kairomones for diabroticine beetles. Proc. Natl. Acad. Sci. USA 77, 3769–3772. Mishra, R. K., and Sharma, R. N. 1989. Host preference by adult Aspongopus janus F. Indian J. Entomol. 51, 480–481. Napompeth, B. 1982. Biological control research and development in Thailand. Proc. Int. Conf. Plant Prot. Tropics, 1982, 301–323. Priesner, H., and Alfieri, A. 1953. A review of the Hemiptera and Heteroptera known to us from Egypt. Bull. Soc. Fouad 1er Entomol. 37, 1–19. Rana, G., and Mondelli, D. 1985. Solanum nigrum L. and Ecballium elaterium Rich., hosts of virus pathogenic to cultivated plants in Apulia. Informator-Fitopatol. 35, 43–46. Wapshere, A. J., Delfosse, E. S., and Cullen, J. M. 1989. Recent developments in biological control of weeds. Crop Prot. 8, 227–250. Wilcoxon, F. 1947. Probability tables for individual comparisons by ranking methods. Biom. Bull. 3, 119–122. Zohary, M., and Dothan, F. N. 1978. ‘‘Flora Palaestina. Part three (Text).’’ Jerusalem Academic Press, Jerusalem.