Zvi Mendel, Klil Adar, David Nestel and Ezra Dunkelblum
Plant Protection Institute, Agricultural Research Organization, The Volcani Center, Bet Dagan 50250, Israel
Abstract - The capture of adult Elatophilus hebraicus (Heteroptera, Anthocoridae) in traps baited with the sex pheromone of the Israeli pine bast scale, Matsucoccus josephi (Homoptera, Matsucoccidae), enabled the study of population trends of the predator as related to those of its prey. Traps were exposed at monthly intervals in stands of Pinus halepensis and P. brutia ssp.brutia for a period of 27 months. Fourier series were used to modulate population trends. The population density of M. josephi increased in March-April and in August and October. A steep increase in trap catch of E. hebraicus was noticeable during May and June. The rise of the predator population related positively, but only to a limited extent (r2=0.40) to the increase in prey density in the previous spring. On an annual base, an inverse relationship was found between the mean densities of M. josephi and E. hebraicus . Population trends of both prey and predator varied slightly between regions, but not between host plant species. We also used pheromone-baited traps in additional areas of the Palaeartic region where other Matsucoccus species occur. It was found that the range of E. hebraicus and M. josephi coincides with that of P. brutia ssp.brutia in the East Mediterranean. In Pinus pinaster stands in Portugal, two other predators, Elatophilus crassicornis and Hemerobius stigma (Neuroptera, Hemerobiidae) were caught in traps baited with the pheromone of M. josephi . These predators are now under appraisal to augment the natural enemy fauna of M. josephi in Israel.
Key words - Elatophilus, Matsucoccus , Pinus, sex pheromone, population monitoring, kairomone, biological control
Introduction
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The Israeli pine bast scale Matsucoccus josephi Bodenheimer and Harpaz is the major pest of pine in Israel (Mendel et al. 1994a). In the Eastern Mediterranean, the scale occurs in the natural stands of P. brutia ssp. brutia at moderate densities, and damage is practically nil. Conversely, high population densities have been observed in plantations and natural forests of P. halepensis in Israel, Jordan and Turkey (Mendel 1992; Mendel et al. 1994a) and in plantations of P. brutia spp. eldarica P. brutia in Israel (Golan et al. 1983; Mendel et al. 1994b) leading to severe injury in these forests.
The predator Elatophilus hebraicus P. brutia Pericart (Heteroptera, Anthocoridae) is the most common natural enemy of M. josephi in Israel. Like other members of the genus, E. hebraicus feeds only on Matsucoccus E. hebraicus and reproduces only on pine (Mendel et al. 1991).
Direct sampling of Matsucoccus on adult trees is tedious and impractical, especially in the case of a multivoltine species such as M. josephi. Because the occurrence of distinct symptoms of injury appear after the scale population has reached its peak, the level of injury does not provide a reliable measure for its population density. Sampling of E. hebraicus in adult pine forests is even more complicated. Hence, one of the major topics regarding the study of pine bast scales of economic importance concerns the development of simple and reliable techniques for monitoring populations trends. Pheromone-baited traps are considered as an efficient tool for monitoring and constitute an important component in any management program for the control of insect pests. In view of this, efforts have been made in the last decade to identify and make available the female sex pheromones of the three pernicious pine bast scales M. josephi, M. feytaudi and M. matsumurae (M. resinosae M. matsumurae) (Lanier et al. 1989; Einhorn et al. 1990; Dunkelblum et al. 1995).
It is known that some natural enemies of insect pests respond to sex or aggregation pheromones of their prey. There are several examples of kairomonal response by egg parasitoids of lepidopteran pests and by parasitoids of armored scales to the sex pheromones of their hosts (e.g. McClain et al. 1990; Noldus et al. 1991). However, until now, there has been no evidence that insect predators use sex pheromones of their prey as kairomones. Recently, we have shown that males and females of E. hebraicus use the sex pheromone of M. josephi as a long-range kairomone (Mendel et al. 1995). Furthermore, we have found that E. hebraicus is also attracted to the sex pheromones of the allopatric M. feytaudi and M. matsumurae (Dunkelblum et al. 1996).
The availability of the sex pheromone of M. josephi has allowed us to use the pheromone trap as a tool for studying the population trends of E. hebraicus in pine forests and for defining its geographical distribution. Taking into account that E. hebraicus is attracted to the sex pheromones of other pine bast scales, these traps have been also used to identify potential predators of M. josephi occurring in the natural area of other Matsucoccus spp. The objective has been to select and subsequently to introduce into Israel those predators which display kairomonal response to the sex pheromones of M. josephi.
Pheromone traps and insect capture
Traps baited with a 56% E:44% Z mixture of the racemic sex pheromone of M. josephi, (2E,6E,8E)-5,7-dimethyl-2,6,8-decatrien-4-one) and (2E,6Z,8E)-5,7-dimethyl-2,6,8-decatrien -4-one) (Dunkelblum et al. 1996), were exposed at monthly intervals in stands of P. halepensis and P. brutia ssp. brutia during 27 months, in several regions in Israel. White plastic triangular traps with a replaceable sticky bottom were equipped with a rubber dispenser (Maavit Products, Tel Aviv, Israel) loaded with 500 µg of the racemic pheromone mixture. Four traps were attached to the north side of living trees for about 4 weeks in each plot. The trapped insects were examined under magnification at the Volcani Center. The number of M. josephi males/trap/day and of E. hebraicus individuals/trap/day was determined for each plot and sampling interval.
Considering that both insects are multivoltine, with several overlapping generations, Fourier series were applied to the time series data and used to modulate population trends. Fourier analysis is based on a theory proposed by J. B. Fourier that any periodic wave can be represented as a sum of harmonic terms with an angular frequency (Bigger 1973). Numbers of annual generations of prey and predator were determined by plotting the annual accumulated degree-days with those required to complete a single generation of M. josephi (Bodenheimer & Neumark 1955) and E. hebraicus (Carmi 1990). Generations of the scale were counted from the first population peak in April and generations of the predator from the first population peak in March.
Geographic distribution of E. hebraicus and attraction of other potential predators of M. josephi
Traps baited with the sex pheromones of M. josephi were tested in other areas of the Palaeartic region where other Matsucoccus spp. had been previously recorded. At least five traps were used on each site, mostly during spring or early summer. Traps were distributed in the following countries: Japan (1, M. matsumurae ), China (2, M. matsumurae ), Jordan (4, M. josephi ), Lebanon (5, M. josephi ), Cyprus (4, M. josephi ), Turkey (3, M. josephi ), Greece (mainland 1, M. pini; Crete, 2 M. josephi ), Italy (2, M. feytaudi ), France (2, M. feytaudi ), Spain (3, Pinus halepensis stands with no record of Matsucoccus ), Portugal (4, M. feytaudi ) and England (1, M. pini). Numbers of study sites and Matsucoccus species occurring in each country are in brackets. The sticky bottoms with trapped insects were shipped to Israel and examined in our laboratory.
Population trends of M. josephi
Seven overlapping generations for each year were calculated for M. josephi. Results showed that the population density increased in March/April, mid October and early December, and remained low during the rest of the year. This general pattern was consistent in the different regions and for stands of different pines (Figure 1). The average number of males varied between locations, although modulation of the scale population, according to Fourier series, shows that the seasonal fluctuations are approximately the same among pine species in the various regions of Israel (Figure 2).
Although M. josephi is a multivoltine species and is active throughout the year, our results indicate two main periods of high reproduction, as demonstrated by peaks of male catch. The two waves of male flight are probably related to the period of cambial activity of P. halepensis and P. brutia ssp. brutia in spring (March/June) and autumn (September/October) (Liphschitz et al. 1984). Growth ceases in mid-summer and mid-winter. (Oppenheimer 1945; Liphschitz & Lev-Yadun 1986). Width growth activity of both pine taxa in Israel is similar (Liphschitz & Mendel 1989).
Figure 1 (left) Population fluctuations of males of M. josephi as determined by pheromone traps in three regions in Israel during 1993 to 1995 in stands of P. halepensis ((DELTA)) and P. brutia (o)
Figure 3 (right) Population fluctuations of adults of E. hebraicus as determined by pheromone traps in three regions in Israel during 1993 to 1995 in stands of P. halepensis ((DELTA)) and P. brutia (o)
Attraction of E. hebraicus to pheromone traps and population trends.
Adults alone were caught in traps baited with M. josephi sex pheromone traps. The behavior of the individual bugs approaching traps showed that most of the bugs aggregated on the stem, in the vicinity of them, and were caughty on the sticky bottom after landing on the external surface of the trap. As a rule, bugs entered by walking or jumping, but not by flying directly into the trap. In all sampling plots and seasons, the sex ratio of the trapped individuals was markedly male-biased, with males making up of 74 to 90% of the catch. Despite the high numbers of larvae that could be easily located on stems and crowns, these were never observed among the aggregating bugs on the stem near the trap, nor were they caught in the trap.
Our findings suggest that despite the similarity of feeding behavior and habitat of larvae and adults of E. hebraicus, only adults respond to the sex pheromone of M. josephi. The male-biased sex ratio may be explained by the lower flight capability of the females. The female mates soon after the emergence from the fifth larval instar. The development of the ovaries, as indicated by the increased volume of the abdomen,
Figure 2 Fourier analysis of population trends of M. josephi in seven regions in Israel (above)
Figure 4 Fourier analysis of population trends of E. hebraicus in seven regions in Israel (below)
occurs within a few days, with an apparent increase in body weight. No changes in body dimensions were observed for males (Carmi 1989). The oviposition period may extend between 1 and 3 months and corresponds to almost the entire life span of the female. Under controlled conditions, the life span of males is similar to that of the females (Mendel et al. 1995).
Population trends of E. hebraicus
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There are 8-9 annual generations of E. hebraicus at low altitudes and 6-7 generations at 700m or more above sea level in the Upper Galilee. Population trends of the predator slightly varied between regions but not between host plant species, despite the distinct differences in susceptibility to M. josephi of the examined pine species (see Figure 3). Modulation, according to Fourier series (Bigger 1973), showed that population density of E. hebraicus increased only in the first half of the summer (mid-May to mid-July) (Figure 4). Excluding the Upper Galilee, where the bug population remained relatively high in the second half of the summer, very few E. hebraicus were caught between August and March in all study areas.
The relationship between spring peak (March) of M. josephi (males/trap/day) and that of E. hebraicus (adults/trap/day) in early summer (May) seemed to be only partially related (r2= 0.40) (Figure 5). However, based on the mean annual catch, the ratio E. hebraicus /M. josephi was inversely related (r2=0.76) to the density of M. josephi (males/trap/day) (Figure 6).
The above findings indicate that the pheromone trap of M. josephi is a useful device for studying the population trends of its main predator E. hebraicus. Furthermore, the pheromone traps enabled us for the first time to study the seasonal history of the predator population, and to estimate the effect of geographical factors and pine species on its populations. The in situ study of populations of predatory bugs in general, and species of Elatophilus in particular, is limited due to the lack of reliable sampling procedures. The trap has a special advantage for a multivoltinuous species such as E. hebraicus. Other congeneric species have a smaller number of annual generations: Elatophilus nigricornis Zetherstedt has three generations in Italy (Covassi et al. 1991) and E. inimica Drake and Harris has only two generations in the Southeastern USA (Lussier 1965). It seems that the increase of the population of E. hebraicus is related to the concomitant increase of the prey population in spring, whereas the predator population density remains low during the second half of the summer and autumn, not responding to the population buildup of M. josephi. We assume that the bug population is adversely affected by the high temperatures during the summer at low altitudes, whereas lower temperatures at higher elevations account for the relatively high population in Upper Galilee. Based on the density relationship between prey and predator, it appears that the response of the bug population to the change, i.e. increase, in the scale population is in fact limited. Therefore E. hebraicus is unable to prevent the heavy damage caused by M. josephi.
Figure 5 Relationship between peak density of M. josephi males in March and peak density of E. hebraicus adults in May (above)
Figure 6 Relationship between mean annual density of M. josephi males and E. hebraicus /M. josephi ratio (below)
Geographic distribution of E. hebraicus
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The predator, E. hebraicus, was always caught together with its prey M. josephi. It was found in P. brutia ssp. brutia stands in western Turkey, Lebanon, and Cyprus and Crete. On P. halepensis, E. hebraicus was trapped only in Israel and Jordan, but not in mainland Greece or the West Mediterranean. Further, it was never caught outside the natural range of P. brutia and P. halepensis. The range of E. hebraicus most probably overlaps the natural distribution of M. josephi and its principal host P. brutia ssp. brutia. Therefore, it may be found also in pine forests on the offshore islands along the Anatolian coast, in Syria, northern Lebanon and in natural relicts of northern Iraq and eastern Turkey. Our findings suggest that E. hebraicus does not occur in southern Europe (mainland Greece, Spain, Southern France, Italy) or North Africa due to the absence of the scale. We assume that its occurrence in the P. halepensis forests in Israel and Jordan is the result of its introduction from Cyprus together with M. josephi (Mendel et al. 1994a) or of its natural spread from northern Lebanon.
Attraction of potential enemies of M. josephi to its sex pheromone
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Pheromone traps exposed in the area of distribution of M. josephi did not attract other predatory species other than E. hebraicus. Traps baited with the sex pheromones of M. josephi, M. feytaudi or M. matsumurae (Dunkelblum et al. 1996), exposed in various countries, outside the range of M. josephi did not attract any known or potential predators of Matsucoccus with the exception of Portugal. Traps, baited with these three pheromones, exposed in stands of P. pinaster in Portugal attracted significant numbers of two predators: Elatophilus crassicornis Reuter and Hemerobius stigma Stephens (Neuroptera, Hemerobiidae). We have recently observed that adult E. hebraicus are also attracted to the sex pheromones of M. feytaudi and M. matsumurae despite the fact that E. hebraicus is associated only with M. josephi (Dunkelblum et al. 1966).
The association of Hemerobius spp. with Matsucoccus (Bean & Godwin 1955; Siewniak 1976; Ming et al. 1983; Covassi et al. 1991) and Elatophilus (Mendel et al. 1991; Lattin & Stanton 1993) is well documented. Here, we have shown for the first time that these predators respond to kairomones in order to find their prey. Furthermore, it seems that the mode of attraction of E. hebraicus, E. crassicornis and H. stigma is the same, and that all three predators are strongly attracted to the sex pheromones of the three studied Matsucoccus spp. The attraction of actual and potential prey species to the sex pheromones of different Matsucoccus spp. may be based on chemical similarity of the pheromones, or on the fact that the kairomonal response of the predators evolved during speciation of the genus Matsucoccus (Dunkelblum et al. 1995, 1996).
The predators E. crassicornis and H. stigma are now under appraisal as a means to augment the enemy fauna of M. josephi in Israel. Since E. hebraicus is also attracted by the sex pheromone of the allopatric scales, M. feytaudi and M. matsumurae, it may, therefore, be considered as a candidate to improve the biological control in the pine forests injured by these pests. Future biological control projects will benefit from the use of traps baited with the sex pheromones of Matsucoccus spp. in order to indentify potential predators and introduce them into forests endangered by Matsucoccus pests.
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