Dispersal and Opposition Strategies in Chrysoperla Carnea

Progress in World's Neuropterology. Gepp J-, if. Aspöck & EL Hòìzet ed, 265 pp^ 1984, Graz. Dispersal and Opposition Strategies in Chrysoperla carnea By Peter DUELLI (Berkeley and Basel) Summary The flight and oviposition behavior of the holarctic common green lacewing Chrysoperla carnea (STEPHENS) (Neuroptera: Chrysopidae), has been investigated in alfalfa fields in the Califor- nia Central Valley and in laboratory experiments. In the first two nights after emergence the adult lacewings perform straight downwind disper: sal flights. Take-off behavior is elicited by the decrease in illumination at sunset. Since neither take- off nor the flight duration appear to be influenced by so-called "vegetative" stimuli such as food or mating partners, these pre-ovipository fligths are termed "obligatory migration flights". For the Central Valley an average initial flight distance of 40 km per night was estimated. In the third night after emergence the lacewings start to react anemochemotactically to food kairomons signalling the presence of honey dew. Males and females on their downwind flight are induced to land and approach the food source in a low, stepwise flight against the wind. Females mate in the third or fourth night after emergence and may deposit the first eggs on day 5. The consequence of the pre-ovipository migration flights is that very few females will deposit their eggs in the habitat in which they emerged. Consequently, virtually all eggs deposited in a particular field most probably stem from immigrant females. But even after mating and the onset of oviposition the dispersal activitiy continues. Reproductively active lacewings also take off every night after sunset and perform "appetitive downwind flights" until they enter the scent plume of a food source. Depending on the availability of food a single female may deposit up to 30 eggs per day before moving on after the next sunset. The whole lacewing population thus moves downwind in a continuous "rolling" movement. The strategy of obligatory nomadism and the spreading of the egg supply over a large area allows Chrysoperla carnea to thrive in cultivated environments, where the changes in habitat suit- ability are unpredictable and drastic. Introduction The common green lacewing, Chrysoperla carnea (STEPH.), is the most widespread and the most abundant lacewing species (KILLINGTON 1936, TJEDER 1966, ASPÖCK et. al. 1980). In cultivated areas of the holarctic region it is by far the most important lacewing with regard to predatory impact on pest arthropods (NEW 1975, TULISALO and TUOVINEN 1975). Lacewing adults are generally considered to be weak fliers. Still, some species such as C. carnea manage to disperse into and colonize the most isolated cultivated fields in the vast arid parts of the Western USA or even to establish populations on small islands in the Mediterra- nean Sea and the Atlantic Ocean (ASPÖCK et al. 1980, OHM pers. comm.). At least for C. carnea and some other species associated with cultivation, strongly developed dispersal strategies have to be assumed in order to explain the various records from isolated or inhospitable localities as well as of sometimes rather spectacular population densities of adults in places where no immature stages were found. The spatial and temporal aspects of the flight behavior and the dispersal activities of C. carnea were investigated in field experiments in the Central Valley of California and in laboratory experiments at the University of California, Berkeley. This article reviews the results of these experiments and integrates them with previous knowledge to present a review of our DUELLI P.: Dispersal and Opposition Strategies in Chrysoperla carnea 134 present understanding of the dispersal strategy of C. carnea. Moreover, in a chapter on the "continuous nomadism" of lacewings in a post-migratory stage, previously unpublished data are presented and discussed. Review In earlier publications on Chrysoperla carnea (STEPHENS) a number of different names were used for the same species in Europe {Chrysopa carnea STEPHENS; C. vulgaris SCHNEIDER) and North America (Chrysopa plorabunda FITCH. ;C. californica COQU.). Recently, the generic name had to be changed also due to splitting of the former genus Chry- sopa into various genera which previously had been considered to be subgenera (SÉMÉRIA 1977, CANARD and LAUDEHO 1978). A. Dispersal The dispersal properties of a population can be expressed as the sum of individual move- ments in the time span of one generation. The discrete units of this quantification are the vec- tors (directions and distances) of individual translocations from the place of hatching of a female larva to the places where the eggs of the next generation are deposited (DUELLI 1981a). Lacewing larvae are general predators on small soft bodied arthropods (refs. in BAL- DUF 1939, ICKERT 1968). Food location is tactile and food recognition seems to be based on contact chemo- and taste receptors (BÄNSCH 1964, ARZET 1973). When hungry, the larvae of C. carnea can be very mobile (BOND 1979) but compared to the effect of adult flight activities their movement is irrelevant with regard to the dispersal of a population. In an alfalfa field with an abundant supply of food the average total dispersal distance of the three larval instars was found to be less than 1 meter. In a field with only 1—2 aphids per stem the average dispersal distance was 1.7 meter (DUELLI 1981a). The pertinent questions therefore concern the endogenous and exogenous stimuli which govern the individual movement of females during their search for food, mating partners and oviposition sites. 1. Flight phenology Most lacewing species overwinter in the larval or pupal stage, whereas in C. carnea and a few other species the adults go into diapause in fall. Diapause induction is triggered by shor- tening or short day light cycles (refs. in TAUBER and TAUBER 1976, 1978) and is partly depending on ambient temperatures (HONEK 1973). The precise stimuli and physiological mechanisms of diapause induction are unknown. After diapause termination in December or January, the dormancy is prolonged into a quies- cence, until the rising temperatures in spring allow for mating and oviposition (TAUBER and TAUBER 1970). In California, C. carnea was caught in sticky traps all year round, even in December and January (DUELLI in prep), whereas in Central Europe flight activity ceases in November and resumes in March or April (HONEK 1977, BOWDEN1979, EGLIN1980). The number of generations per year can vary from 1—2 in Central Europe (e. g. ZELENY 1965, HONEK 1977) to 7 in Israel (NEUMARK 1952). Lacewing flight activity is mainly nocturnal (LEWIS and TAYLOR 1965). Catches on sticky traps in Central California (DUELLI 1980a) have shown a flight activity pattern similar to the one described for hemerobiids (BANKS 1952) : Flight activity begins shortly before sunset, reaches a maximum 1-2 hours after sunset and then decreases steadily towards sunrise. A tentative interpretation for such an activity diagram could be based on diel wind speed mea- surements (Fig. 1). The flight speed of lacewings (0.75 m/sec, see Fig. 4) is slow compared to the average wind speeds measured in the Central Valley. The number of lacewings caught in DUELLI P.: Dispersai and Ovipositìon Strategies in Chrysoperia carnea 135 km/h •o 0a) a (0 Ü O 0) O) T3 Iu (0 5- -5 12 12 Fig. 1 : Correlation between air movement and trap catches of Chrysoperia carnea in the California Cen- tral Valley. Wind profile based on a 20-day average from wind speed recordings of TAMAKI, SMITH and HAGEN (1961, impubi.). Flight activity diagram from DUELLI (1980a). Wind Food Fig. 2: Schematic illustration of the anemochemotactic approach of a C. carnea adult in an "appetitive stage" to an attractive food source. sticky traps depends strongly on the air flow through these traps because at higher wind speeds more air is filtered (DUELLI 1980b). Thus the slow winds in the early morning are at least partially responsible for the apparent decrease in flight activity. Lacewings seem to be reluctant to fly in full daylight. When disturbed, they only fly for a few meters to reach the nearest shelter. In early spring or late fall, when night temperatures are often too low for flight activity, lacewings can be observed to start flying in the late after- noon. In California the flight activity of C. carnea in late fall does not seem to end as an all-or- none response to changing day length as suggested by BOWDEN (1979) for lacewing populations in England. By restricting their flight to the nocturnal hours, lacewings escape the predatory activity of optically oriented diurnal hunters such as dragonflies, asilid flies and other predatory insects, as well as birds. To avoid bats at night, lacewings have depeloped an escape mecha- DUELLI P.: Dispersal and Opposition Strategies in Cbrysoperla carnea 136 nism similar to the one in many moths: upon hearing the sonar cry of a bat, lacewings perform an elaborate "drop dead" behavior (MILLER and OLESEN 1979). The tympanal organ which is sensitive to sounds in the range of 13 to 120 kHz (MILLER 1971) are located at the base of the forewings (MILLER 1970). 2. Food Location Only about half of all lacewing species investigated so far are predatory in the adult stage (HAGEN and TASS AN 1972). The adults of C. carnea feed on the honeydew of aphids and other homopteran insects, as well as on nectar and pollen (refs. in HAGEN et al. 1970, SHEL- DON and MAC LEOD 1971). An attractive substance in natural and artificial honeydew has been found to be indol acetaldehyde, a volatile breakdown product of the amino acid tryptophan (VAN EMDEN and HAGEN 1976). In the field, large numbers of adult C. carnea can be accumulated overnight by spraying an artificial food mixture of brewers yeast (containing tryptophan), sugar and water on the crop (HAGEN et al.

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