Biology and Life Tables of Galendromus helveolus (Acari: Phytoseiidae) on Florida Citrus

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1 Biology and Life Tables of Galendromus helveolus (Acari: Phytoseiidae) on Florida Citrus SARA CACERESl ANDCARL G CHILDERS University of Florida, Institute of Food and Agricultural Sciences, Citrus Research and Education Center, Lake Alfred, Florida Environ.Entomol.20(1): (1991) ABSTRACT The biology of Galendromus helveolus (Chant) (Acari: Phytoseiidae) was studied using Eotetranychus sexmaculatus (Riley) (Acari: Tetranychidae) as the food source. Larvae developed to protonymphs without feeding. The total developmental times of G. helveolus were 12.36, 9.66, 5.63, 4.61, and 4.84 d at 18, 20, 25, 30, and 32"C, respectively, at 76% RH. A higher percentage of eggs (32%) did not hatch and higher larval mortality (37%) occurred at 32"C. The intrinsic rates of natural increase (rm) at 16, 20, 25, 30, and 32"C were 0.100,0.153,0.286,0.327, and 0.144, respectively. The optimal temperature range for G. helveolus was considered to be between 25 and 3O"C. The sex ratio of G. helveo/us was strongly female biased (females/total = 0.82). G. helveolus was able to survive and reproduce on Panonychus citn (McGregor), Eutetranychus banksi (McGregor), and Tetranychus urticae Koch (Acari: Tetranychidae) with a daily rate of oviposition of 2.21, 1.96, and 2.25, respectively, compared with 2.37 when fed only Eotetranychus sexmaculatus. Females of G. helveolus were observed to feed and survive for 10 d on Phyllocoptruta o/eivora (Ashmead) (Acari: Eriophyidae), but no eggs were laid. In addition, G. he/veo/us protonymphs and deutonymphs were observed feeding on P. oleivora. A low percentage of the predator eggs (8%) was able to develop to the adult stage on P. oleivora compared with 82% when provided Eotetranychus sexmaculatus and 70-80% when provided Panonychus citn, Eutetranychus banksi, or T. urticae. KEY WORDS Arachnida, Galendromus helveolus, spider mites, citrus rust mite MOST OF THE PHYTOSEIIDMITESof Florida were described by Muma (1955, 1961, 1962, 1963, 1965), Muma & Denmark (1968, 1969, 1970), Denmark (1965, 1977, 1982), De Leon (1959, 1960, 1962), Chant et al. (1959), and Chant (1965). These authors presented mainly taxonomic information and suggested that additional biological research was needed (Muma & Denmark 1970). It is impossible to evaluate the biological control potential of a predator without knowing its feeding habits, life cycle, and general biology (Muma 1969). Determining such parameters as reproductive capacity, searching abilities, and rates of development should lead to a better understanding of a predator's po.. tential and the possibility of predicting its ability to suppress mite populations below economic levels (Helle & van de Vrie 1974). Galendromus helveolus (Chant) has been collected in Florida and Central America (Muma'1955, Chant & Baker 1965). This species was originally described as Typhlodromus floridanus Muma. (Muma 1955). A series of scientific name changes occurred (Chant 1959; Muma 1961, 1963; Chant & Baker 1965) with the name G. helveolus (Chant) designated by Denmark (1977). Galendromus helveolus is one of the most promising phytoseiids found on Florida citrus. Muma I InstitutoNacionalde TecnologiaAgropecuaria(INTA)e.e. 5 (3432) BellaVista,Corrientes(Argentina). (1970), Sandness & McMurtry (1970), and Tanigoshi & McMurtry (1977) recognized the potential of G. helveolus as a predator of tetranychid mites on citrus and on the avocado brown mite, Oligonychus punicae (Hirst). G. helveolus was capable of reducing infestations of Eotetranychus sexmaculatus (Riley) but was probably ineffective in the natural control of either Eutetranychus banksi (McGregor) or Panonychus citri (McGregor), because it appeared to require dense localized colonies of prey for population development (Murria 1970). Life table studies were undertaken to determine the developmental biology and behavior of G. helveolus at different temperatures when fed E. sexmaculatus. Survivability and reproduction of G. helveolus using P. citri, E. banksi, Tetranychus urticae and Phyllocoptruta prey were evaluated. Materials oleivora (Ashmead) as and Methods Mite Cultures. G. helveolus stock cultures were maintained using all stages of E. sexmaculatus as the food source. Both species of mites were collected from a grapefruit grove located in Fort Ogden, DeSoto County, Fla., in May 1988 and The mite cultures were established on the underside of excised leaves of Citrus sinensis (L.) Osbeck using a modified leaf arena technique X(91( $02.00(O 1991 Entomological Societyof America

2 February 1991 CACERES& CHILDERS:BIOLOGYANDLIFE TABLESOF G. helveolus 225 (Abou-Setta & Childers 1987a); they were maintained at 25 C. This method permitted the use of an entire leaf in a 9-cm diameter plastic Petri dish with adequate air exchange that prevented relative humidity from reaching saturation. The units containing the spider mite and predator were maintained in an environmental chamber at 25 C under dark conditions. The arenas were placed within closed plastic boxes (Rubbermaid; 32 by 22 by 11 cm; ~7.7-liter volume) and each contained ~ 1.7 liter of saturated salt solution of NaCI that provided 76% RH at a temperature range of 15 to 350C (Winston & Bates 1960). Each box held seven dishes (28 arenas). Relative humidity within the plastic boxes was measured with a thermocouple probe for temperature and humidity (Digi-Sense, Cole-Parmer Instrument, Chicago, Ill.). Influence of Temperature on Development. Mature, but not senescing, medium-sized leaves were divided into four parts using a barrier of Canada Balsam and Castor bean oil (ratio, 1.5:1) (Abou-Setta & Childers 1987b). Each part was infested with E. sexmaculatus. The predator study was begun after the colonies were established in all arenas. Eggs of the predator were collected on folded pieces of black paper (~1 cm 2 ) from the stock culture of G. helveolus. The black papers with the attached eggs of G. helveolus were then placed on clean leaf arenas at 25 C. Egg hatch and subsequent development were recorded every 8 h. Six newly hatched eggs were placed individually on (a) circular black paper (9-cm diameter divided into six parts using a mixture of Canada balsam and castor bean oil, (b) clean leaves divided into six parts with the same method, and (c) divided leaves infested with all stages of E. sexmaculatus to determine if the larval stage feeds. They were maintained at 25 C and observed daily. This experiment was replicated five times. The developmental stages of individuals were observed daily at 0600,1400, and 2200 hours. Each developmental stage change that occurred before observation was recorded at half the time interval between observations. The presence of an exuvium was used as the criterion for successful molting to the next developmental stage. The characteristics observed were incubation period of the egg stage, duration of protonymphal and deutonymphal stages for males and females, mating, and preoviposition periods during the adult stage. Life Tables. Eotetranychus sexmacuiatus was used as prey for G. helveolus to develop life tables at 20, 25, 30, and 32 C at 76% RH. Newly mated females of G. helveolus were confined in groups of five per arena with all stages of the prey. The number of arenas varied per temperature as follows: 9 at 16 C, 8 at 20 C, 10 at 25 C, and 5 at both 30 and 32 C. Each group of females was observed daily for egg deposition. Eggs were collected daily from one arena per temperature and reared to the adult stage. Sex ratios of the progeny were then determined by visual observation. Egg collection was stopped after 100 eggs were obtained. Life table parameters were calculated using the Life 48 computer program (Abou- Setta et al. 1986). Food Range. Other mite species, including P. citri, E. banksi, T. urticae, and P. oleivora, were evaluated to determine the ability of G. helveolus to feed and develop on them as the sole food source. Panonychus citri was reared using a modified leaf arena technique (Abou-Setta & Childers 1987a). Eutetranychus banksi and P. oleivora were supplied daily from field collections. Tetranychus urticae was maintained in the greenhouse on 'Henderson Bush'lima beans and supplied daily. Newly mated G. helveolus females were separated from the stock culture and provided all stages of one of the phytophagous mite species. The average daily oviposition rate for the first 12 d was calculated for females on each food source. Results and Discussion Life Cycle and Behavior. Galendromus helveoius had the four immature developmental stages characteristic of phytoseiids: egg, larva, protonymph, and deutonymph. The eggs were translucent and colorless when female G. helveolus fed upon E. sexmaculatus, light orange when they fed upon E. banksi, and pink when they fed upon P. citri. Variation in the color of eggs of Phytoseius macropilis (Banks) also was observed to depend upon the kind of food consumed by the female (Prasad 1967). Recently emerged adults were transparent and off-white, light tan when they fed on E. sexmaculatus, dark tan when they fed on T. urticae and E. banksi, and bright red when they fed on P. citn. Larvae moved slowly and rested most of the time. Feeding was not observed during this stage, and 100% (n = 30) were able to molt to the protonymph on clean paper surface, clean leaves, or when provided various stages of E. sexmaculatus. McMurtry et al. (1970) suggested that it may be advantageous if the larva did not have to find food, assuming that it had lower searching ability than the protonymph. Protonymphs moved faster than larvae and required food to develop to the deutonymph stage. Galendromus helveolus protonymphs died after h when prey was not provided. The ability to molt to the deutonymph on leaves alone was reported for Amblyseius ruhini Swirski & Amitai (Swirski et al. 1967). Females deposited their eggs in clusters. It was common to observe groups of four or five females around a cluster of as many as 40 eggs. The females moved only when searching for food or when disturbed. The eggs were laid among the webbing of the prey on the leaf surface, near the midrib of the leaf or other protected areas with an abundance of prey.

3 226 ENVIRONMENTALENTOMOLOGY Vol. 20, no. 1 Table I. Duration (in days) of different stages of G. helveolus at different temperatures 22 M Temp,"C n Min. Max. Mean SO n Min. Max. Mean SO Egg Larva Protonymph Oeutonymph Influence of Temperature on Developmental Times. The duration of development of each stage decreased as temperature increased except at 32 C, where a slight increase in developmental time was observed (Table 1). Total developmental times were slightly shorter in males at 18, 20, and 25 C, respectively, and similar for both sexes at 30 and 32 C (Table 2). Previous reports of developmental times for G. helveolus fed E. sexmaculatus were 7.8 d for females and 6.5 d for males at 26.7 C (Muma 1970). Tanigoshi & McMurtry (1977) re.. ported 8.35 d for females and 8.40 d for males fed Oligonychus punicae (Hirst) between 22 and 26 C In this study, the egg stage required 35.6% of the total time to develop, and the larval stage re qui red 13.1% of the total. The protonymph and deutonymph stages had similar durations. This time distribution of life stages corresponded to other species of Phytoseiidae (Sabelis 1985). A high percentage of G. helveolus eggs (32%) did not hatch at 32 C and high mortality (37%) occurred during the larval stage and in molting to protonymphs. Sex Ratio. The sex ratio of G. helveolus was strongly biased for females (females/total = 0.82) over the first 12 d of oviposition at 25 C when fed E. sexmaculatus. This was expected in a specialized predator that uses a high-density prey such as E. sexmaculatus. Tanigoshi & McMurtry (1977) reported a 0.64 sex ratio for G. helveolus when fed O. punicae. A tertiary sex ratio of 0.80 was reported for G. helveolus by Muma (1964) who interpreted this bias as evolution toward parthenogenesis or a single fertilization requirement. The sex ratio was 0.62 on the first day for five females. Higher percentages of males produced during the first days have been reported for other phytoseiids: Typhlodromus caudiglans Schuster (Putman 1962), Amblyseius andersoni (Chant) (Amano & Chant 1978), A. bibens Blommers (Schulten et al. 1978), and Euseius scutalis (Athias- Henriot) (Bonfour & McMurtry 1987). A son-first pattern favors the mother's fitness by ensuring fertilization of her daughters at the earliest possible time (Sabelis 1985). Reproductive Potential: Life Table Parameters. The effect of temperature on life table parameters is shown in Table 3. The preoviposition period decreased with increased temperature except at 32 C, while the oviposition period decreased with increasing temperature. The age specific fecundity and survival are shown in Fig. 1. Table 2. Total developmental time (in days) of G. helveolus at different temperatures Temp, 2 d "C n Mean SD n Mean SD

4 February 1991 CACERES& CHILDERS:BIOLOGYANDLIFE TABLESOF G. helveolus 227 Table 3. Life table parameters of G. helveolw at 76% RH at different temperatures Parameter Temperature, "C Developmental time (days) Preoviposition period (days) Mean total fecundity (eggs/9) Net reproductive rate (Ro) Generation time (days) (T) Intrinsic rate of increase (r m ) Finite rate of increase (exp r m ) Sex ratio (females/total) The intrinsic rate of natural increase, r m (as calculated using the method of Abou-Setta et al. (1986)) obtained at 30"C was compared with an r m value of at 25 C. The net reproductive rate (R.,) was higher at 25 C (28.06) because of the longer ovipositional period; 300C provided a higher r m regardless of the lower Ro. The shorter developmental time at 30 C was responsible for this effect. The earlier an egg was laid in the life, the greater was the contribution of that particular egg to the value of r m (Birch 1948). The highest r m was obtained at 30 C despite decreases in survival and reproduction later in life (Fig. 1). Similar r m values were reported for A. deleoni Muma & Denmark (Saito & Mori 1981) and A. chilenensis (Dosse) at 25 C (Ma & Laing 1973). The r m value for G. helveolus with O. punicae as the food source was between 22 and 260C (Tanigoshi & McMurtry 1977). ~ S _ C '-" >-. :1\.. 30 C < 1.4.I~ ~, :l '-" < :l C < r.ol --.\ ~ :::!! r.ol ::::.v ':\ r.: > 0.5 :; ';;) \\. D:: ::J rn 1.0 r.ol..\ C r.:l \ :a.....:l 0.5.\-...~:... \. < :::!!,.....:::.:..::.:::.:-..~ r.ol 0.0 r.: l6 C D AGE IN DAYS Fig. 1. Age-specificfecundity and survival of G. helveolus at constant temperatures (m".~; I" -). Feeding Behavior. Females and nymphs of G. helveolus were observed to feed on their own eggs in the absence of food. The same was observed for this species by Tanigoshi & McMurtry (1977). Cannibalism may provide a means for the stronger individuals to survive, when no other food sources are available. If neither alternative food nor water sources are available, cannibalism can be expected (Sabelis 1981). Galendromus helveolus protonymphs and deutonymphs preferred eggs and protonymphs of E. sexmaculatus when all prey stages were available. Adult G. helveolus fed on all stages of E. sexmaculatus, including adults, but they showed a preference for nymphs. The same was observed for G. helveolus feeding on O. punicae (Tanigoshi & McMurtry 1977). The webbing produced by E. sexmaculatus did not affect G. helveolus. The characteristic of long dorsal and lateral setae seems to be associated with "highly effective" species specialized in tetranychid predation and capable of developing in dense webbing of prey species (McMurtry 1982). Sabelis (1981) suggested that these setae serve as a wedge in the sticky web during forward locomotion of P. persimilis and A. bioons. Galendromus helveolus was able to survive and reproduce on P. citri, E. banksi, and T. urticae. Considering that the profitability of each prey type would be expressed in terms of reproduction success, the numbers of eggs laid per G. helveolus female at 25 C and 76% RH over 12 d were obtained (Table 4). The oviposition rates for G. helveolus fed upon P. citrt and E. banksi were 2.21 Table 4. Average daily rate of oviposition of G. helveolub on different phytophagous mites of citrus at 25"C and 76% RH during the first 12 d of oviposition Prey Eotetranychus sexmaculatus Panonychus cit" Eutetranychus banksl Tetranychus urtlcae Phyllocoptruta oleloof'a Oviposition rate MeanO 2.37a 2.21a a O.OOb a Means not having the same letter are significantly different (P ~ 0.05; ANOVA). SD

5 228 ENVIRONMENTAL ENTOMOLOGY Vol. 20, no. 1 and 1.96, respectively. They were not significantly different. Eggs of G. helveolus reached the adult stage in 6,8, 7, and 8 d when fed E. sexmaculatus, P. cieri, T. urticae, and E. banksi, respectively. Most G. helveolus eggs reached the adult stage on the spider mites tested (70-80% on P. citri, E. banksi, and T. urticae; and 82% on E. sexmaculatus). Feeding by G. helveolus was observed on eggs and immatures of all four tetranychid mite species. Nine of 13 newly mated G. helveolus collected from the stock culture being fed E. sexmaculatus were able to survive for 10 d when only P. oleivora was offered as food. No eggs were oviposited, except for 2-3 produced the first day as a result of previous feeding on E. sexmaculatus. Moraes & McMurtry (1981) described the following types of behavior depending on the level of hunger when a predator touched a prey: avoidance with evasive movement, indifference, casual Qhase, and emphatic chase. They observed the first three types of behavioral responses by A. citrifolius (Denmark & Muma) soon after the predator had fed. Most of the time, G. helveolus females showed indifference when they touched P. oleivora, including walking on the rust mites and then continuing to search. Finally, they fed on P. oleivora when they were unable to locate suitable prey. In addition, G. helveolus protonymphs and deutonymphs were observed feeding on P. oleivora. Only 6 of 75 eggs (8%) of G. helveolus reached the adult stage when provided with P. oleivora as the only food source. Mortality occurred during the protonymphal stage and when deutonymphs were molting to adults. Although G. helveolus immatures and ovipositing females were observed to feed on their own eggs in the absence of food, the 2-3 eggs produced from previous food sources were not consumed by the female when P. oleivora was present. According to the terminology of Overmeer (1985), the term alternative food should not be applied to P. oleivora, because the predator was not able to survive and reproduce. The term "additional" or "supplementary" food was suggested for this type of response. It was clear that G. helveolus will not feed on P. oleivora when tetranychid prey are available. This supplemental food may have value as a means of survival when suitable prey are not available. Its potential value should not be ignored, because recently emerged and newly mated females of G. helveolus were able to survive only 24 and 48 h, respectively, in the complete absence of food at 25 C. This predator species has been collected in every month of the year (Muma & Denmark 1970). This could be due to its ability to survive and reproduce on P. citri and E. banksi and to survive on P. oleivora. Acknowledgment We thank Mohamed Abou-Setta, University of Florida, and James A. 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