Aphytis Melinus Research Articles

Temperature-Dependent Size and Development of California Red Scale (Homoptera: Diaspididae) and Its Effect on Host Availability for the Ectoparasitoid, Aphytis melinus DeBach (Hymenoptera: Aphelinidae)

Developmental rates of individual stages of California red scale, Aonidiella aurantii (Mask.) and Aphytis melinus DeBach were determined at 17, 20, 25, 26.7, 30, and 33°C. A linear degree-day model estimated the scale's developmental time in the field more closely (4–7% of observed) than did either the polynomial (16–19%) or thermodynamic models (17–21%). The model overestimated male duration by 20%. Although actual duration of California red scale stages that are susceptible to parasitism by A. melinus decreased with increasing temperature, their durations relative to other stages increased. The size of non-growing stages (second molt and prepupa) decreased with increasing temperature. High temperatures also reduced the size of growing stages (second and third instars) and therefore affected the period during which scales were vulnerable to A. melinus . High temperatures also reduced the proportion of the scale population available to A. melinus for female progeny production. When developing scales were exposed to variable temperatures, average temperature during the developmental period predicted the size of second molt and prepupal stages. Based on developmental time, A. melinus has several generations to everyone of its host, thus its effectiveness as a biocontrol agent depends on a continuous supply of suitable scale stages to serve as hosts, especially for daughters.

Sublethal Effects of Pesticides on the Parasitoid Aphytis melinus (Hymenoptera: Aphelinidae)

Sublethal effects of insecticides on the parasitoid Aphytis melinus DeBach were investigated. Longevity, daily rate of progeny production per female, and size and sex ratio of offspring were measured for parasitoids exposed to rates near the LC50’s of carbaryl, chlorpyrifos, dimethoate, malathion, and methidathion. Survivors of the exposure to carbaryl exhibited no significant sublethal effects. Exposure to each of the organophosphorous materials reduced longevity by 73-85% and temporarily depressed progeny production. Chlorpyrifos also shifted the sex ratio of offspring toward more males. The strength and variability of sublethal effects found in this and other studies indicate that sublethal effects must be considered to evaluate accurately the selectivity of pesticides in favor of parasitoids.

Non-target effects of dimethoate and acephate against Eusieus tularensis Congdon and Aphytis melinus DeBach on lemons in California

Understanding the non-target effects on pesticides used in crop production systems is essential for the development of IPM programmes in those systems. Two organophosphates for control of citrus thrips in citrus were compared at commercial field rates in a lemon orchard for their effect against a predacious mite and a parasitic wasp, both of which have a significant role in coastal lemon IPM. Dimethoate and acephate greatly reduced populations of both beneficial insects, holding them to below detectable levels for 8 weeks post-treatment. Moreover, citrus red mite, a secondary pest, developed beyond the economic treatment threshold in both the dimethoate and acephate plots 10 weeks and 4 weeks post-treatment, respectively. In addition to predatory mite mortality, hormoligosis is suspected in both instances, and especially with acephate, of causing this secondary pest problem.

Residual Toxicity of Acaricides to Three Beneficial Species on California Citrus

Impacts of field-weathered residues of nine acaricides (amitraz, avermectin + oil, chlorobenzilate + oil, cyhexatin, dicofol, fenbutatin-oxide, oil, oxythioquinox, and propargite) on survival of selected beneficial arthropods of citrus ( Aphytis melinus DeBach, Cryptolaemus montrouzieri Mulsant, and Euseius stipulatus Athias-Henriot) were evaluated in two field trials. Concurrent with bioassays, foliar residue analyses were performed and were correlated with mortality. With the exception of moderate (causing amitraz > oxythioquinox > cyhexatin > chlorobenzilate + oil > propargite (least persistently toxic).

Foraging Behavior of Aphytis Melinus: Effects of Patch Density and Host Size

The influence of size and density of the California red scale (Aonidiella aurantii) on the behavior of the parasitoid Aphytis melinus was examined. Aphytis appears to employ a simple strategy of searching the same amount of time in patches of different host density, and as a result the same fraction of hosts was encountered at all host densities. Only the first few of the encountered hosts were actually parasitized, however, probably due to egg depletion. Different instars of the California red scale yielded Aphytis progeny of different size and sex ratios. Small hosts, such as the second instar, produced small Aphytis with a male—biased sex ratio. The third instar, a larger host, produced much larger Aphytis and a more balanced sex ratio. Perhaps because of its low quality, the second instar was parasitized at a lower rate than the third instar, in both the field and laboratory. Parasitism on second instars in the field was lower when third instars were abundant. Host size distribution may have significant effects on Aphytis size and sex ratio, with implications for biological control of the red scale.

On Assessing the Role of Spatially-Heterogeneous Density-Independent Host Mortality on the Stability of Host-Parasitoid Systems

In an already classical work, Beddington et al. (1978) analysed several cases of successful biological control and suggested that their common feature was a displacement to a new equilibrium with a strong depression of the host population (low q values, q = N*/K, where K is the equilibrium density before the introduction of the parasitoid and N* the one afterwards). Since then, one of the aims of theoretical studies on hostparasitoid dynamics was considered to be the identification of processes that lead to the stability of these systems at low q values (for a review see Hassell 1978). Spatial heterogeneity in the distribution of hosts and parasitoids has been singled out as the most important process leading to the stability of host-parasitoid systems at low values of q, either when it results in spatial density-dependent (Hassell and May 1973, 1974, Murdoch and Oaten 1975, Murdoch 1977) or inverse density-dependent mortality of the host (Hassell 1984). Parasitoid aggregation, which leads to spatially-heterogeneous but density-independent mortality, has also been identified as an important stabilizing mechanism (May 1978, Murdoch et al. 1984). This view has been challenged by Murdoch et al. (1984, 1985, Reeve and Murdoch 1985) on the grounds that neither stability nor sufficiently strong spatial differences in host mortality are common features of successful biological control. Murdoch et al. (1984, and Reeve and Murdoch 1985) devised two methods (one graphical, the other statistical) to measure the strength of spatially-heterogeneous, density-independent host mortality based on May's (1978) model. In this model, host and parasitoid survival rates are considered as density-independent and f, the fraction of hosts evading parasitism (see Appendix), is given by the zero term of the negative binomial distribution (f = (1 + a P,/k)-k, where a is the search efficiency, k determines the degree of clumping in parasitoid attacks and P, is the mean predator density at generation t). This model is stable if and only if k < 1. A similar conclusion is attained for realistically low values of q with models in which density dependent survival is represented by a logistic function (May et al. 1981). The methods proposed by Murdoch et al. allow determination of whether k is lower than 1 in natural host-parasitoid systems. They used these methods in two studies of systems formed by scale insects and introduced parasitoids. In the system formed by the olive scale (Parlatoria oleae Culvee) and two introduced parasitoids (Murdoch et al. 1984), the host density was reduced from over 200 cales per twig to one or two per 50 twigs, (q = 0.0001). Whilst Murdoch et al. (1984) suggested that the system is unstable, Huffaker et al. (1986), on the basis of a long-term study, argued in the opposite sense. The second system (the California red scale, Aonidiella aurantii, and its parasitoid Aphytis melinus) is said to be stable (Reeve and Murdoch 1985), and the data provided by the authors suggest a q value between 0.1 and 0.01. No clear evidence of density-dependent or inverse density-dependent mortality was found in either system. In both host-parasitoid interactions k is greater than 1, and so Murdoch et al. (1984, Reeve and Murdoch 1985) concluded that spatial heterogeneity does not play a key role in the stability of either system. Their conclusions are discussed in the following paragraphs. May (1978) proposed the negative binomial distribution to describe parasitoid aggregation on phenomenological grounds (it fits many data sets), but pointed out that it could be derived by supposing that the parasitoids are gamma distributed among spatially discrete patches and the searching of parasitoids within patches is totally random. The same conclusion was reached by Murdoch et al. (1984; Chesson and Murdoch 1986): If ut is the number of parasitoids in the vicinity of each host and s(u) the probability density function of ut, then the average or expected proportion of unparasitized hosts (f) is given by

An Invulnerable Age Class and Stability in Delay-Differential Parasitoid-Host Models

Reviewing some examples of biological control of insect pests, Murdoch et al. (1985) concluded that success was commonly characterized by unstable populations of pest and parasitoid or predator and, perhaps, by a tendency toward local extinction. The only exception appeared to be control by the parasitoid Aphytis melinus of red scale, a pest insect on citrus trees in coastal southern California. A classic example of biological control, this system appears to persist around a low, stable equilibrium density of pest and parasitoid. It is, of course, difficult to determine whether or not a population in the field is stable. The difference between the conditions of biological control in different scale insects, however, lends support to the idea that some pests may be controlled in a stable manner and others not. The olive scale in groves in northern California seems unstable in that all scales in twig samples, in trees sampled, and in whole groves can be parasitized, with the populations showing a tendency toward local extinction; in one grove, density fluctuated strongly and not apparently around an equilibrium (Murdoch et al. 1984). In southern California citrus groves, however, A. melinus appears unable to parasitize more than about 30% of the red scale in any small sample, and living scales are found in most samples. In addition, long-term data on scale populations (DeBach 1974) show only small fluctuations around the mean density. Reeve and Murdoch (1985) confirmed that both scale density and the parasitism rate are remarkably constant, and they later showed (Reeve and Murdoch 1986) that the per capita rate of change of the scale population is density-dependent. These observations do not establish rigorously that this system is stable, but at least the contrast with olive scale and other examples discussed by Murdoch et al. (1985) suggests that the maintenance of relative constancy and the avoidance of local extinction in red scale require an explanation. Two other differences between these two scale-insect systems may help explain the contrast in their dynamic behavior: the presence or absence of an invulnerable

Aggregation of parasitoids and density‐independence of parasitism in field populations of the wasp Aphytis melinus and its host, the red scale Aonidiella aurantii

ABSTRACT. Repeated counts were made of the number of adult Aphytis melinus (DeBach) wasps per fruit on Valencia oranges in an orchard over two successive periods. Resulting rates of parasitism per fruit were measured at the end of each period. For both periods, corresponding to high and low mean numbers of adult parasites, there was a significant positive regresssion of adult wasps per fruit on the number of available hosts per fruit. However, there was a high level of variability about the regression, and the overall aggregative response appears to be weak. For both periods, rates of parasitism per fruit were independent of host density per fruit and they showed a high level of variability at all densities. Similar patterns were found in another, commercial, orchard over a wide range of mean host densities. There was no evidence for aggregation of parasites or density dependence of parasitism at a patch size corresponding to the whole tree. Suggestions based on some host‐parasitoid models, that aggregations of parasite attack in areas of high host density are necessary for effective biological control, are rejected. However, the model of Hassell (1982), incorporating aggregation of parasites and limitations on the effectiveness of the parasite, seems to fit the data quite well.

Intraspecific Variation in Levels of Pesticide Resistance in Field Populations of a Parasitoid, Aphytis melinus (Hymenoptera: Aphelinidae): The Role of Past Selection Pressures

Thirteen populations of Aphytis melinus DeBach, a key biological control agent of California red scale, Aonidiella aurantii (Maskell), were collected from the citrus-growing regions of California. Each population’s history of exposure to insecticides was estimated by determining history of insecticide use at both local (in-grove) and regional (county-wide) geographical scales. Concentration/mortality regressions for five insecticides widely used in citrus were estimated for the populations. For each chemical, substantial variability existed in the responses of different populations. LC50’s were correlated with both in-grove and county-wide pesticide use histories; patterns of variability were best explained by results of a multiple regression analysis that combined the influences of these two histories. Roles of food limitation, migration, and host distribution in determining patterns and rates of evolution of pesticide resistance in arthropod biological control agents are discussed.

Biological Control by the Parasitoid Aphytis melinus, and Population Stability of the California Red Scale

(1) The California red scale, a citrus pest, is under successful biological control by its parasitoid Aphytis melinus. Scale abundance is relatively stable, in comparison to other insect populations, and the rate of change of total scale abundance is density dependent. (2) Using data from a lemon grove, we have examined the scale-Aphytis system for several potential stabilizing mechanisms. These were: (a) density-dependent parasitism by Aphytis, in which parasitism rates vary in response to changes in scale density through time, (b) density-dependent sex-ratios in Aphytis, where the proportion of female progeny is influenced by both scale and parasitoid density, and (c) a spatial refuge from parasitism. (3) No evidence was found for the first two mechanisms. Parasitism rates were influenced only by rainfall, while sex-ratio was not significantly affected by any of the variables examined. (4) Strong evidence exists for a refuge from parasitism, which may account for the apparent stability of the system.

Toxicity of Major Citrus Pesticides to Aphytis melinus (Hymenoptera: Aphelinidae) and Cryptolaemus montrouzieri (Coleoptera: Coccinellidae)

The 48-h residual toxicity of major pesticides used for control of citrus thrips, Scirtothrips citri (Moulton), and California red scale, Aonidiella aurantii (Maskell) , in California was evaluated for adults of Aphytis melinus DeBach and Cryptolaeumus montrouzieri Mulsant. Rates as high as 4-fold the highest recommended field rate revealed little effect of sabadilla to A. melinus or of sabadilla, dimethoate, formetanate, or chlorpyrifos to C. montrouzieri . Concentration-mortality regressions were quantified for the remainder of the materials; these showed toxicity (at LC50) of chlorpyrifos > carbaryl > dimethoate > acephate > parathion > formetanate > methidathion to A. melinus and carbaryl > acephate > parathion > methidathion to C. montrouzieri .

Technique for Evaluating Residual Toxicity of Pesticides to Motile Insects

A-general method useful in evaluating residual toxicity of pesticides to motile arthropod species while minimizing pesticide fumigation effects is described. Data obtained using the method are presented. Both the pesticide tolerance to acephate and longevity are compared for adults of the citrus thrips, Scirtothrips citri (Moulton); Aphytis melinus DeBach, an aphelinid parasitoid; and Cryptolaemus montrouzieri Mulsant, a coccinellid predator.

Residual Toxicity of Four Insecticides Used for Control of Citrus Thrips (Thysanoptera: Thripidae) on Three Beneficial Species in a Citrus Agroecosystem

Impact of field residues of four insecticides (acephate, dimethoate, formetanate hydrochloride and sabadilla) commonly used in California for control of citrus thrips ( Scirtothrips citri ) was evaluated for three species of natural enemies ( Aphytis melinus, Cryptolaemus montrouzieri , and Euseius stipulatus ) that are important biological control agents on citrus. Foliage samples were collected at regular intervals from pesticide-sprayed trees, and analyses for toxic extracuticular residues and bioassays for 48-h acute effects were performed. Of the three species tested, E. stipulatus appeared the most susceptible and C. montrouzieri the least susceptible. Acephate had the longest residual impact on A. melinus and C. montrouzieri; formetanate had the longest impact on E. stipulatus . Mortalities in the dimethoate treatment occurred for a shorter period of time than in the acephate treatment for all species. A. melinus mortalities in the formetanate treatment were similar to those in the dimethoate treatment Mortalities of C. montrouzieri in the formetanate treatment were very low. Sabadilla residues showed little toxicity to the species tested.

Competitive Exclusion of Aphytis lingnanensis by A. melinus: Potential Role of Host Size

Aphytis melinus, an ectoparasitoid of California red scale, Aonidiella aurantii, replaced A. lingnanensis, another ectoparasitoid, in certain California citrus areas during the 1960's following the former's introduction in 1956—1957. We show that, although both species attack a similar range of host sizes, A. melinus utilizes a smaller threshold scale size for the production of daughters. Size differences of scales within a citrus tree (largest scales occur on fruits, smallest on wood, and intermediate on leaves), seasonal variation in scale age structure, multivoltinism of the scale and parasitoid populations, seasonal availability of citrus fruits (the substrate that supports the largest scales), differences in the temperature tolerances of the two parasitoid species (A. lingnanensis is less tolerant), and differences in their vagility (A. melinus is more vagile), probably all favor A. melinus in competition with A. lingnanensis. A. lingnanensis does not realize its fecundity advantage in female progeny when these species compete because A. melinus preempts California red scale before it grows into the size range preferred by A. lingnanensis. Collection records through 1972 show that A. melinus continued to extend its geographical range coastally in southern California through 1972.

Flight Response of Aphytis melinus (Hymenoptera: Aphelinidae) and Scirtothrips citri (Thysanoptera: Thripidae) to Trap Color, Size, and Shape 1

Aphytis melinus DeBach responded significantly more to green and yellow traps than to white, blue, fluorescent yellow, clear, black, or red; Scirtothrlps citri (Moulton) responded significantly more to fluorescent yellow, than to any of the other colored polyvinyl rectangular traps. A. melinus did not distinguish between opaque and translucent colored rectangles nor between different trap shapes. S. citri responded significantly more to translucent yellow and green than to opaque yellow and green. S. citri also preferred triangular, elliptical, and rectangular shapes over circular and square ones. Smaller traps were significantly more efficient in trapping A. melinus than the 386.1-cm2 rectangular traps; however, there was no significant difference between S. citri catches on the basis of number per unit area.

Energy comparison in three cases of pesticide versus bio-control pest management

The deletrious effects of chemical pesticides on many plant and animal populations have been described. Rapidly increasing costs of convenient energy sources have increased cost-benefit ratios of chemical pesticides used. We compared biological control techniques versus chemical pesticides for inputs of human labor, machinery, and fuel per hectare at each pertinent step of the production procedure. Three pest-crop systems were selected and all applicable factors converted into kilocalories of energy required. These were: (1) citrus-red scale ( Aonidiella aurantii), using oil as a chemical and the predator wasp, Aphytis melinus as a biological control; (2) greenhouse cucumbers-white fly ( Trialeurodes vaporariorum), using Orthene or the wasp, Encarsia formosa; (3) tomato-hornworm ( Manduca quinquemaculata) using Sevin or the bacterium, Bacillus thurengiensis. For citrus-red scale, the pesticide required 5 248 405 kcal, the use of A. melinus 100 793 kcal; for cucumber-white fly the pesticide required 29 300 kcal, E. formosa 154 500 kcal; for tomato-hornworm the pesticide 26 693, B. thurengiensis 207 240 kcal.

Host selection and egg allocation behaviour by Aphytis melinus and A.lingnanensis: comparison of two facultatively gregarious parasitoids

Abstract. 1. The ovipositional and egg allocation behaviour of individual females of Aphytis melinus DeBach and A.lingnunensis Compere were compared.2. Both Aphytis species exhibit the same behavioural sequence during oviposition.3. Aphytis melinus laid most of its female eggs on the dorsum of a scale‐insect beneath its cover, and most of its male eggs under the scale‐insect's body. Aphytis lingnanensis also oviposited both dorsally and ventrally on scale‐insect hosts, but female and male progeny arose with equal frequency from eggs laid in both locations.4. Both A.melinus and A. lingnanensis are facultatively gregarious parasitoids. The degree of gregariousness depends on host size, i.e. the larger the host, the more the Iikelihood that several eggs will be deposited at each visit by the parasitoid.5. When two eggs were laid during the same host visit, both A.melinus and A.lingnanensis laid one female and one male egg more often than would be expected under an assumption of random allocation of sexes.6. Because A.melinus successfulIy utilize smaller hosts than A.lingnanensis to produce progeny, these parasitoids should not be considered ecological homo‐logues, as suggested by DeBach &amp; Sundby (1963).

Sensitivity of Selected Nontarget Insects 1 to the Carrier of Dipel 4L® in the Laboratory 2

A mixture of water and the carrier of Dipel 4L® (3:1) was applied to selected insect predators and a parasite in a controlled laboratory environment. Corrected mortality from the 9.4:liter/ha application never exceeded 2.1% for any species. The 18.7-liter/ha rate resulted in a statistically significant increase in mortality for Chrysopa carnea Stephens and Hippodamla convergens Guerin and Meneville adults 3 and 7 days after treatment, but not for C. carnea larvae or Aphytis melinus De Bach adults. Corrected mortality for the higher application rate never exceeded 13.4% for any species.

Effects of constant and variable temperature extremes on sex ratio and progeny production by Aphytis melinus and A.lingnanensis (Hymenoptera: Aphelinidae)

Abstract. 1. Aphytis melinus DeBach and A.lingnanensis Compere were exposed to temperature extremes during development, mating, preoviposition, and opposition to assess the effect of these exposures on the progeny per female and F1 sex ratio.2. Both species showed a significant reduction in the expected proportion of F1 females when the male parent was exposed to 32°C during its development.3. The proportion of females was further reduced when mating and/or oviposition occurred at 32°C and these effects on sex ratio appeared cumulative.4. Preovipositional exposure of the females of both species to 38°C, 2°C or ‐2°C for 1 5, 4 or 7 h in most cases significantly reduced the expected proportion of females.5. In general our results showed that the sex ratios of A.melinus was less severely affected by these exposures than were those of A.lingnanensis.6. It is suggested that this difference may be one factor explaining the ability of A.melinus to exclude A.lingnanensis from the climatically more extreme areas of southern California.

Tolerance of Aphytis melinus DeBach (Hymenoptera: Aphelinidae) to 20 orchard chemical treatments in relation to integrated control of red scale, Aonidiella aurantii (Maskell) (Homoptera: Diaspididae)

The toxicity of 20 orchard chemicals at field rates was determined for all stages of Aphytis melinus in the laboratory. The insecticides Bacillus thuringiensis + surfactant, endosulfan, pirimicarb: the miticide cyclosulfyne : the herbicides and plant growth regulators dimazide, diuron, gibberellic acid: the fungicide copper oxychloride: the nutrient mixture zinc oxide, manganese sulphate and urea : and the surfactant nonyl phenol ethoxylate (9 Eto) appear safe for use in citrus groves containing A. melinus. The herbicide dalapon + surfactant and the fungicide Bordeaux mixture were moderately toxic to immatures, and the herbicide bromacil was moderately toxic to adults. However, it is considered that careful application of these treatments is safe. The organophosphorus insecticides azinphos-methyl, maldison and parathion; the insect growth regulator hydroprene; and the miticide cyhexatin were highly toxic to either juvenile or adult stages. White oil, recommended for integration with A. melinus, was highly toxic to immature stages. This anomalous result is discussed in terms of spray distribution and integrated control of red scale on citrus