1.  Introduction

Armored scale insects (Hemiptera: Diaspididae) are one of the most important groups of agriculture pests. Many species are highly destructive to fruit trees and ornamental plants [1]. The latania scale (LS), Hemiberlesia lataniae(Signoret) is one of the most serious diaspidid pests worldwide [2-5].

Natural enemies of diaspidid species include pathogens, predators, and parasitoids. Pathogens and predators are rather non-specific and often depend on high prey densities. Parasitoid is able to remarkably reduce population levels of scale insects [5]. Parasitoids as abio-control agent provide an opportunity for significant reduction in LS populations [6,7]. The parasitoid, Aphytis diaspidisHoward (Aphelinidae: Hymenoptera) is among the main mortality factor regulating the populations of several diaspidid species including LS [7-9]. The signiphorid parasitoid, Signiphora sp. was recorded for the first time attacking latania scale in Mansoura region [10]. The Signiphora faxGirault was also recorded for the first time on ornamental plants in the Alexandria district, Egypt [11], S. flavellaand S. perpaucaon LS in Queensland [12], S. merceti(Malenotti) on Hemiberlesia rapax(Comstock) in Italia [13] and New Zealand [14] and S. flavopalliata reared from Aspidiotus neriiBouch., Hemiberlesia latastei(Ckll.), Chrysomphalus aonidum(L.), Aonidiella aurantii(Mask.) and Coccus hesperidumL in Argentine Republic [15]. Generally, the most of Signiphora species are known to be specific on some hemipterous pests especially Bemisia sp. and there is lack in evaluating the potential role of this species against diaspidid scale insects.

There are some searching characteristics that should be evaluated in in the biocontrol agent prior to its use in the biological control programs. They include characteristics which will tend to reduce the average population density of the host, such as tendency to aggregate where host density is high (density-dependent response) [16,17]. Most of the previous research have been evaluated the response of natural enemies to the populations of their hosts under lab conditions, however few studies assessed their behavioural responses under natural conditions. Studying the relation between the parasitoid and its insect hosts during the different season of the year from field samples will help in determining the suitable time for either control procedures or augmentative releases. Therefore, the study was conducted to determine the reaction response of latania scale density for two parasitoids under field conditions.

2.  Materials and Methods

Density - dependent response

The density- dependent response was determined for A. diaspidisand Signiphora sp. in response to H. lataniaeon fig, guava, and loquat trees during 2017. Five trees of each host plant infested with parasitoid’s host were marked and numbered at the experimental farm belonging to the farm of Mansoura University. In each sample, ten heavily infested leaves were collected from each tree of fig and guavas and five twigs (20 cm long) from loquat trees. Samples were collected every two weeks during the periods of December-February (winter generation), March- May (spring generation) and June–August (summer generation) of 2017 season. The numbers of 2nd instar and adult stages of the insect host per each tree and sample were recorded under laboratory conditions using binocular microscope. Then, the infested samples were kept in Petri-dishes or transparent containers until emergence of parasitoids. These parasitoids were separated, identified, and counted. The efficiency of parasitoid was estimated as k-value (killing power) according to [18] as follow:

               N

 K = log_e ----------

              S

where, N is the initial number of hosts, and S is the number of unparasitized hosts. To determine the relationship between the logarithm of the host density and the efficiency of parasitoid (K-value), linear regression analysis was performed using Excel program.

3.  Results

3.1.  Density–dependent response

3.1.1.   A-Winter generation

The density-dependent response was determined for the parasitoids, A. diaspidisand Signiphora sp. on different host plants by plotting the values of killing power (K) for each parasitoid against the logarithm of host density (log p). The regression analysis indicated that the k-values of A. diaspidisand Signiphora sp. were relatively more correlated with the host density on loquat trees during winter season (Table 1). Both parasitoid species showed a tendency to aggregate where host density is high. The k-values of the parasitoids, Signiphora sp. were more correlated with the density of H. lataniaethan A. diaspidisduring winter generation. In opposite, the k-values for A. diaspidiswere negatively correlated with the host density of H. lataniaeon fig samples. The k-values of both parasitoid species positively correlated with the density of H. lataniaeon loquat samples (Table 1). The values of Killing for both parasitoids in relation to the host density on guava, fig, and loquat during spring generation are also presented in Figures 1,2,3.

3.1.2.  B- Spring generation

The parasitoid A. diaspidisexhibited a positive density-dependent response to the density of H. lataniaeon loquat, whereas it disappeared on guava during spring generation as well as on fig due to leaves fall season. On the other hand, the Signiphora sp. exhibited strong responses to the density of H. lataniaefrom guava and loquat samples, with determination coefficient was the highest for the parasitoid from guava samples (Table 2). The values of Killing for both parasitoids in relation to the host density during spring generation are also presented in Figures 4,5.

3.1.3.   Summer generation

The parasitoid response to its host varied on host plants. The parasitoid, A. diaspidisexhibited a positive density-dependent response for H. lataniaeon loquat and fig trees with determination coefficients of 0.7 and 0.6, respectively, whereas this response was negative on guava trees with weak determination coefficient. The parasitoid Signiphora sp. showed a density-dependent response to density of H. lataniaeon fig samples with a determination coefficient of 0.81, an inverse density-dependent response on guava with a determination coefficient of 0.71, and a weak response on loquat (Table 3). The values of Killing for both parasitoids in relation to the host density during spring generation on guava, loquat, and fig are presented in Figures 6,7,8.

 From the previous results, it could be concluded that, the parasitoid A. diaspidis showed a positive density- dependent response to host density on loquat during, winter, spring, and summer generations. While, the parasitoid, Signiphora sp. showed differences in his response on the tested host plant species. It showed the highest density dependent response on loquat, guava, and fig during winter, spring, and summer, respectively.

4.      Discussion

The response of parasitoids to H. lataniae populations varied from season to season, from plant to plant, and from species to species. Unfortunately, the scientific literature for Signiphoraspp. is very rare. The ectoparasitoid A. diaspidis responded inversely to H. lataniae populations in guava samples during both winter and summer; however, these relations were not strong enough. In opposite, this response was positive to host density on loquat samples in all tested seasons with higher R² during spring and summer seasons [19]. Reported a type II response for A. diaspidis to H. lataniae densities. For a type II response, there should be a decline in the proportion of host parasitized as the density increases, so that the linear term should be negative. Although the parasitoid exhibited an inverse parasitism to H. lataniae densities under laboratory conditions, this relation could be changed according to several factors: one of them is host plant [19-22] and inter specific competition among parasitoid species on available stages. Parasitoids show different functional responses to host on different host plants. This is related to host size, to differences in defensive and other behaviors of the host, and to effects on searching time of parasitoids [23]. Another factor is the weather factors [e.g., 24-27]. This could be confirmed by the response of Signiphora sp. to H. lataniae populations, in which Signiphora sp. responded positively to populations of its host on guava samples during winter and spring and negatively during summer. The coefficient of determination represents this relation more appropriately during spring and summer. Similarity, both parasitoid species responded negatively to host populations on fig leaves during winter, and positively during summer with a coefficient of determination seems to be high. The same relation has been obtained from loquat samples, but this relation was weak during summer season. The variation in parasitoid responses could be attributed to several reasons, among them the effects of interspecific competition and host plant species. Almost, both parasitoid species showed a tendency to aggregate where host density is highest on loquat.

5.      Conclusion

Parasitoids show different behavioural responses to host scale on different host plants. The hardness of scale cover, host size, plant cues might be among the main reasons for such variations. Both parasitoid species have a complementary effect of its host. In other words, the response of each parasitoid species to its host and the alternation in this response from season to season, and from host to host could confirm this synergistic effect in regulating the host population.

Figure 1: The relation between k-values of the parasitoids, A. diaspidis and Signiphora sp., and the logarithm of the insect host, H. lataniae density on guava trees during winter generation..

Figure 2: The relation between k-values of two parasitoids, A. diaspidis and Signiphora sp., and the logarithm of the insect host, H. lataniae density on fig trees during winter generation.

Figure 3: The relation between k-values of two parasitoids, A. diaspidis and Signiphora sp., and the logarithm of H.lataniae density on loquat trees during winter generation.

Figure 4: The relation between k-values of two parasitoids, A. diaspidis and Signiphora sp., and the logarithm of the insect host, H. lataniae density on loquat trees during spring generation

Figure 5: The relation between k-values of two parasitoids, A. diaspidis and Signiphora sp., and the logarithm of the insect host, H. lataniae density on guava trees during spring generation.

Figure 6:The relation between k-values of two parasitoids, A. diaspidis and Signiphora sp., and the logarithm of the insect host, H. lataniae density on guava trees during summer generation.

Figure 7: The relation between k-values of two parasitoids, A. diaspidis and Signiphora sp., and the logarithm of the insect host, H. lataniae density on loquat trees during summer generation.

Figure 8: The relation between k-values of two parasitoids, A. diaspidis and Signiphora sp., and the logarithm of the insect host, H. lataniae density on fig trees during summer generation.

Table 1: The relation between k-values of the parasitoids, A. diaspidis and Signiphora sp. and the logarithm of H. lataniae density on three different host plants during winter generation.

Table 2: The relation between k-values of the parasitoids, A. diaspidis and Signiphora sp. and the logarithm of H.lataniae density on three different host plants during spring generation.

Table 3: The relation between k-values of the parasitoids, A. diaspidis and Signiphora sp. and the logarithm of H.lataniae density on three different host plants during summer generation.

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