Laboratory Studies on the Host Preference of Cotton Mealybug ‘Phenacoccussolenopsis’ Tinsely (Hemiptera: Pseudococcidae) in Khartoum State, Sudan

The mealybug which was not well known to many courtiers since, today it became very dangerous pest. It originated in Central America, and spread to more than 11 countries in South America, Asia, Africa and Australia¹

It is reported that a number of mealybug species cause damages to over 202 plant species belonging to 55 families across the globe ², ³, ⁴. They parasitize on a wide range of host plants; causing significant damages to numerous plants including field crops (Cotton, Sesame, Sunflower), vegetables (Okra, Tomato, Eggplant), ornamentals, weeds, bushes, and in particular the plants which belong to the families of Malvaceae, Solanaceae, Asteraceae, Euphorbiaceae, Amaranthaceae and Cucurbitaceae ⁵, ⁶, ⁷.

Studies revealed that the pest attacks weeds throughout the year, and migrate to other crops through natural carriers (wind, water, bird and human being). It spread between continents through international trade ⁸. One of the researcher reported that high population of mealybugs can cause serious losses to a wide range of crop plants through fruit, flower/leaf drop, fruit/flower deformation and development of discolored welts on the rind of the fruit, flower etc. He also added that in the last 30 years, outbreaks of mealybugs caused alarming damage to crops in the United States of America (USA), New Zealand and France ⁹.

Ibrahim, et. al., (2015) are the first to identified P. solenopsis as a new insect pest attacking tomato plants in Egypt.¹⁰. In Queensland, the pest was first identified in 2009, and considered as minor pest, but it became a major pest and widely spread to affect the cotton crop¹¹.

Recently, the Cotton mealybug has acquired the status of major insect pest in the world, and therefore, the entomologists are facing difficulties in managing this pest due to its polyphagous nature ^{1, 12}. It attacks weeds year around and migrate to other crops through natural carriers (wind, water, bird and human being). Moreover, it has the ability to increase rapidly and spread to cover vast areas in a relatively short period of time ¹³. Furthermore, the pest is protected from insecticides and natural mortality factors by the presence of white powdery and waxy secretion ⁹. Thus, it causes a significant damage to numerous crops including Cotton, Okra, Tomato, Eggplant, Sesame, Sunflower and Ornamental shrubs⁷.

In Sudan, infestation with Cotton Mealybug was reported from different States in the country. A survey conducted in Gezira and Khartoum States in 2015 showed that at least 26 host plant species belonging to 16 plant families were identified as host plants ^[14]../

This study was conducted in the laboratory of the College of Agricultural Studies, Sudan University of Science and Technology at Shambat during 2017.

A culture of Cotton mealybug, (Phenacoccussolenopsis) was prepared by collecting adult stage from infested vegetables, ornamental shrubs and weeds from different sites in Khartoum State. They were reared in plastic containers (15cm diameter by 7cm deep) under laboratory conditions maintained at temperature of 25 to 28^oC and relative humidity (RH) 25-28 %. The females were reared from the newly emerged nymphs and adults of P. solenopsis.

In this study, three vegetables, two ornamentals, two weeds and one field crop were studied. Where a total of 8 common host plant species available in Khartoum State, namely the Okra (Abelmoschus esculentus), Cotton (Gosspiumsp), Egg plant (Solanum melongena), Tomato (Lycopersiconesculentum), Lantana (Lantana camara), Raba (Zaleyapentandra ) and Hambouk (Abutilon pannosum), were collected.

The host plant species were evaluated against 1^st, 2^nd, and 3^rd nymphal instar and adult female, where healthy twigs of each selected plant species with at least five tender leaves were collected, washed and dried to remove moisture from the surface of the leaves. In this study the Eickhoff, T.E., Baxendale (2005)¹⁵ method was followed.

A Plastic dish (34 cm. diameter by 13 cm. deep) was used as a study arena. 2 cm. diameter holes were drilled in the outer circle of the dish (8 holes). The twigs were inserted in vials (2 cm. diameter by 9.3 cm. deep) containing fresh organic peat moss wetted with water. The vials were randomly inserted in the holes of the dish. A counted number of Cotton mealybug (200 individual of each stage in a Petri-dish (15x2 cm.) were put in the centre of the dish and the whole dish was covered with muslin cloth following multi-choice experiment under laboratory conditions, at temperature of 25 -28^oc and relative humidity27- 28%.

The food preference for the P. solenopsis among the tested plant species were compared with the Chinese rose (as a control), and proportion of mealybug was determined. The tested plant species were observed after 2 , 8 and 24 hours after release.

Data regarding feed preference of P. solenopsis among tested plant species in the laboratory were statistically analyzed by Software (Statistic 8 -Software, 2003) and subjected to analysis of variance (ANOVA) under completely randomized design (CRD). Means were also compared following the least significant difference test (LSD) at probability level of 1%.

This study revealed variation between the host plant species (cotton mealybug) (P. solenopsis) and attractiveness. However, the proportions of all nymphal instars varied significantly (P value = 0.01) with respect to plants species and observation intervals (Table 1, Table 2, Table 3).

After two hours of release, the attraction of all the nymphal instar and adult of P. solenopsis revealed non-significant differences between the host plants as compared with the control (Chinese rose). The mean proportion of the first nymphal instar ranged from 1.3 to 25.3, on Raba and Hambouk respectively, where the attraction of the second nymphal instar, ranged from 0 to 9 on Egg plant and Hambouk, respectively. However, the third nymphal instar, displayed low levels of attraction on the tested plants with a range of 0 to 2 on Eggplant and Lantana, respectively. For the attraction of the adult of the P. solenopsis, low population of adult stage was recorded ranging from 3 to 0.67 on Egg plant and Cotton, respectively (Table 1).

At eight hours after release, there is a highly significant different between the two (Lantana, and Cotton) and Raba. The proportions of P. solenopsis ranged from 0 to 50 on Raba and Lantana respectively. Nevertheless, Lantana possessed high population proportion followed by Cotton 45. The proportions of the second instar settled on the different host species ranged from 0 to 16.33 on Raba and Lantana respectively with non- significant differences as compared with the control. The proportion of the third nymphal instar ranged from 0 to 11.33 on Raba and Okra, respectively with significant differences. However, non-significant differences between the attractiveness of all tested plants and the control were recorded; with proportions for the adult stage ranged from 0.33 to 5.67 on Raba and Cotton, respectively (Table 2).

697230624078000The occurrence of the first instar after 24 hours of release ranged from 0.33 to 62.33 on Raba and Okra and high significant difference between Okra and Lantana, Eggplant and Raba was recorded. The proportions for the second instar on the different host species ranged from 0.33 to 38.67 on Raba and Okra, and non-significant differences between Okra, Cotton (31.33) and Hambouk (27.33) were recorded. Moreover, high significant difference was noticed in the proportions of the 3rd instar on Okra compared with other plants and the control ranged between 0.33 and 43.67. For the Adult stage, Okra showed significant difference compared with other host plants, and the range was 0 to 33 on Egg plant and Okra respectively (Table 3).

From figure 4, it is clear that, the settlement of all stages (1st, 2nd, 3rd nymphal instar and adult females) of P. solenopsis increased with increasing time on the host plant species belonging to family Malvaceae (Table). These observations were supported by of one of the researcher (12) who stated that once the mealybug attached to the host, it inserts proboscis and starts feeding on the plant. It remains attached on the plant. Therefore its numbers increase arithmetically with the passage of time. Nevertheless, there is a clear consistent preference of all stages of P. solenopsis.

Generally, Okra appeared to be highly preferred over the selected host species compared with the control (Figure 1, Figure 2, Figure 3 and Figure 4), followed by Cotton and Hambouk, except the 3rd nymphal instar where Hambouk was preferred more than Cotton (Figure 3). This finding disagrees with the findings of other researcher ¹⁶ who studied host preference of P. solenopsis using 25 plant species, and recorded that, the proportions of the 1st and 3rd instars were maximum on cotton. Other studies ¹⁷,¹⁸, ^{5, 19} reported that, the most favorable host plants of the cotton mealybug are Cotton, Eggplant, Sunflower, Chinese rose and Lantana¹⁹.

Figure 1.Attraction of the 1st nymphal instars of cotton mealybug to selected plant species, compared with Chinese rose (control), at different interval time after release - Khartoum- Sudan (2018).

Figure 2.Attraction of the 2nd_ nymphal instar of cotton mealybug to selected plant species, compared with Chinese rose (control), at different interval time after release- Khartoum- Sudan (2018).

Figure 3.Attraction of the 3rd_nymphal instar of cotton mealybug among selected plant species as compared with Chinese rose (control) at different interval time after release- Khartoum- Sudan (2018).

Figure 4.Attraction of adult females of cotton mealybug to selected plant species, compared with Chinese rose (control), at different interval time after release- Khartoum- Sudan (2018

This study showed that the first instar of P. solenopsis moves quickly towards the vials containing the host plant species more than other stages, where the mean population of the first instar was high in all observation intervals as compared with other developmental stages (Table 1, Figure 1). These results are confirmatory to the findings of one of the researchers ²⁰ who studied host plant preference and mortality of P. solenopsis on different plant species.

However, although both Tomato and Egg plant are belonging to the same family (Solanaceae), but tomato is more preferred by the host than Egg plant, because the first plant is succulent.

The response of the pest to the various host species is significantly different. The more suitable and preferred the host, the more is the fecundity of the pest in the locality and the same environmental conditions ²¹. It is reported that the insect selection and utilization of a host plant depends upon both biophysical and biochemical factors ¹⁹.

The dietary requirement and fitness of phytophagous insect pests depends upon the nutrient chemistry of the host plant reflected that the quality and quantity of the food affects the food selection behavior, survival and reproduction of phytophagous insect pests. Sucking insects, including mealybug, are commonly attracted towards succulent plants that enriched with chlorophyll ²⁰.

The selection of host plant by the pest is often divided into ‘host plant finding’ and ‘host plant acceptance, where the volatile chemicals guide phytophagous insects to their host plants ²². The insects assess the plant with respect to its suitability as host species and also its nutritional suitability. However, the plant secondary compounds play a significant and dominant role in host plant selection by a behavioral response of the insects to these chemicals ²³.

The preference of Phenacoccus solenopsis for the family Malvaceae is confirmed by the finding of one of the researcher ²¹ who study, under field conditions, the effect of host plant species, and season in three locations in Pakistan (Alipur, Multan, Faisalabad) on the fecundity of P.solenopsis , using 10 alternative host plants to determine its relative preference and suitability as feed. His results indicated that the host plant species belonging to the family Malvaceae (Cotton, Okra and China rose) has higher number of crawlers per female which varies significantly between different months according to the temperature and relative humidity. On contrast, locality has a non significant effect on fecundity.

All plant species which belong to the family Malvaceae such as Okra, cotton and Hambouk are the most favorable to Cotton mealybug. Plants species belonging to the families Verbenaceae (Lantana) and Solanaceae (Tomato) appeared to be an intermediate to attractiveness, while Egg plant (Solanaceae) and Raba (Aizoaceae) appeared to be the least preferred host plants at temperature of 25 to 28oC and relative humidity (RH) 25-28 %..

The study recommends further study on food preference and attitude of P. solenopsis under both; different climatic conditions in the laboratory and field to understand the magnitude of pest damage to economic crops so that to propose strategies for managing this serious pest.

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