Biology of Planococcus citri (Risso) (Hemiptera: Pseudococcidae) on Five Yam Varieties in Storage

Yam is an important staple cash crop, which constitutes 53% of total root and tuber consumption in West Africa. It is a cheap source of carbohydrate in the diets of millions of people worldwide and in tropical West Africa. However, attack by Planococcus citri results in shriveling of the tubers, making them become light and unpalatable. They also lose their market value. The total number of eggs laid, incubation period, developmental period and adult longevity of P. citri on stored yam Disocorea species were studied on five yam varieties namely Dioscorea rotundata var. Pona, Dioscorea rotundata var. Labreko, Dioscorea rotundata var. Muchumudu, Disocorea alata var. Matches and Dioscorea rotundata var. Dente in the laboratory with ambient temperatures of 26.0 ̊C 30.0 ̊C and relative humidity of 70.0% 75.0%. The mean life spans of the female insect that is from hatch to death on Dioscorea rotundata var. Pona, Dioscorea rotundata var. Labreko, Dioscorea rotundata var. Muchumudu, Disocorea alata var. Matches and Dioscorea rotundata var. Dente were 62.4, 63.5, 66.8, 67.3 and 69.8 days, respectively and those of the male were 23.8, 25.3, 29.6, 30.2 and 33.5 days, respectively. The adult female lived longer than the adult male. The findings showed that citrus mealybug preferred Dioscorea rotundata var. Pona to the rest of the varieties, as it developed faster and laid more eggs (497.0 eggs) on it than on the other varieties. This implies that citrus mealybug multiplies faster and destroys Dioscorea rotundata var. Pona tubers in storage faster than on the other varieties. Therefore, Dioscorea rotundata var. Dente should be the preferred yam for long-term storage.


Introduction
Planococcus citri commonly known as citrus mealybug is a polyphagus species known from all zoogeographical regions [1].It belongs to the Class Insecta, Order Hemiptera, Family Pseudococcidae, Genus Planococcus and Species citri [2].The citrus mealybug (a short-tailed mealybug) is a soft-bodied pseudococcid.Its life cycle (egg-to-egg-laying adult) duration ranges from 20 to 44 days [3].The citrus mealybug female can produce about 600 eggs that are laid in groups in cottony structures called ovisacs.Generally, within two to 10 days, new mealybugs (crawlers) emerge and start to go out of the ovisac [4].
The male P. citri has four nymphal stages during its development and each larval stage is separated by a molt.The male nymph is more elongated or narrower and often occurs in a loose cocoon.Based on laboratory studies on coffee leaves, the first male nymphal stage lasts for seven to 14 days, the second, six to 16 days, the third, two to three days, and the fourth, one to six days [4].Approximately four days into the male second instar, a black tinge that is easily discernible develops around the insect body.Two days later, the nymph starts spinning cocoon around itself.This cocoon is continuously spun increasing in density until the winged adult mealybug is ready to emerge two molts later [4].The adult P. citri male is a tiny two-winged gnat-like insect.It has an elongated body, reddish in colour and has two long filaments at the rear end.Male mealybugs live for two to four days after the final larva molt (a short-lived insect) [3] [4].
The female has three nymphal stages called instars with each larval stage separated by a molt.The immature or nymph resembles adult female in appearance except that it is smaller.The first nymphal stage lasts seven to 17 days, the second five to 13 days, and the third five to 14 days [4].Females live for about 88 days as adults and may start laying eggs 15 to 26 days into her adult life [4].
The eggs are oblong in shape, pale yellow or pink in color and are in clumps of five to 20 inside egg sacs composed of white cottony-waxy filaments.The first-instars (crawlers) are very active, oval in shape, yellow in color with six segmented antennae which lack waxy coating on the body and are often found congregated upon the egg sac.The second-instar larva is bigger than the first instar.It is oval in shape, yellow in color and has six segmented antennae.The third-instar larva resembles the larger adult female and is recognized readily by its yellowish body colour, seven segmented antennae and its larger anal ring; [3] [5].
The adult P. citri female has an elongated oval body about 3 mm long and 1.5 mm in width, flat with 18 pairs of short-waxy filaments around the margin of the body 17 of which are equal and the anal one slightly longer (no more than double the length of the others).The body color is mainly brownish yellow, covered with powdery wax through which distinguished transverse lines of segmentation appear.It is wingless and has eightsegmented antennae [3].
The citrus mealybug attack new shoots and leaves of a wide range of greenhouse and indoor plants including apple, avocado, citrus, English ivy, ficus, gardenia, jasmine, oleander and orchard crops in many temperate and tropical regions [6].The nymphs and adult females damage hosts by sucking out plant sap, by excreting honeydew on which sooty mould can grow and by causing distorted growth and premature leaf senescence with their toxic saliva [7] [8].A very late but major sign of their presence is the appearance of sooty mould on infested hosts.Black sooty mould fungi are detrimental to plants because they cover leaves, thus reducing photosynthesis and inducing plant stress.
Yams are monocotyledonous of the family Dioscoreaceae and genus Dioscorea and consist of more than 600 species but only six species are important as staple crops consumed in West Africa [9] [10].Dioscorea rotundata Poir (White yam) and Dioscorea alata L. (Water yam) together make up about 90% of world production of food yams [11].The preferred yam species in West Africa is the white yam (D. rotundata) since this generally produces tubers with a high dry-matter content that is good for pounding fufu [9].Water yam (D. alata, native to Southeast Asia) is gaining popularity in sub-Saharan Africa because it is easier to propagate than the native white yam [9].
In Ghana, yam is a very important indigenous subsistence and cash crop that is now the most popular nontraditional export food crop, despite years of scientific neglect [12].P. citri is now one of the major pests of stored yam tubers in Ghana.Infestation of stored tubers by P. citri results in shriveling of the tubers, making them become light, fibrous and unpalatable and therefore, lose their market value.Their attack also predisposes the yams to fungal infection leading to rotting of tubers, and badly affected tubers fail to sprout if planted [13].
Generally, control of mealybugs with insecticides is difficult because they tend to hide in protected locations and form dense colonies.The mealybugs' waxy covering also helps to protect them from chemical contact, suc-cessful control requires repeated insecticide applications directed against immature stages of mealybugs [14].
There is, however, not much information on the biology of P. citri on stored yam either locally or from the West African sub-region.The goal of this study therefore, was to determine the relative susceptibility of the five major yam varieties in Ghana to P. citri attack.The specific objectives were to determine the incubation period, development, longevity, and reproduction parameters of P. citri on the yam varieties.The results obtained in this study may provide useful information for designing a comprehensive pest management program for citrus mealybug on stored yam.

Test Yam Samples
Four varieties of D. rotundata (Pona, Labreko, Dente and Muchumudu) and one of D. alata (Matches) were selected based on consumer preference for the studies.These varieties were randomly selected from improved yam barns in Ejura in Ejura-Sekyedumase District a predominately yam growing area in the Ashanti Region and were maintained in the screen house of Faculty of Agriculture, Kwame Nkrumah University of Science and Technology, Kumasi.

Laboratory Culture of P. citri
P. citri adults were collected from yam barns and introduced onto tubers of D. rotundata var.Dokoba to ensure that mealybugs used for subsequent experiments developed from a common food source (standardization of mealybugs).Three tubers of D. rotundata var.Dokoba were placed in 30 × 30 × 60 cm wire mesh cage with five replications and the mealybugs allowed to oviposit for 72 hours after which the mealybugs were removed from the tubers with a standard aspirator leaving the egg sacs "in situ".
The eggs were allowed to develop into adults and produced ovisacs.50 eggs were collected using a paintbrush (No.000) (American Painter 4000, Loew-Cornell Incorporated Englewood Cliffs, New Jersey) and placed on the head region of each of the five test yam varieties (D. rotundata variety Pona, D. rotundata variety Labreko, D. rotundata variety Dente, D. rotundata variety Muchumudu and D. alata variety Matches) in 30 × 30 × 60 cm wire mesh cage with five replications.The eggs were allowed to develop on the test varieties into adults and the resulting eggs from them were used for the experiment.This was to ensure that any changes in behavior associated with the change in host varieties were eliminated [15].

Inoculation of Test Varieties
The test yam tubers were infested with 24-hour-old mealybug ovisacs.The ovisacs were teased open with blunt probes under a stereo microscope (×15) and 10 eggs were counted and refolded into the cottony ovisac and placed on the head region of each of the five test yam varieties (D. rotundata var.Pona, D. rotundata var.Labreko, D. rotundata var.Dente, D. rotundata var.Muchumudu and D. alata var.Matches) in the 30 × 30 × 60 cm wire mesh cage.Each 30 × 30 × 60 cm wire mesh cage had three tubers of each variety with five replications.
The development times of individuals were determined for each nymphal instar.They were observed daily under a dissecting microscope and the presence of exuviae was used to identify each stage.Daily monitoring continued until maturity.Upon emergence of adult females, one male was added with a fine hairbrush.The onset of oviposition was ascertained by the presence of an elongated white cottony egg sac extending beneath and behind the female.During the oviposition period, freshly laid eggs in ovisac were removed daily and counted until all the females died.Different parameters such as the incubation period, developmental period, adult longevity, total number of eggs laid and life span were determined.

Experimental Design and Data Analyses
The experimental design used for the biology of P. citri was completely randomized design with five treatments (varieties) and five replications.The data were analyzed by analysis of variance (ANOVA) using the statistical Package GenStat (GenStat Software for Personal Computer/Windows XP Release 7. 22 DE, 2008, VSN International Limited).

Female Egg Incubation Period
The mean incubation periods of female eggs (eggs that developed into females) on D. rotundata var.Pona, D. rotundata var.Labreko, D. rotundata var.Muchumudu, D. alata var.Matches and D. rotundata var.Dente ranged between 10.8 and 11.4 days (Table 1).The longest mean incubation period of female egg was recorded on D. rotundata var.Dente and the shortest on D. rotundata var.Pona.There were no significant differences (P > 0.05) among the mean incubation periods of P. citri female eggs on the yam varieties.

First Female Instar
The duration of P. citri female first instar on D. rotundata var.Pona, D. rotundata var.Labreko, D. rotundata var.Muchumudu, D. alata var.Matches and D. rotundata var.Dente ranged between 7.9 days and 12.8 days (Table 1).The duration of the first instar was longest on D. rotundata var.Dente (12.8 days) and shortest on D. rotundata var.Pona (7.9 days).There were significant differences (P < 0.05) among all the yam varieties but there was no significant difference (P > 0.05) between the duration of P. citri female first instar on D. rotundata var.Muchumudu (11.1 days) and D. alata var.Matches (11.2 days).

Second Female Instar
The mean duration of P. citri second instar female on D. rotundata var Pona, D. rotundata var Labreko, D. rotundata var.Muchumudu, D. alata var.Matches and D. rotundata var.Dente ranged between 6.9 and 11.2 days (Table 1).The longest mean duration of second instar female was recorded on D. rotundata var.Dente and the shortest on D. rotundata var.Pona.There was no significant difference (P > 0.05) between the duration of the second instar female bred on D. rotundata var.Muchumudu and D. alata var.Matches but this differed significantly (P < 0.05) from the rest of the yam varieties which also differed significantly (P < 0.05) from one another.

Third Female Instar
The mean duration of P. citri third female instar on D. rotundata var.Pona, D. rotundata var.Labreko, D. rotundata var.Muchumudu, D. alata var.Matches and D. rotundata var.Dente ranged between 9.6 and 12.9 days (Table 1).The longest mean duration of the third female instar was recorded on D. rotundata var.Dente and the shortest on D. rotundata var.Pona.There was no significant difference (P > 0.05) between the duration of the third instar female bred on D. rotundata var.Muchumudu and D. alata var.Matches (Table 1) but this differed significantly (P < 0.05) from the rest of the varieties.There was no significant difference (P > 0.05) between the duration of the third instar female bred on D. rotundata var.Pona and D. rotundata var.Labreko.

Total Female Larval Developmental Period
The total P. citri female larval developmental period on D. rotundata var.Pona, D. rotundata var.Labreko, D.  1) but this differed significantly (P < 0.05) from the rest of the varieties which also differed significantly from one another.

Mean Number of Eggs Laid
The mean number of eggs laid by P. citri on D. rotundata var.Pona, D. rotundata var.Labreko, D. rotundata var.Muchumudu, D. alata var.Matches and D. rotundata var.Dente ranged between 257.0 and 497.0 eggs (Table 2).The highest mean number of eggs was laid on D. rotundata var.Pona and the lowest on D. rotundata var.Dente.There were significant differences (P < 0.05) among the mean number of eggs laid on all the yam varieties (Table 2).

Adult Female Longevity
The

Male Egg Incubation Period
The mean incubation periods of male eggs (eggs that developed into males) on D. rotundata var Pona, D. rotundata var.Labreko, D. rotundata var.Muchumudu, D. alata var.Matches and D. rotundata var.Dente ranged between 10.5 and 11.2 days (Table 3).The longest mean incubation period of male egg was recorded on D. rotundata var.Dente and the shortest on D. rotundata var.Pona.There were no significant differences (P > 0.05) among the mean incubation periods of P. citri male eggs on the yam varieties (Table 3).

First Male Instar
The mean duration of P. citri first instar male on D. rotundata var.Pona, D. rotundata var.Labreko, D. rotundata var.Muchumudu, D. alata var.Matches and D. rotundata var.Dente ranged between 7.6 and 11.5 days (Table 3).The duration of the first instar male was longest on D. rotundata var.Dente (11.5 days) and shortest on D. rotundata var.Pona (7.6 days).The difference between the duration of the first instar male P. citri  on D. rotundata var.Muchumudu and D. alata var.Matches was not significant (P > 0.05) but it was significantly different from the rest of the varieties which also differed significantly (P < 0.05) from one another (Table 3).

Second Male Instar
The duration of P. citri second instar male on D. rotundata var.Pona, D. rotundata var.Labreko, D. rotundata var.Muchumudu, D. alata var.Matches and D. rotundata var.Dente ranged between 8.7 and 12.5 days (Table 3).The duration of the second instar male was longest on D. rotundata var.Dente (12.5 days) and shortest on D. rotundata var.Pona (8.7 days).The difference between the duration of the second instar male P. citri on D. rotundata var.Muchumudu and D. alata var.Matches was not significant (P > 0.05) but it was significantly different from the rest of the varieties which also differed significantly (P < 0.05) from one another (Table 3).
There was no significant difference (P > 0.05) between P. citri pre-pupa duration on D. rotundata var.Muchumudu and D. alata var.Matches but this differed significantly (P < 0.05) from the other varieties.There was also no significant difference (P > 0.05) between P. citri pre-pupa duration on D. rotundata var.Pona and D. rotundata var.Labreko but this differed significantly (P < 0.05) from the pre-pupa mean duration on D. rotundata var.Dente.

Mean Male Larval Developmental Period
The mean male larval developmental period of P. citri on D. rotundata var.Pona, D. rotundata var.Labreko, D. rotundata var.Muchumudu, D. alata var.Matches and D. rotundata var.Dente ranged between 21.3 and 32.0 days (Table 4).The mean male larval developmental period was longest on D. rotundata var.Dente (32.0 days) and the shortest on D. rotundata var.Pona (21.3 days).There were significant differences (P < 0.05) in the mean male larval developmental period of P. citri among all the yam varieties but there was no significant difference (P > 0.05) between the mean male larval developmental period on D. rotundata var.Muchumudu (28.0 days) and D. alata var.Matches (28.7 days).
The longest mean longevity was recorded on D. rotundata var.Pona and the shortest on D. rotundata var.Dente.There were no significant differences (P > 0.05) among the mean longevity of P. citri adult male on D. rotundata var.Muchumudu, D. alata var.Matches and D. var.Dente but these differed significantly (P < 0.05) from the mean longevity of P. citri adult male on D. rotundata var.Pona and D. rotundata var.Labreko which also differed significantly (P < 0.05) from one another (Table 4).

Discussion
The different parameters measured differed on the different yam varieties.It is clearly seen that D. rotundata var.Pona was the best of the five hosts studied for the development and total number of eggs laid by P. citri.D. rotundata var.Dente was the least suitable compared to the other hosts.
Several researchers including Woets and van Lenteren [16] have reported variations in biological parameters such as oviposition period, developmental period and longevity of insects on different hosts [17].These researchers attributed differences in whitefly populations on different host plants to a combination of the effect of the host plant on lifespan and the developmental rate of the insect.
Sakurai et al. [18] reported that the quality of food and environmental factors like temperature and humidity play an important role in the different aspects of the biology of coccinellid beetles.Metspalu et al. [19]) also observed that the amount and composition of nutrients particularly vitamins contained in plants considerably influence the development of caterpillars of Large White Butterfly ( Pieris brassicae).
Hogendrop et al. [20] showed that leaf nitrogen concentration affected citrus mealybug life history parameters with those mealybugs, which fed on plants containing the highest leaf nitrogen being the most fecund, being the largest in size and having the shortest developmental times.Working with P. citri, Yang and Sadof [21] reported that growth, development and fecundity of the mealybug differed substantially when fed on red, yellow, or green leafed Coleus blumei (Bentham).Therefore, the difference in parameters of P. citri reared on the five yam varieties might be due to the difference in the physical and chemical qualities of the different yam varieties.
The nutrient compositions of the five yam varieties (D. rotundata var.Pona, D. rotundata var.Labreko, D. rotundata var.Muchumudu, D. rotundata var.Dente and D. alata var.Matches), although not determined in this experiment, may be different from one another.Several researchers have shown that the nutrient compositions of white and water yams differ slightly [22]- [25].These differences in the nutrient compositions can influence the performance of P. citri on the five yam varieties to varying degrees in terms of oviposition period, developmental period, longevity, and total eggs laid.In this experiment, the larval developmental periods of female P. citri on the five test varieties were in the increasing order of D. rotundata var.Pona (24.4 ± 0.2 days), D. rotundata var.Labreko (26.7 ± 0.2 days), D. rotundata var.Muchumudu (31.9 ± 0.5 days), D. alata var.Matches (32.7 ± 0.3 days) and D. rotundata var.Dente (37.0 ± 0.5 days).These differences in the developmental periods might be due to difference in the nutrient contents of the varieties.
Goldasteh et al. [26] observed that the mean duration of first instar, second instar and third instar female of P. citri bred on red variegated coleus, Solenostemon scutellarioides (L.) Codd [previously Coleus blumei (Bentham)] in the laboratory lasted 6.40 ± 0.21, 8.72 ± 0.25, 9.48 ± 0.31 days, respectively with the mean female larval period, that is (1st instar to 3rd instar), being 24.6 ± 0.66 days.These durations are similar to those obtained on D. rotundata var.Pona but shorter than the results obtained on the other varieties.However, Cloyd [27] reported 33.7 days as the development period of P. citri on red variegated coleus, and this is comparable to the mean female larval period on D. rotundata var.Muchumudu (31.9 ± 0.5 days) and D. alata var.Matches (32.7 ± 0.3 days) but shorter than that on D. rotundata var.Dente (37.0 ± 0.5 days).
According to Goldasteh et al. [26] the mean female longevity of P. citri bred on coleus in the laboratory was 32.70 ± 0.89 days.This result is similar to the result obtained on D. rotundata var.Dente (32.9 ± 0.5 days) but shorter than the results obtained on the other varieties.
The mean male longevity of P. citri bred on coleus in the laboratory lasted for 1.67 ± 0.10 days [26].This result is comparable to the results obtained on all the varieties except D. rotundata var.Pona, which had the highest adult male longevity of 2.46 ± 0.04 (Table 4).Based on laboratory studies on coffee leaves, adult male mealybugs lived for two-four days after the final nymphal molt [4] and this is comparatively similar to the results of this study.

Conclusion
The results indicate that the development, longevity, and the total number of eggs laid by P. citri are influenced by yam variety and the order of preference of P. citri for development, longevity and oviposition is D. rotundata var.Pona followed by D. rotundata var.Labreko, D. rotundata var.Muchumudu then D. alata var.Matches and the least being D. rotundata var.Dente.

Recommendation
From the studies, D. rotundata var.Dente was less susceptible to P. citri infestation.Research can be carried out to ascertain the shelf life of D. rotundata var.Dente when infested with P. citri.Also D. rotundata var.Pona was the most susceptible to P. citri infestation.Investigation can be carried out to ascertain the shelf life of D. rotundata var.Pona when infested with P. citri.

Table 1 .
Mean duration (in days) of female Planococcus citri reared on tubers of five different yam varieties.var.Muchumudu, D. alata var.Matches and D. rotundata var.Dente ranged between 24.4 days and 37.0 days (Table1).The longest total larval developmental period was recorded on D. rotundata var.Dente and the shortest on D. rotundata var.Pona.There was no significant difference (P > 0.05) between the total larval developmental period on D. rotundata var.Muchumudu and D. alata var.Matches (Table lsd = least significant difference.CV = Coefficient of Variation.P = Probability of ANOVA.rotundata

Table 2 .
Mean numbers of eggs laid and mean longevity (in days) of female Planococcus citri reared on tubers of five different yam varieties.
lsd = least significant difference.CV = Coefficient of Variation.P = Probability of ANOVA.

Table 3 .
Mean development time (in days) of male Planococcus citri reared on tubers of five different yam varieties.

Table 4 .
Mean total larval developmental period and longevity of P. citri males reared on tubers of five different yam varieties.
lsd = least significant difference.CV = Coefficient of Variation.P = Probability of ANOVA.