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Electronic Journal of Polish Agricultural Universities (EJPAU) founded by all Polish Agriculture Universities presents original papers and review articles relevant to all aspects of agricultural sciences. It is target for persons working both in science and industry,regulatory agencies or teaching in agricultural sector. Covered by IFIS Publishing (Food Science and Technology Abstracts), ELSEVIER Science - Food Science and Technology Program, CAS USA (Chemical Abstracts), CABI Publishing UK and ALPSP (Association of Learned and Professional Society Publisher - full membership). Presented in the Master List of Thomson ISI.

Volume 11
Issue 2
Topic:
ELECTRONIC
JOURNAL OF
POLISH
AGRICULTURAL
UNIVERSITIES
. , EJPAU 11(2), #24.
Available Online: http://www.ejpau.media.pl/volume11/issue2/art-24.html


 

ABSTRACT

In the present study honeydew excretion process was investigated using modified hygrothermographs for daily measures, following the method applied in the 80’s by Koteja. Diurnal activity of honeydew excretion in different developmental stages of Coccus hesperidum was analyzed. Maximum activity of Coccus hesperidum honeydew production has been observed at afternoon and night hours. The difference size and density of honeydew droplets in relation to the scale insects developmental stage and diurnal rhythm of Coccus hesperidum has been noted.

Key words: .

INTRODUCTION

Honeydew is a substance typical of hemipteran insects [13]. The scale insects, larval stages and preovipositing females in particular, excrete high amounts of sugar-rich honeydew [13].

Coccus hesperidum belongs to most burdensome scale insects species of glasshouses in Poland. It commonly occurs in majority of glasshouses where by mechanical and physiological damaging weakens plants’ condition and deteriorates their decorative values [8,9,14].

Literature data [4,9,11,14] and personal observations show that the first evidence for this pest presence on the plant is often the honeydew it excretes. Larvae and females from family Coccidae produce a lot of honeydew that coats the plant surface as air-impermeable layer, later covered by saprophytic fungi, dust and other pollutions. Sweet honeydew attracts ants and other honeydew-eating insects which presence additionally reduces the plant’s aesthetics.

Although C. hesperidum is one of the most common and burdensome species of scale insects inhabiting ornamental glasshouse plants, little is known on its indirect harmfulness.

METHODS

The analysis of honeydew excretion activity was performed following the method previously applied by Koteja [12]. In the study, three modified types of a hygrothermograph for daily measures were used where on the cylinder instead of paper, normally serving for recording temperature and humidity readings, an X-ray film was fixed. The film has good absorbance capacity of water contained in honeydew what prevents the destruction of solidifying.

Above the cylinder, rotating with a constant velocity, a transparent plastic plate was attached. Fragments of plants with coccids were then fixed to the interior side of the plate. On one hygrothermograph 6 to 8 individuals of developmental stages (L1, L2, ♀) was observed. Every instar be noted by period about 2 weeks, with three replicates. A total number of 200 individuals had been subjected to the study. Honeydew excretion activity was investigated for larval stages and females of C. hesperidum. Activity – a frequency of honeydew excretion expressed as mean number of honeydew droplets excreted by particular developmental stages in time unit (one hour). Mean value of honeydew excretion activity, droplet diameter, droplet volume and the density of honeydew droplets were calculated from above 50 individuals of each instars C. hesperidum. The study involved one host-plant species, Ornitogalum sp. The observation was conducted in a laboratory (LD± 12:12), for 24 continuous hours. Air humidity and temperature measured by a hygrothermograph during the experiment was 35-55%, 12-22°C, respectively.

The aim of the study was to find out whether there are any differences in honeydew excretion activity or in the size of the honeydew droplet excreted by Coccus hesperidum, in relation to diurnal rhythm and particular developmental stage of the insect.

RESULTS

Honeydew excretion activity in relation to the developmental stage
The highest mean honeydew excretion activity – 5.9 droplets per hour showed first larval stage (L1), slightly lower value reached second larval stage – 5.2 droplets per hour. Both larval stages showed a continuous activity of honeydew excretion during a day, proceeding with a differentiated intensity.

The number of droplets excreted by L1 and L2 fell within a respective threshold values 2.8-7.8 droplets/hour and 3.7-6.8 droplets/hour.

The lowest honeydew excretion activity, not exceeding 1 droplet per hour, showed females of C. hesperidum. On average, one female of this species extracted 0.3 honeydew droplets in time unit (Fig. 1, Table 1, Phot. 1, 2).

Photo 1. Coccus hesperidum L. – L1, L2 and female (photo K. Golan)

Photo 2. Crawlers of C. hesperidum and droplets of honeydew on leaf (photo K. Golan)

Diurnal rhythm
Honeydew excretion activity in the scale insect species studied was changing during the day, peeking at night hours.

In larval stages a visible decrease in honeydew excretion activity took place between 7.00 and 14.00 with the minimum at 12.00-13.00 (2.8 droplets/hour) for first larval stage and at 9.00-10.00 (3.7 droplets/hour) for second larval stage. Starting from 14.00 an increase in honeydew excretion activity was noted with maximum at 21.00-22.00 for L1 – 7.8 droplets/hour and at 19.00-20.00 for L2 – 6.8 droplets/hour. Larvae maintained a high honeydew excretion activity till early-morning hours (Fig. 1).

Fig. 1. Twenty-four hours activity of honeydew excretion in different developmental stages of Coccus hesperidum (mean value calculated from above 50 individuals of each instars)

The analysis of diurnal activity of C. hesperidum mature females revealed that it was considerably lower as compared to larval stages. In females, several intervals in honeydew excretion were observed. However, owing to the generally low females’ activity it was difficult to distinguish any regular diurnal changes in this process. Mean honeydew excretion activity measured between 13.00 and 17.00 ranged from 0.3 to 0.5 droplets/hour. It was slightly higher at 19.00-20.00 (0.8 droplets/hour) and at 23.00-24.00 (1 droplets/hour).

The size and density of honeydew droplets in relation to the scale insects developmental stage
Comparing the size of honeydew droplets between these extracted by C. hesperidum larval stages and by females it is noticeable how the diameter and volume of a droplet increase following the insect’s ontogenesis. Smallest honeydew droplets are extracted by first larval stages, and the bigger the honeydew droplet diameter the lower the number of droplets per surface unit-density (Table 1).

Table 1. Mean ( SE) honeydew excretion activity, droplet diameter, droplet volume and the density of honeydew droplets in respect to particular developmental stages of C. hesperidum*

Developmental stage

Mean honeydew excretion activity (droplets/hour)

Mean honeydew droplet diameter
(mm)

Honeydew droplet volume
(mm3)

Honeydew droplets’ density (droplets/cm2)

L1

5.9

0.19

0.0036

5.69

L2

5.2

0.60

0.1130

4.99

Females

0.3

0.62

0.1247

0.3

*Mean results calculated from above 50 individuals of each instars

DISCUSSION

In foreign literature considerably much attention is devoted to honeydew chemical composition [5,6,7,10,18,22], to its role in honeydew honey production [19,20,21] or to relationship between honeydew-producing and honeydew-eating insects [1,3]. Within the group of honeydew-producing insects these are aphids that are described most frequently. The scale insects, in comparison, have drown much less attention.

Works concerning aphids have proved the increase of honeydew droplet volume in respect to successive developmental stage of the insect [15]. Similar relations were observed in the present study. According to Nishida and Kuramoto [17] the number of honeydew droplets extracted by Dysmicoccus neobrevipes (Hemiptera; Coccinea; Pseudococcidae) decreases in respect to the insect’s age. This correlation is related to metabolism, alimentation and feeding – the amount of the food up-taken decreases when coccids reach maturity. Similar studies on honeydew excretion concerning aphids showed that the number of honeydew droplets and their size increased in respect to the proceeding ontogenesis, and honeydew excretion activity did not change when females reached maturity and started producing larvae [15,16]. Unlike aphids, scale insects stop feeding after becoming mature [17]. In scale insects from the family Coccidae dorsal cuticle of female bulges intensively when the ovisac is being formed while the ventrial cuticle remains membraneous and elevates as more and more eggs are produced, and the female dies [13]. According to Beardsley [2], the food-intake of scale insects females increases together with the progressing maturity. Similar results were obtained in the present study in females from the family Coccidae.

Comparing to second instars, adult females of C. hesperidum showed a decrease in honeydew excretion activity what resulted in a less intensive intake of plant sap. Koteja [12] noted in his studies that honeydew excretion activity has a diurnal cycle with a peek at night hours. In the present study the analysis of honeydew excretion activity in larvae and females of C. hesperidum has also revealed the increase in honeydew excretion at night hours, what was noticeable particularly in the case of larval stages. According to Koteja, diurnal changes in honeydew excretion activity are closely related to physiological activity of a host plant. Nutritive accumulated in plants during a day are then sucked in with sap, transformed in bug’s digestive system and extracted in a form of honeydew droplets.

CONCLUSIONS

Honeydew droplets excretion on the hygrothermograph’s cylinder marked the X-ray film with tracks specific for particular developmental stages. Under laboratory conditions, honeydew excretion activity of C. hesperidum larvae and females followed the diurnal rhythm. Highest amounts of honeydew were extracted at afternoon and night hours. Successive developmental stages of C. hesperidum individuals extracted honeydew droplets of bigger size but less numerous on surface unit (density).

ACKNOWLEDGMENT

The study was financed from the means of the Ministry of Science and Higher Education in the years 2006-2009 as a research project.

REFERENCES

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Accepted for print: 17.04.2008



Responses to this article, comments are invited and should be submitted within three months of the publication of the article. If accepted for publication, they will be published in the chapter headed 'Discussions' and hyperlinked to the article.


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