Food consumption of the coccinellid predator , Stethorus punctillum reared on the two-spotted spider mite , Tetranychus urticae under different constant temperatures

Food consumption of the predator, Stethorus punctillum Wiese reared on immature and mature stages of the prey, Tetranychus urticae (Koch) was studied under laboratory conditions at three different constant temperatures (15, 25 and 35oC). The larval instars consumed more immature prey individuals at 15oC, followed by 35 oC then 25oC. Adult males of the predator consumed more individuals of immature preys at 35oC, followed by 25 oC then at 15oC, while the predator females relatively consumed more prey individuals than the males, at the same temperatures. When the larvae of the predator reared on mature stages of the prey, they consumed more prey individuals at 15oC, followed by 25oC then at 35oC. The adult males and females of the predator consumed almost the same numbers of the prey individuals. They consumed the highest number of preys at 35oC, followed by 25oC, while the lowest consumed number was recorded at 15oC.


INTRODUCTION
Tetranychus urticae (Koch) (Acari: Tetranychidae) is one of the most polyphagous species of the Tetranychidae, attacking vegetables in greenhouses and of several other agricultural outdoor crops causing economic damage (Ripa et al., 2006).T. urticae is adapted to various environmental conditions and is distributed worldwide, causing loss of yield and quality or the death of the plants by sucking out the contents of plant sap (Geest, 1985;Granham, 1985;Rott and Ponsonby, 2000 a&b).
The present laboratory work was carried out to study some biological aspects of Stethorus punctillum and its efficacy as a biological control agent under different rearing conditions.

MATERIALS AND METHODS
The predator, Stethorus punctillum, was collected from different plant leaves, especially the castor oil plant leaves, which were infested with the red spider mite, T. urticae.Adults of S. punctillum beetle reared on potted bean plants artificially infested with T. urticae maintained for 2-3 months before testing their efficiency.Wooden cage (50x50x50 cm) were used for maintaining the culture of S. punctillum and T. urticae.Cages were covered with nylon cloth on top and glass door in the front for daily services and watching.
The prey consumed by S. punctillum was studied through out the larval and adult stages at each tested temperature (15, 25 and 35ºC).The predator was offered 50 immature and 30 mature prey individuals daily.Fifteen replicates were carried out for each tested temperature.

A-Larval stage of S. punctillum 1-Feeding on immature T. urticae
Leaf disc (2.5cm) was artificially infested with fifty immature of prey individuals.Each newly hatched larva of the predator was kept separately on the leaf disc in the experimental cell (4x3x3cm).The cell was covered and incubated at constant conditions (25±2ºC, 18L: 6D photoperiod and 65±5% RH).The cell was checked daily and the number of consumed preys was calculated.The predatory larva was transferred to new fresh leaf disc infested with the same number of immature prey.This procedure was repeated until the pupation of the predator's larva.

2-Feeding on mature T. urticae
Leaf disc (2.5cm) was infested artificially with thirty mature prey individuals.Each newly hatched larva of the predator was kept separately on the leaf disc in experimental cell.The cell was covered and incubated as mentioned above.This procedure was repeated until the pupation of the predator larva.

B-Adult stage of S. punctillum Feeding on immature and mature T. urticae
Leaf disc (2.5cm) was artificially infested by fifty immature prey individuals.Each newly emerged predator's adult was kept separately on the leaf disc in the experimental cell.The cell was covered and incubated as mentioned before.The previous steps were repeated with substitution the immature preys with thirty mature individuals.The experimental cells checked daily until the death of the predator's adult.The whole experiment was replicated five times at each tested temperature.
Statistical analysis: Data were subjected to analysis of variance (ANOVA) using "SPSS" a computer statistical program.Mean values were compared using Duncan's Multiple Range test.

Food consumption A-Larval stage of S. punctillum
Data in Tables (1 and 2) showed that food consumption of the predator was markedly affected with temperature under investigation.When the predator was fed on immature stage of T. urticae, the highest number of consumed preys was recorded at 15ºC for all larval instars, followed by 35ºC then 25ºC.Statistically, there were significant differences (P<0.01) between the number of consumed preys at each temperature.The corresponding figure when the predator fed on mature stage, it was observed that the highest number of consumed prey was recorded at 15ºC for all larval instars, followed be 25ºC then 35ºC.Statistically, there was a significant difference (P<0.01) between the number of consumed preys.

B-Adult stage of S. punctillum
Feeding on immature and mature stages of T. urticae Data in Table (3) showed that adult male of the predator consumed the highest number of the prey at 35ºC, followed by 25 then 15ºC, for either immature or mature prey.The female predator relatively consumed more prey individuals than the male at the same tested temperatures.Statistically, there was a significant difference (P<0.01) between the number of consumed preys (Table 3).

DISCUSSION
The present study examined the activity of the coccinellid predator, S. punctillum (the mite destructor) under three different constant temperatures (15 ºC, 25ºC and 35ºC).Generally, it was observed that, the degree of temperature had different impacts on its activity and the food consumption rate.The optimum activity of this predator under laboratory conditions was around 20 ºC and 25ºC.Whereas it reached, the maximum at the highest tested temperature (35ºC) and it's minimum at 15ºC.These results are in agreement with Shoeib (2001).
At 15ºC, the development of the larval instars became sluggish, and larvae takes longer periods to metamorphose because of the low temperature, so, the larval instars (1 st to 4 th ) consumed more preys.This consumption rate was more than the other rates at 25 ºC and 30°C, respectively.It was observed that the predator's larvae consumed somewhat more preys, which may be referred to its need to cumulate high quantity of its food requirement to be able to proceed with its development and metamorphose to the pupal stage.At 15°C, no results were manipulated by other authors.Our obtained results at 25 ºC and 35ºC, are in agreement with the results reported by Zhou et al. (1991); Iskander et al. (1994) and Ragkou et al. (2004) for their work on S. punctillum and Kheradpir et al. (2006) for their study on S. gilvifrons.

2-Feeding on mature T. urticae
Our results revealed that, at 15ºC, the four larval instars of the predator, also, consumed more number of preys than those eaten at either 25ºC or 35ºC.These results were in agreement with Iskender et al. (1994) for S. punctillum at the same temperature.
It is clear that, the predator's larvae consumed more preys at 15ºC than other tested temperature, that is because of under this degree of temperature, the rate of development of the larvae became slower than at other tested temperatures, so that, the larvae spent longer periods and needs to prey on more spider mites to fulfil its requirement for development and metamorphoses.The aforementioned results indicated that the larval stage of the predator were more active and consumed more prey individuals at higher temperatures than at low one.

B-Adult stage of S. punctillum
In the present study, it could be concluded that the adult males and females of the predator consumed the lowest number of preys (immature and mature preys) at 15ºC.While the highest number of consumed preys (immature and mature preys) was recorded at 25ºC and 35ºC, respectively.Adult females were relatively consumed more number prey individuals than males at the same tested temperatures.These results are in accordance with those reported by Lui and Lui (1986) for S. siphonulus Kapu; but were mismatched with those reported by Iskander et al. (1994) for S. punctillum; Chang & Leu (1986) for and Liu (2002) for S. siphonulus on citrus red mite, Panonychus citri Our obtained results were contradicting with those reported by Shin et al. (1991) on their work on Naher and Haque (2005) on S. punctillium as a predator for T. urticae.
It could be concluded that the optimum temperature for mass-rearing the predator S. punctillium under laboratory conditions was 25 ºC ±2ºC.

Table ( 1
): Average number of Teteranychus urticae immature stages consumed by Stethorus punctillum larval instars at different constant temperatures

Table ( 3
): Average number of consumed Teteranychus urticae mature and immature stages by Stethorus punctillum adult stage at different constant temperatures