An article presented at the Second International Conference on Mathematics and Natural Sciences. ITB Bandung, 28/10/2008
A research gas been conducted to study development and predation of a syrphid fly Ischiodon spp. on cowpea aphid Aphis craccivora Koch. It used a randomized complete .block design. Treatments contained six prey populations. i.e.: 20, 40, 60, 80, 100, and 120 aphids. Each was replicated four times. Results performed that various aphid population densities did not influence the predatory life span. However, the body sizes varied at different treatments. Greater number of aphids as the prey tended to increase amount of consumed preys by the predator, ranging from 15.5 – 67.75 aphids per predator. The highest predation reached 80% on 40 aphid population.
Key words: Ischiodon spp, Aphis craccivora, development, and predation.
Among agricultural crops, cow pea is a vegetable plant cultivated both either at low or high land and farmers like to grow it. People choose cow pea as vegetable food due to its delicious taste, containing A, B, and C vitamins (Deptan, 1983 in Herminanto and Sehat, 2003).
Farmers often get disadvantages caused by pest and disease attacks from planting to harvest. One of the important pests on cow pea is cow pea aphid (Aphis craccivora Koch.). It invades plant tips and young pods.
Controls to this pest commonly use mechanical and chemical techniques. Control to the pest mechanically requires many labors and is expensive for wide areas. When seen from pest population suppression. Chemical control gives rapid action, particularly if applied at wide areas. However, besides giving advantages unwise uses of chemical insecticides may cause negative effects (Sudjarwo and Herminanto, 2003).
Dilemma between needs and environmental safety grows idea of developing biological control to insect pests as a crucial component for integrated pest management. Uses of natural enemies such as predators, parasitoids, and pathogens have developed until currently for controlling agricultural insect pests. Among known predators, a syrphid fly Ischiodon spp. is a potential natural enemy for the control of cow pea aphid Aphis craccivora population.
In nature, the predator is capable to survive and to develop well without any specific treatment. Abundance of the prey in the field may attract the predator to come and to stay at certain plant becoming a host of the pest.
This research was conducted to know development and predation of the syrphid fly Ischiodon spp. in controlling populations of the aphid A. craccivora on cowpea plants.
The research was carried out in the Laboratory of Plant Pest and glasshouse in the Department of Plant Pests and Diseases Faculty of Agriculture Jenderal Soedirman University Purwokerto from May to August 2006.
Materials used were cowpea plants, cowpea pods, cowpea aphid A. craccivora, syrphid fly Ischiodon spp., and equipments for the experiment. The predator and its prey were collected from the field at vicinity of cowpea plants at Karangwangkal Village North Purwokerto Subdistrict Banyumas Regency. Then they were reared in the laboratory and the glasshouse.
A randomized complete block design was used to study the development and predation of the predator. Treatments consisted of six population densities of the prey i.e.: 20, 40, 60, 80, 100, and 120 aphids. Each was replicated four times. Predation arenas were made from plastic tubes infested with the aphids suitable with the treatments and cowpea pod cuts for prey food, and then new predatory larvae were put individually in them. The arenas were subsequently covered with flimsy cloth. Variables observed were sizes, life duration, and predation by the predator. Obtained data were analyzed by using F test at 5 and 1% levels and when significant then continued by LSD at 5%.
The first instar larvae had white colour and approximately 0.5 mm long. It hid in aphid colony, did not eat much and did less motion. The development of this predatory larva to further instars was indicated by colour alteration from white to cream or green on the dorsal body. White line on above body part also emerged. Larva activities decreased when it began to pupate and to leave its prey moving to dry leaf or soil. This situation was similar to Berry and Coop (2000) statement that the syrphid fly pupates in soil or on leaf surface. Table 1 performs the duration of predatory development from eggs to emerged adults.
Table 1. Duration of the predatory development
In growing, the predatory larvae require preys for food to survive. This natural enemy in hunting its preys actively move and creep around aphid colonies. They have a unique way to prey the food. When finding preys, they will hit, catch, and bite them. Then, they lift the caught preys up, pierce and suck body liquid of the preys (Putra, 1994).
Table 1 shows that the developmental time of the predatory larvae (1st – 3rd instars) reached 4.75 – 5.00 days. This result differs from observation done by Berry and Coop (2000) reporting that duration of the predatory larvae last for 2 – 3 weeks. This difference may be due to some factors. Moschetti (2003) informed that life cycle of the syrphid fly was affected by temperature, kinds of preys, and searching capacity of predator. Observations performed that the predatory life cycles ranged from 12.5 to 13.5 days. Changes in prey densities did not influence the life cycle. In Malaysia, the life cycle reaches 16 – 28 days (Malaysia Tropical Fruit Information System, 2006).
Various prey populations significantly affected body length of the predator (Table 2). The third instar larvae tended growing better when treated in 120 aphids than other treatments indicated by their longest size reaching 10.12 mm long. Adult sizes were not directly influenced by the treatments, but due to premature body sizes.
Table 2. Length measurements of the predator
Beck (1965 in Wahyuningsih, 1992) stated that nutrition composition and quantity of consumed food might affect young larvae growth and development, including their sizes.
Table 3. Weight of the predator
Results of observations and analyses performed that consumed aphids by the hoverfly larvae were strongly different among treatments, as seen in Table 4. High density of provided preys tended to increase consumed aphids by the predator, ranging from 15.50 to 67.75 aphids. Adversely lower predation percentage occurred on greater quantity of provided preys, but they are indifferent statistically.
Table 4. Predation by syrphid fly larvae
Regression analyses on relationship between amount of provided aphids and the predatory body length at the third instar larvae shows the equation: Y = 7.77 + 0.01 X and r = 0.646 (r 0.05 = 0.811 and r 0.01 = 0.917). Relationship between predatory weight of the 3rd instar larvae and provided aphids is explained by the equation Y = 15.22 + 3.23 X and r = 0.749. This implies that increase in the provided aphids does not correlate significantly.
1. Developmental times of the predatory eggs, larvae, and pupae were 2.25 – 2.50, 4.75 – 5.00, and 5.25 – 13.50 days. High density of the prey tended to increase body sizes of the predatory larvae.
2. Increase in the provided aphid densities caused to increase aphids consumed by the predator. The highest predation occurred on low prey density reaching 80% on 40 aphids provided.
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