Population fluctuation of some insect pests infesting sugar beet and the associated predatory insects at Kafr El-Sheikh Governorate

摘要

The experiments were performed on sugar beet crop at the farm of Sakha Agricultural Research Station in Kafr El-Sheikh Governorate throughout two successive seasons (2011 and 2012) to study the population fluctuations of the main insects of sugar beet plants and their associated predators. The results showed that the sugar beet plants were harbored three main insect species i.e. Cassida vittata )Vill); Pegomyia mixta Vill and Scrobipalpa ocellatella Boyd. Four predatory species were associated with the three insect pests i.e. Coccinella undecimpunctata L.; Scymnus sp. , Paederus alfierii Koch. and Chrysoperla carnea (Steph.). Results revealed that the highest infestation levels of these insect pests were occurred during March and April in both seasons. The population densities of these pests were relatively higher during first season than the second one. Larvae of C. vittata showed two of peaks seasonal abundance in both seasons during late February and late March in the first season (10 and 230 larvae/25 plants, respectively) and during early and mid-April in the second season (29 and 35 larvae/25 plants, respectively). Also results showed that during the first season peaks occurred about one month earlier than the second season as there were manifest declines in the temperature throughout the second season. Larvae of P. mixta recorded three peaks at late December, early February and mid-March (15, 72 and 143 larvae/25 plants, respectively) during the first season. Also during the second season three peaks were recorded at mid-December, late January and mid-March (13, 22 and 114 larvae/25 plants, respectively) with considerable decline in larval population throughout the second season. Larvae of S. ocellatella occurred were during the period from December till late May on sugar beet plants with apparent oscillations. The number of larvae gradually increased till it reached 12 larvae/25 plants (mid-April 2011) and 9 larvae/25 plants (early May 2012). Peaks were unclear because the mean number of larvae was relatively few. Results concerning the four predator species found associated with these insect pests showed that during the first season the population of C. undecimpunctata increased gradually till it reached the highest peak at the second week of May (59 beetles/25 plants). The population of C. carnea was fluctuated till it reached the highest peak at the second week of April (12 Larvae/25 plants). Both Scymnus spp. and P. alfierii were found with few numbers. During the second also the population of C. undecimpunctata started to build up at the second week of April till the end of the season. The highest mean number of C. undecimpunctata was 21 beetles/25 plants were occurred at the third week of May. The population of Scymnus spp. also increased gradually till the end of the season with the highest number (30 beetles/25 plants) was occurred at the third week of May. The larval population of C. carnea started to increase gradually till it reached the highest peak at the second week of May (19 larvae/25 plants). P. alfierii was represented with few numbers. These results revealed that the appearance of C. undecimpunctata , Scymnus spp. and P. alfierii on sugar-beet plants followed the appearance of P. mixta and S. ocellatella and most probably these predators were fed on other hosts before attacking other insect pests such as aphids and lepidopterous larvae. The synchronization of S. ocellatella larvae and their predators occurred during April and May during the both seasons.