Pest status and management options for plutella xylostella l. (lepidoptera: plutellidae) in Iringa and Morogoro, Tanzania

Abstract

Plutella xylostella L. is the pest of economic importance in production of Brassica crop. This pest can cause up to 90% loss if not controlled. The pest has been reported to exist in Tanzania but the actual assessment of its abundance and inflicted damages are lacking. This has made production of brassica to be dependent on a number of pesticide applications to allow harvesting of marketable heads. Interrupted mating through overhead irrigation has been reported to minimize pest damage but the exact timing for irrigation has never been established. The present study aimed at understanding the pest status of P. xylostella in major production areas and exact the irrigation water based management as potential alternative to pesticides. Specifically, the study sought to: (1) determine the spatial distribution of P. xylostella in cabbage growing areas of Iringa and Morogoro Regions, and (2) determine the effect of timing of overhead irrigation on occurrence and perpetuation of P. xylostella on cabbage crop. Methods A survey was conducted in Iringa and Morogoro Regions from May 2017 to July 2017 to assess pest status and its distribution in tandem with the first specific objective. A total of 80 respondents’ were interviewed using questionnaires and field visits made for diagnostic assessments of 80 brassica farms owned by the respondents. Assessments of the abundance and damages inflicted by P. xylostella were done. Collected data included; respondents’ demographic variables, awareness of the pest and its seasonal occurrence, practiced control measures against the pests, grown varieties, field incidences of the pests, damage severities, and general management of the crop. The collected data were descriptively analyzed by using the Statistical Package for Social Science (SPSS 16.0) software. In addition, Chi square tests (p = 0.05) were used to test the statistical significances among variables. Regression (R2) analyses were performed to determine the relationship between P. xylostella incidence and altitude, cropping system, varieties commonly grown, previous crop and crop growth stage. Obtained results were presented in tabular form and graphs. The second objective was accomplished through a field experiment set at SUA Horticultural Training Farm. The effect of timing of overhead irrigation on occurrence and perpetuation of P. xylostella on cabbage crop was investigated. The experiment was laid out in a two factor Split plot arranged in a Randomized Complete Block Design (RCBD) and replicated thrice. Main-plot consisted of two Brassica spp namely; Chinese cabbage (Brassica rapa) and Head cabbage (Brassica oleraceae var. capitata) while, Sub-plot consisted of three time intervals for sprinkler irrigation namely; T1 (5 pm), T2 (6 pm), T3 (7 pm) and Control (plants treated with insecticide-Phosphorothioate and irrigated using watering can). Data collected on pest incidences, developmental stages and magnitude of damages was subjected to Analysis of variance (ANOVA) and means separated using The Least Significant Difference Test (LSD; p = 0.05) was also performed as means separation test. Genstat v.16 Statistical package (VSN International) was used for statistical analyses. In addition, relationships among test variables and P. xylostella were established through regression (R2) analysis was performed using MS-Excel v.16 software. Findings Results from comparative analyses among surveyed wards in Iringa and Morogoro Regions suggested no significant differences (χ2 = 12.27, df = 16, p = 0.73). Thus, brassica crop in almost all fields succumbed equally to P. xylostella. Recorded field incidences were (67.2%) and (50.2%) in Iringa and Morogoro regions respectively. Detailed analyses of the four surveyed wards in Iringa indicated that Mangalali had the highest incidence (93.3%) iv while Ulanda had the lowest (47.3%). In Morogoro region Mlali had the highest incidence (79.0%) while Dumila had lowest incidence (37.3%). Up to 66.7% of assessed fields were infested with P. xylostella in combination with other pests, while 28.9% of the sampled fields were infested with P. xylostella alone. Further analysis suggested a significant relationship (R2 = 0.032, p = 0.019) between mono-cropping system and P. xylostella incidences. The pest incidences increased with the crop growth stage (R2 = 0.031, p = 0.02). There was a positive significant relationship between larvae counts and damage score for Head cabbage (R2 = 0.42, p = 2.23 x 10-5) and Chinese cabbage (R2 = 0.43, p = 1.22 x10-5). Control plots recorded significantly lower larvae count (1.63), adults count (1.46) and damage score (2.07) than T1 (1.83 larvae, 1.60 adults, 3.01 damage score), T2 (1.81 larvae, 1.64 adults, 3.22 damage score) and T3 (1.80 larvae, 1.64 adults, 3.07 damage score). The crop type had a significant (LSD = 63, p < 0.013) effect on yield. Chinese cabbage recorded higher yield (578 kg Ha-1) than Head cabbage (461 kg Ha-1). Conclusions The survey established that P. xylostella was abundantly existent in Iringa and Morogoro regions wherever Brassicas are grown. Brassica production was not without the application of insecticides in fear of damages inflicted by P. xylostella. Interrupted mating through irrigation water somehow reduced the pest incidences but no significant variation was recorded with varied timing of irrigation. Insecticides application significantly minimized the number of P. xylostella and its effects on brassicas. Thus, the application of insecticides remained one most effective control strategy against P. xylostella.