Environmental and cassava variety influence on dynamics and impacts of cassava green mite, Mononychellus tanajoa (Bondar) (Acarina: Tetranychidae) in Tanzania

Abstract

In Tanzania, cassava (Manihot esculenta Crantz) is an integral component of most cropping systems and is among the most important staples in many zones. It is an essential component as a food security crop and provides useful opportunities for extending labour use and sometimes considered as a famine reserve when cereals fail due to its drought tolerance. Cassava is attacked by several pests, among which cassava green mite, Mononychellus tanajoa (Acarina: Tetranychidae) is most important. Mononychellus tanajoa can cause a reduction of about 50% in leaf weight and up to 80% tuber yield loss. It is therefore a pest that can pose multi–dimensional problems that require multi–dimensional solutions. The current cassava productivity in Tanzania is near the lower end of internationally reported yields. Mononychellus sp. was first reported in Tanzania in 1972 at Ukerewe Island. At present cassava green mites have spread throughout the country and are prevalent in all the cassava growing zones of Tanzania. The current study aimed at establishing M. tanajoa pest status and distribution in the three major cassava growing zones. It also studied the damage inflicted on some commercial and local cassava varieties and examined the influence of the environment and seasons on the pest in Tanzania. Specifically the study aimed to i) determine the spatial distribution of M. tanajoa on commonly grown cassava varieties in the Eastern, Lake and Southern zones of Tanzania ii) evaluate the environmental influence on occurrence and perpetuation of M. tanajoa on selected cassava varieties and iii) assess the abundance and associated damage of M. tanajoa on commercial and popular cassava varieties in the Lake zone. The studies were conducted in the three major cassava producing zones (Lake, Eastern, and the Southern Zones) of Tanzania. The surveys were conducted in farmers’ fields during 2015 and 2016 dry seasons in the three Zones. A total of 5400 plants in 180 fields were assessed during 2015 and 2016 for M. tanajoa infestation and associated damages. Collected data were subjected to non-parametric analysis in SPSS and Kruskal-Wallis test was used to compare between the pest counts and distributions. Weather parameters were analyzed against the pest counts and associated damages. Results revealed that M. tanajoa counts were significantly (P<0.05) varied with the Lake and Southern zones recording the highest numbers compared to the Eastern zone. The assessed cassava varieties significantly (P <0.05) influenced M. tanajoa counts while cropping systems had no effect on insect counts in both 2015 and 2016. The relationship among the environmental variables, rainfall (β = -0.406, p<0.0.007), maximum temperature (β = -14.35, p<0.010) except relative humidity (β = -0.0054, p<0.524) were significant (P <0.05) and negatively related to M. tanajoa counts in 2015 while in July 2016, only rainfall (β = -0.1091, p<0.001) was significant (P <0.05) and negatively correlated to M. tanajoa. The survival, perpetuation and distribution of M. tanajoa were attributed to the differences in varietal preference, crop age and weather variables. Field experiments were conducted to evaluate the environmental influence on pest occurrence and associated damages at three (3) sites namely Ukiriguru, Ng’ombe (Misungwi district) and Kishiri (Kwimba district) in the Lake zone during the 2014/2015 and 2015/2016 cassava growing seasons. Cassava varieties were established in a Split Plot Design and replicated three times. Treatments included the experiment location (main factor) and varieties (sub-factor) under natural infestation by the mites. Kyaka variety was considered as resistant check to M. tanajoa. The findings from this study suggested that location within a zone has limited influence (P>0.05) on M. tanajoa counts. The highest counts were recorded during the dry season in both years. Young, succulent and actively growing foliage were highly infested compared to the mature and old foliage. Almost all the tested cassava varieties sustained great counts of M. tanajoa and were likewise susceptible except Kyaka and Meremeta varieties. The tested weather variables negatively (rainfall) or positively (temperature) affected the survival, perpetuation of and subsequent damages by M. tanajoa in both years. The interaction effects of time (months) and locations on M. tanajoa damage was significant (P<0.05). Another field experiment was set for two consecutive years (2014/2015 and 2015/2016) at Ukiriguru to determine the response of the eight commercial cassava varieties and one local variety (used as susceptible check) to M. tanajoa. The experiment was laid out in a completely randomized block design (CRBD) with nine varieties as treatments in three replications. Results indicated that Liongo Kwimba recorded significantly (P<0.05) highest damage, leaf malformation and subsequently low yield (9.51ton/ha) compared to other varieties especially Kyaka and Meremeta. The least preferred and damaged by the pest, Kyaka variety recorded the highest yield (43.235 ton/ha). Leaf damage and malformation were significantly (P<0.05) higher in 2016 compared to 2015. Highest leaf damage and malformation were recorded in June to August and the lowest in March to May with varied fluctuations among varieties in the rest on the months. Based on the findings, it was concluded that M. tanajoa was present in all cassava growing areas of the country and most of the grown varieties succumbs to the pest. The environment and weather influence on the M. tanajoa counts and damages were varied across sites with rainfall and maximum temperature affecting the pest negatively while relative humidity and minimum temperature promoted the pest survival and perpetuation. Most of recently released commercial varieties also succumbed to M. tanajoa except Kyaka and Meremeta that were resistant. Therefore, promotion of these varieties across the country will be imperative for improved cassava productivity in Tanzania.