Records of Arthropod Species Sampled from Avocado Plant (Persea americana Mill) in Small-scale Agro-ecosystems at Taita Hills and Mount Kilimanjaro

Avocado, Persea americana Mill, plays a central role in distribution of both beneficial and detrimental arthropods thereby influencing local species diversity in agro-ecosystems adjacent to Afromontane forests at Mount Kilimanjaro in North-eastern Tanzania and Taita Hills in South-eastern Kenya. However, little is known about arthropod species that inhabit avocado trees in the two study areas despite the fact that the crop forms the major part of agro-ecosystem in the East African highlands. A novel survey was, therefore, carried out for two years between August 2012 and July 2014 to establish arthropod species in avocado orchards along South-eastern slopes of both Mount Kilimanjaro and Taita Hills. A total of sixty one species of arthropods were recorded from the avocado crop through fruit observation and canopy sampling. The present arthropod checklist provides baseline knowledge for scientists in evaluating beneficial and pest status of each species inhabiting avocado plant in the East African agro-ecosystems.


INTRODUCTION
Avocado, Persea americana Mill (Lauraceae), is an important crop in the world as it enhances both agroforestry conservation concept and nutritional security (Griesbach, 2005;Bergh, 1992). The avocado trees thrives well in agro-ecosystems with relatively high altitude between 1000m a.s.l and 2600m a.s.l that receive average annual precipitation ranging from 120mm to 160mm and average temperature of 21 0 C (Griesbach, 2005;Wasilwa et al., 2004;Whiley, 2002). Such ecosystems are located near indigenous forests with favourable agricultural conditions as exemplified by Afrotropical highlands at Mount Kilimanjaro and Taita Hills where avocado is the dominant fruit crop (Griesbach, 2005;Wasilwa et al., 2004). However, the potential land area available for avocado farming along slopes of Mount Kilimanjaro in Northeastern Tanzania and Taita Hills in South-eastern Kenya is shrinking as a result of ecological degradation (Conte, 2010) and human activities. The envisaged reduction of avocado orchards in these East African highlands will not only affect distribution of arthropod species but also livelihood of local farmers that depend on avocado fruits as a source of cash and nutritious food (Hemp, 2009). Eight five percent of avocado production in Kenya and Tanzania is at small scale level with number of trees per farm varying from three to twelve where the crop is grown mainly for subsistence and local markets (Griesbach, 2005;Wasilwa et al., 2004). Unlike commercial plantations, the small-holder avocado cropping systems do not utilized modern-day agricultural technology leading to poor farming practices, possible increase in arthropod pests and reduction of related natural enemies (Ware et al., 2016;Ware et al., 2012;Mwatawala et al., 2009, Griesbach, 2005Bale et al., 2002;Bergh, 1992). Moreover, there is limited information on arthropods inhabiting avocado crop in Kenya and Tanzania. This paper was, therefore, initiated to provide a checklist of arthropods sampled from different parts of avocado crop in farmlands at Mount Kilimanjaro and Taita Hills. Establishing checklist of arthropods inhabiting avocado plant in East Africa can furnish important baseline information on pest and beneficial status of each species

Study areas
The study was carried out in avocado farmlands adjacent to East African montane forests at Mount Kilimanjaro in North-eastern Tanzania and Taita Hills in South-eastern Kenya (Fig 1a and b, respectively). Avocado crop is grown in the two study transects along altitudinal gradient from 900 to 2000m a.s.l. and form major part of agro-ecosystem in the region. Mount Kilimanjaro and Taita Hills are the first uppermost elevated montane forms inland from the Indian Ocean and these highlands are important catchment areas for surrounding lowland areas of Moshi and Voi in Tanzania and Kenya, respectively (Hemp 2006a;Hemp 2006b;Bytebier, 2001;Bennun & Njoroge, 1999). The two study areas; Taita Hills and Mount Kilimanjaro, are situated 90km apart and both are about 150km from Indian Ocean. Mount Kilimanjaro study area is located between 03 0 378 ' S, 37 0 450 ' E and 03 0 481 ' S, 37 0 456 ' E in North-eastern Tanzania (Fig 1a). Mean elevation of Mount Kilimanjaro study area is 1372.69m a.s.l, average annual rainfall was 107.83mm, an average annual temperature of 20.14ºC with a mean annual humidity of 78.97%. Taita Hills study area is located in South-eastern Kenya, 25km west of Voi town in the Taita-Taveta County between 03 0 481 ' S, 38 0 378 ' E and 03 0 402 ' S, 38 0 296 ' E (Fig 1b). Mean elevation of Taita Hills study area is 1397.02m a.s.l, average annual rainfall was 135.19mm; mean annual temperature was 19.56ºC with a mean annual humidity of 81.46%.

Sampling design
Species of arthropods were sampled randomly from avocado plants for two consecutive years between August 2012 and July 2014 along each study transect; Mount Kilimanjaro and Taita Hills. A transect comprised of fifteen blocks with each block consisting of at least a hundred avocado trees. During each survey, five avocado trees from the hundred sampling unit of the plants at each block were randomly examined for arthropods using protocol described by Ekesi et al (2006), Stibick (2006), Palmer (1990) and Moritz et al (2013). Leaves and flowers were gently shaken on a tray to sample species that inhabits plant parts as described by Palmer (1990). However, avocado fruits were observed for other arthropod species as described by Ekesi et al (2006). Some species were handpicked from the avocado the plant using fine forceps and aspirator following protocols described by Millar et al (2000). The collected specimens were preserved in vials containing 60% ethyl alcohol and later taxonomically identified at the National Museums of Kenya (NMK) entomology laboratory in Nairobi.

Statistical analysis
Rank abundance test was used to categorize arthropod species based on their population using Biodiversity-R software (R Development Core Team, 2012). Species accumulation curves were used to compare if the observed species richness at two study areas along slopes of Mount Kilimanjaro and Taita Hills reach saturation point (R Development Core Team, 2012). In order to check for the completeness of sampling, observed numbers of species were compared with projected ones (R Development Core Team, 2012). The estimated species richness was constructed for each study area using non-parametric estimators; Chao, Jackknife 1 and Bootstrap (R Development Core Team, 2012; Crawley, 2007;Crawley, 2005).

III. RESULTS
A total of sixty one species of arthropods were recorded inhabiting avocado plants in the farmlands adjacent to Afromontane forests within Mount Kilimanjaro in Northeastern Tanzania and Taita Hills in South-eastern Kenya.

IV. DISCUSSION
Our results revealed that the Asian invasive fruit fly (Bactrocera dorsalis), False codling moths (Thaumatotibia leucotreta), common blossom thrips (Frankliniella schultzei) and the Greenhouse thrips (Heliothrips haemorrhoidalis) are the most abundant arthropod species inhabiting avocado plant parts at Mount Kilimanjaro in Tanzania and Taita Hills in Kenya. Bactrocera dorsalis was recorded from ground collected ripened avocado fruits whereas Thaumatotibia leucotreta was sampled from immature avocado fruits confirming them as fruit a scenario that has been reported by Mwatawala et al (2009) and Prinsloo and Uys (2015), respectively. Frankliniella schultzei was recorded exclusively feeding on flower resources, however, Heliothrips haemorrhoidalis was recorded from leaves and young fruits proving to be pests of avocado crop as reported in Palmer (1990) and Prinsloo and Uys (2015). Beneficial or pest status of each of the species sampled was not evaluated in this novel paper since our study focused on generating the checklist of arthropods that inhabits avocado plant. However Thaumatotibia leucotreta are economically important pests of avocado fruits. Moreover, beneficial insects such as honey bees (Apis melifera) was exhaustively described pollinating avocado flowers at Taita Hills in South-eastern Kenya by Luvanga (2015). The failure of the species accumulation curves to reach saturation point indicated that more species were likely to be found if additional sampling effort continued at Taita Hills transect than at Mount Kilimanjaro study area. This hypothesizes that if surveys are conducted over multiple years, the species accumulation curves for arthropods are expected to reach saturation point probably due to a larger proportion of rare species which are often recorded in single encounters (Novotny and Basset, 2000). Most of the rare species in this study were Thysanopteran non-pest thrip species that contributed to the high projection of species richness for Taita Hills using Chao; a non-parametric estimator (R Development Core Team, 2012). Taita Hills study areas. The present checklist provides baseline data for scientists to identify species of economic importance which can assist in designing strategies for avocado pest control and pollination services in East Africa. In order to enhance avocado farming strategies at the two study areas, spatiotemporal data is required for each species recorded. This study, therefore, recommends research on how the most abundant arthropods (Bactrocera dorsalis, Thaumatotibia leucotreta, Frankliniella schultzei and Heliothrips haemorrhoidalis) impacts on avocado farming at Mount Kilimanjaro and Taita Hills. The study, further, recommends research on possible beneficial insects such as Apis mellifera (Hymenoptera: Apidae), Oecophylla longinoda (Hymenoptera: Formicidae) and Camponotus maculatus (Hymenoptera: Formicidae) in the avocado orchards along slopes of Mount Kilimanjaro and Taita Hills.