Preliminary Studies of Insect Diversity and Abundance on Twelve Accessions of Tomato , Solanum lycopersicon L . Grown in a Coastal Savannah Agro Ecological Zone

The relative abundance and diversity of insect species were studied for three months, between the months of June and August 2013 on twelve different accessions of tomato (Solanum lycopersicon L.) at the Ghana Atomic Energy Commission’s Biotechnology and Nuclear Agriculture Research Institute (BNARI) farm. The main objectives of the study were to determine the insect diversity and the relative abundance of the insect species on twelve tomato accessions. The field was divided into four replicates each containing twelve different accessions of tomato coded V1, through to V12. In general, there was lower abundance of insect species across tomato accessions. The study revealed higher abundance of Bemisia tabaci (Gennadius) (Hemiptera, Aleyroidae) on accession V11 followed by V1 with V6 having the least number. Bemisia tabaci accounted for 95.5% of the insect counts. Low diversity of insect species was observed across the field of study. B. tabaci, Omocestus viridulus, (Linnaeus) (Orthoptera, Acrididae) and Psylla mali (Schmidb) (Hemiptera, Psyllidae) were observed to occur on all the accessions. Accessions V6 and V11 have the highest diversity of insects while accession V3 has the least diversity. The other major insect pests such as Aphis craccivora (Koch) (Hemiptera, Aphididae), Phenacoccus sp., Podagrica sp. and Zonocerus variegatus (Linnaeus) (Orthoptera, Pyrgomorphidae) recorded very low percentages. The highest record of insect pests was recorded at the fruiting stage of the plant’s development. Fourteen different insect species were recorded giving an indication of the species diversity of the farm.


Introduction
Tomato, Solanum lycopersicon L. (syn.Lycopersicon esculentum Mill.) is one of the most consumed vegetables in the world and global production is estimated at around 136 Billion metric tons per year (FAOSTAT, 2008).Tomato is the third most economically important vegetable crop after potato and onion.Major production countries in descending order include China, USA, India, Turkey and Egypt respectively.Africa contributes 15% of the world's tomato production (FAOSTAT, 2008).Globally, tomato is the most important greenhouse vegetable crop with a production of 720 MT and a total value of $170 Million per year (FAOSTAT, 2008).
Tomato is a dietary source of vitamins especially A and C, minerals and fiber, which are important for human nutrition and health.In addition, tomato is the richest source of lycopene, a phytochemical that protects cells from oxidants that have been linked to human cancer (Giovannucci, 1999).
In Ghana, tomato is a popular vegetable with high per capita consumption as it is used in almost all Ghanaian homes.Tomato production is a source of employment and income to both rural and urban dwellers.It contributes significantly to the economic growth of Ghana and source of foreign exchange.Tomato is the most important crop in recently established dry season gardens in Northern and Upper Regions and in southern Volta Region of Ghana (Obeng-Ofori et al., 2007).
Total production cost for an acre of fresh market tomato is approximately $6,000 to $7,000, with nearly 25 percent of costs related to pest management (Olson et al., 2005).This cost is mainly borne by farmers and this ultimately increases the cost of production of the tomato and farmers find it difficult to break even all due to the damage posed by arthropod pests.
Tomato production in Ghana has been facing many biotic and environmental constraints.Prominent among such constraints are pests and diseases which reduce yields and the quality of marketable fruits.In the tropics, particularly in Ghana, many insect pests are associated directly with tomato damage and yield losses while some others are most important as vectors of diseases (Messiaen, 1992;Tindall, 1983).
In the field, leafminers, stink bugs and fruitworms cause maximum damage to the tomato fruit.However, severe damage may result either from their feeding on the fruit or by spreading diseases.These insect pests attack tomato from the time plants first emerge in the seed bed until harvest (Webb et al., 2001).At least 27 arthropod pests have the potential of seriously reducing both yields and the market value of tomato fruit (Bloem & Mizell, 2004).According to Horna et al. (2008) and Gianessi (2009), fresh tomato yield losses in Ghana can be as high as 64%.
Even though farmers complain incessantly, there are no estimates of abundance and diversity of insects on tomato production in Ghana.This knowledge gap makes it impossible to plan and implement any strategies to control insect pests affecting tomato production in the country.Therefore, quantitative studies on the abundance and diversity of insects in any growing area vis-a-vis reduction in crop yield must be carried out in order to establish economic thresholds necessary for instituting control measures.This information will guide stakeholders and farmers in making informed decisions as to when to institute control measures.It is in this vein that this study was carried out to determine the abundance and diversity of insect species on twelve different accessions of tomato.This would assist us develop an Integrated Pest Management (IPM) strategies for tomato cultivation and ultimately boosting the production of this important commodity.

Study Site
The experiment was conducted at one of the experimental plots at the Ghana Atomic Energy Commission's Biotechnology and Nuclear Agriculture Research Institute farm from June to August, 2013.This period was the major planting season for growing tomato in Ghana.The study site was located about 20 km north of Accra (5º40′36.6″N and 0º11′52.5″W), with an elevation of 76 m above sea level.The vegetation is Coastal Savannah, and the area is characterized by a bimodal rainfall pattern with the major rain season falling between March and June, and a minor rainy season around October.The mean annual rainfall is 810 mm distributed over less than 80 days, and temperatures are moderate with maxima rarely exceeding 32 ºC while the minimum does not fall below 17 ºC.

Land Preparation and Experimental Design
An acre area that has been left fallow for over two years was used.No tomato has ever been planted on any part of the research field.Border effects were considered in setting up the research field to cancel out the activities of suspected insects hibernating in neighboring alternative host plants.Stumps were removed from the field before being ploughed.Disc harrowing was done a week after ploughing.No fertilizer or manure was applied to the experimental plots.Seeds were nursed on 14 th May, 2013 and transplanted on 6 th June, 2013 onto an experimental plot measuring 28 m × 24 m in the center of the one acre area so that the experimental plot was surrounded by a homogeneously managed terrain.Each replicate was allotted a plot size of 143 m 2 .Twelve tomato accessions (treatments) were tested and coded from V 1 to V 12 .The parent plant, Wosowoso, Roma, Wild and New Wild were obtained from a seed shop at Tudu market, Accra, Ghana and Cherry yellow tomato fruits were obtained from Shoprite, Accra Mall, Ghana and seeds extracted.The parent plants were crossed and selection made based on desirable traits to obtain hybrids from which these accessions (V 1 -V 12 ) were obtained.
The experimental plot was subdivided in four sub-plots.Each sub-plot hosted one replicate of all the accessions under observation.Each replicate consisted of 20 plants of the same accession, transplanted with a space of about 0.45 m, along rows that had a distance of 1.5 meters to each other.The experimental treatments were deployed in a Randomized Complete Block Design (RCBD), replicated four times.Blocking was done to cater for any variability in the tomato accession.Plots were separated by 2 meters.Weeding was done by hoe and neither irrigation nor pesticides were used during these experiments.Observations were recorded starting from one month of planting when foliage starts to appear till when harvesting time was due.

Sampling of Entomofauna on Tomato Plants
Weekly scouting for insects was made by visual observations of insect occurring throughout all above ground plant parts of 5 randomly selected plants per accession per replicate (i.e. 5 × 12 × 4 = 240 plants selected).Visual surveys have been shown to be an effective and efficient method for censuring insect species richness and abundance on a variety of host plants including soybean (Mayse et al., 1978a(Mayse et al., , 1978b)).The leaves in each selected plant were observed (naked eye) from the base of the stem to the crown.Plots were visited from 5:00 am to 9:00 am since this is the time most insects were less active.Counting was done on the insect species that were found and recorded on a data sheet.Thirty minutes was spent on each replication to thoroughly take an inventory and observe behavior of the insect species.Only adult insects were collected, recorded and kept in 70% alcohol.Identification to family, genus and species level and curation of the insects were done in the laboratory using insect voucher specimens, CAB International manual keys and descriptions.The data were analyzed by performing an analysis of variance (ANOVA) at 95% level of significance, using the statistical package for agricultural sciences Genstat Software version 12 release 12.1 (Genstat, 2009).The least significant difference (LSD) was used to separate the means of treatments that showed significant "F"values.The diversity of the experimental area was calculated using Simpson index, Ds = 1-Σ (n i (n i -1))/(N(N-1)) where Ds = Simpson's index of diversity; N = total number of individuals of all species; n i = total number of individuals of the species i.

Results
Table 1.Insect species recovered from three developmental stages of tomato in Kwabenya, Ghana

Adult
Table 3. Mean numbers of dominant insect pests counted on twelve accessions of tomato Podagrica sp.2.0 a 2.0 a 0.0 a 1.2 a 0.5 a 3.5 a 1.0 a 0.0 a 0.5 a 0.8 a 2.0 a 3.0 a Psylla mali 2.0 a 3.0 a 3.2 a 1.8 a 3.0 a 3.5 a 7.0 a 2.0a 2.8 a 5.2 a 3.0 a 0.0 a Aphis craccivora 1.0 a 2.0 a 2.2 a 5.2 a 0.0 a 1.8 a 0.0 a 0.0 a 0.2 a 0.0 a 0.0 a 2.0 a Omocestus viridulus 1.0 a 1.0 a 0.2 a 0.8 a 3.8 a 3.0 a 1.0 a 0.0 a 1.2 a 0.5 a 0.0 a 0.0 a Phenacoccus sp.1.0 a 3.0 a 0.0 a 1.5 a 1.0 a 1.0 a 1.0 a 0.0 a 1.2 a 1.2 a 0.0 a 1.0 a Means followed by the same letters within the column are not significant (P < 0.05).
Analysis of variance showed that there was significant difference in the mean number of insect species collected from the twelve different accessions of tomato (P < 0.05).There was no significant difference between the different insect species among the twelve different accessions of tomato except B. tabaci (Table 3).Simpson's index of diversity, Ds = 0.0876.
Occurrence of Podagrica sp. in large numbers may destroy entire leaves.

Abundance of Insect Species on Twelve Different Accessions of Tomato
A total of 10,562 insects were collected belonging to fourteen different families (Table 2).B. tabaci population was 10,087 representing about 95.5% of the total population of insects species collected (Table 4).Other major insect pests namely; A. craccivora has a total population of 0.50% of relative abundance, Phenacoccus sp.(0.46%), O. viridulus (0.53%), Podagrica sp.(0.45%) and P. mali (1.16%) (Table 4).Accession V 11 had the largest population of 1,115 specimens of insects.This is followed by accession V 2 with an insect count of 1,036 while least number of insects was found on V 6 (645).The rest of the accessions V 3, V 4 , V 5 , V 7 , V 8 , V 9 , V 10 , and V 12 had insect counts between nine hundred (900) and seven hundred (700).The total population of insect species varied.No single accession was heavily populated by all insect families (Table 2).

Economic Status of the Identified Insect Species on Twelve Different Accessions of Tomato
The major insect group in the study belongs to the Aleyrodidae family which was B. tabaci.This insect group in their numbers poses a great danger to the development and survival of the tomatoes with respect to the harm they cause to the plant.High numbers of B. tabaci could cause total crop loss due to the sucking behavior and indirect transmission of virus as well as the development of sooty mould on the plant.A large insect group was found to be associated with the tomato accessions (  6).The sucking of sap from the tomato plant by these insects resulted in stunted growth, yellowing of the leaves, wilting, fruit drop, and premature fruit ripening.Defoliators such as Z. variegatus, O. viridulus, and Podagrica sp. were also observed to defoliate the leaves of the tomato plant.The small holes created in the leaves of tomato by Podagrica sp. could ultimately affect the total photosynthetic area of the leaf resulting in poor yield of tomato.Three beneficial insects (predators) were also recorded in this work namely; Camponotus sp., M. religiosa and C. lunata (Table 1).

Abundance of Insect Species on Twelve Different Accessions of Tomato
Accession V 11 which has the highest abundance of insect species may be due to certain attributes of the plant which make it more susceptible to attack by most of the insects in Table 2.There was significant difference in the mean number of B. tabaci at P < 0.05 among the twelve different accessions of tomato (Table 3).The high numbers of B. tabaci is due to the short season after major raining season in the area (Morales & Jones, 2004).B. tabaci numbers increases mostly when there is little or no rainfall.The low percentages recorded by other major insect pests do not pose a major threat to the crop due to their lower abundance.The numbers are not large enough to cause economic injury and ultimately reducing the yield of the plant.In general it can be said that there was low abundance of insect species on the tomato plot, giving an indication that a lot less number of individual insect species is associated with the tomato plant.The low numbers of A. craccivora in this study could be attributed to the predatory behavior of C. lunata.C. lunata tend to feed on A. craccivora and therefore reducing the total population of aphids (Xue et al., 2009;Ofuya, 1989).The least abundance of insect group on accession V 6 may confer tolerant attributes to that accession and therefore enhances it resistance to insect attack.

Diversity of Insect Species on Twelve Different Accessions of Tomato
During the study period, a total of 14 insect species were identified on the tomato plants from transplanting to fruiting (Table 2).The Simpson's diversity index, Ds calculated showed that the experimental area is less diversified.This is due to the fact that species diversity (species heterogeneity) is an expression of community structure.A community is said to have high species diversity if many equally or nearly equal abundant species are present.Conversely, if a community is composed of a very few species or if only a few species are abundant, then species diversity is low.Identification of these 14 different insect species on the tomato accessions within this period of the year gives an indication of how less rich the insect population on tomato in this agro ecological area is.Six of these insect species namely; Z. variegatus, A. craccivora, Podagrica sp., B. tabaci, Dysdercus sp.(Figure 4), and N. viridula, have been documented to be serious pests of tomatoes worldwide (Lange & Bronson, 1981).Minor pests such as O. viridulus, Phenacoccus sp., and G. campestris have also been recorded on some accessions of tomato.The major pest, B. tabaci recorded in this study confirms a similar study by Arno et al. (2008).They found out that B. tabaci is capable of causing severe losses even at low densities due to the range of plant viruses it can transmit.Purcell et al. (1993) identified five different insect pests as major pests and nine beneficial insects on tomato which were mainly parasitoids.The beneficial insects in this study can be exploited and packaged in an IPM strategy to combat the major pests in the study.It can, however, be established that the non-prevalence of fruit worm (Figure 1) and borers could be due to early harvest and short duration of the tomato plant growth.The borers did not have enough time to colonize the tomato plant.Contrary to the results of Mailafiya et al. (2014), who found only three main insect pests namely Helicoverpa armigera Hubner (Lepidoptera: Noctuidae), B. tabaci (Gennadius) and Spodoptera littoralis (Boisd.)(Lepidoptera: Noctuidae) to be present on tomato, B. tabaci seems to be common to this study and this could be due to the invasiveness of this insect.Umeh et al. (2000) described these species as very common on tomato crop in a survey carried out in Oyo State where they recorded seven insect species common on tomato.Three of these recorded insects are present in this study.These common insects shared by the two studies include variegated grasshopper, whitefly and aphids.In Ghana, Sam et al. (2014) recorded five different insect species on tomato namely; B. tabaci, Thrips tabaci Lindeman, A. gossypii, Liriomyza sp. and the tomato fruit worm, H. armigera as the most important insect pests collected on tomato in Kumasi, Ghana.

Conclusion
A large number of experiments carried out, under different conditions, indicated a reduction in pest activity with diverse vegetation if compared with monoculture.The present study showed low diversity of insect species on the tomato plant.Fourteen different insect species have been recorded belonging to fourteen different families.
Additionally, low abundance of insect species was recorded on the tomatoes despite B. tabaci accounting for 95.5% of the insect population.By considering the impact of insect pests on tomato, we can accurately predict pest population dynamics.This should assist research and extension personnel in designing integrated pest management programmes for tomato.

Figure 1 .
Figure 1.Fruit worm and its damage on tomato Figure 2. Zonocerus variegatus on tomato fruit 242.0 b 213.5 b 155.0 b 188.0 b 178.8 b 208.0 b 236.0 b 231.2 b 178.0 b 254.0 b 238.0 b

Table 2 .
List of insect species and their abundance on twelve accessions of tomato in Kwabenya, Ghana