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Effect of fertiliser and mulching on bean infestation and damage by bean fly

S. Byabagambi, S. Kyamanywa and M.W. Ogenga-Latigo
Department of Crop Science, Makerere University, P . O. Box 7062, Kampala, Uganda

The damage caused by the bean fly (Ophiomyia sp.), which is the major insect pest of beans in Uganda, is more serious in poor soils and under drought conditions. There is, however, limited information on cause and effect. This study investigated the relationship between fertiliser and the influence of moisture conservation through mulching on bean fly infestation. Fertiliser application significantly increased bean fly oviposition per leaf, number of pupae per plant and plant mortality. Mulching also significantly reduced bean fly pupae density and bean plant mortality. Mulching increased soil moisture content and there was a significant negative relationship between soil moisture content and bean fly pupae density. The influence of fertiliser and mulching on bean fly damage is discussed based on nitrogen and water relationships in plant growth.

Key Words: Moisture, nitrogen, pest infestation, Ophiomyia sp., Phaseolus vulgaris

Les dégâts causés par la mouche du haricot (Ophiomyia sp.), l’insect-peste majeur du haricot en Uganda, sont plus graves dans les conditions de sols pauvres et de sècheresse. Les informations sur la cause et effet sont cependant limitées. Cette étude a examiné la relation entre l’engrais et l’influence de la conservation de l’humîdité par paillage sur l’infestation de la mouche du haricot. L’application du a augmenté significativement l’oviposition de la mouche du haricot par feuille et le nombre de pupes par plante. Le paillage a réduit significativement la mouche du haricot, la densité des pupes et la mortalité de plantes du haricot. Le paillage a augmenté la teneur en humîdité du sol et une relation négative a été établie entre la teneur en humîdité du sol et la densité en pupes de la mouche du haricot. L’influence de l’engrais et du paillage sur les dégâts de la mouche du haricot est discutée en se basant sur la relation de l’azote et de l’eau dans la croissance de la plante.

Mots Clés: Humidité, azote, infestation de la peste, Ophiomyia sp., Phaseolus vulgaris

The bean fly (Ophiomyia sp.) is one of the most important insect pests of beans (Phaseolus vulgaris L.) in East Africa, and Uganda in particular (Greathead, 1968; Nyiira, 1978; Karel, 1991; Oonyu, 1992; Katwijukye, 1998). The adult bean fly attacks mainly bean seedlings. Infested plants are characterised by yellowing or wilting of leaves, which may drop prematurely, swelling and splitting of stems at the collar, which often result in death, leading to poor plant stand.

The damage by the bean fly is more pronounced in drought conditions or in soils of poor fertility (Greathead, 1968). However, studies by Letourneau (1994) and Byabagambi and Kyamanywa (1997) indicated that nitrogen fertiliser increased population density and incidence of bean fly, yet yield was not affected. On the other hand, mulching and earthing-up reduced bean fly pupae population density, wilting/yellowing symptoms and root damage (Ampofo, 1993; Letourneau, 1994; Byabagambi and Kyamanywa,1997). However, plant characte-ristics responsible for increased bean fly infestation on beans growing under fertilised soils, and reduced infestation in mulched and earthed-up beans are not clear. Because of this paucity of information, studies on effects of mulching and nitrogen-fertiliser application were conducted in green houses at Kabanyolo, Uganda with the objective of understanding the relationship between bean fly damage and fertility levels and mulching.

Two experiments, one on nitrogen fertiliser application rates and the other on mulching were conducted in 1997 in a greenhouse at Makerere University Agriculture Research Institute, Kabanyolo (MUARIK). In both experiments, bean plants were grown in 50 cm diameter plastic basins each filled with 20,000 cc of soil.

Effect of nitrogen fertiliser on bean fly damage. The approach used by Ogenga-Latigo and Khaemba (1986) to provide contrasting levels of soil fertility was adopted for varying nitrogen fertiliser rates. The soils used consisted of an equal mixture by volume of white sand and soil from plots on which beans had previously been grown. Because of the high proportion of sand and the use of exhausted soil in the mixture, this type of soil was considered to be low in available plant nutrients and was taken as a basis on which the following nitrogen-fertiliser treatments were applied:

* Soils with low nutrient level: These consisted of white sand and soil from plots on which bean plants had previously been grown (experimental soils) i.e., control.

* Soils with medium nutrient level: These consisted of experimental soils to which 1.5 gm of urea fertiliser was added. This was equivalent to 30 kg N ha^-1.

* Soils with high nutrient level: These consisted of experimental soils to which 3.0 gm of urea fertiliser was added. This was equivalent to 60 kg N ha^-1.

* Soils with very high nutrient level: These consisted of experimental soils to which 6.0 gm of urea fertiliser was added. This was equivalent to 120 kg N ha^-1.

For all the treatments, an equivalent of 30 kg ha^-1 of K (K₂O) and 30 kg ha^-1 of P (P₂O₅) were added to cater for potassium and phosphorus, respectively. Four basins were used to make up a plot for each of the treatments. The experiment was laid in a completely randomised design with three replications. A 50 cm distance was left in between the different plots and replicates. Twenty seeds of K-131 bean variety were planted in each basin. Five days after bean emergence (DABE), all the basins in their respective treatments were transferred to a bean field to allow for natural bean fly infestation. The basins were kept in the bean field for 20 days and then at 26 DABE, they were transferred back to the greenhouse where data were taken on plant stand and leaf area index: The leaf area index was determined at 26 DABE using the method of Edje and Osiru (1987).

Using a vernier calliper, the diameters of stems (at a height 5 cm from the collar) from the six sample plants were measured and recorded in millimeters (mm). The number of bean fly punctures per leaf were determined at 26 DABE. All leaves on the six plants selected from each treatment were examined using a hand lens, and the bean fly puncture per leaf counted.

Percentage plant mortality was determined by counting all the dead plants at 28, 35, 42 and 49 DABE. The dead plants were brought to the laboratory, dissected and the number of pupae per plant recorded. Subsequently, cumulative percentage plant mortality for all the treatments during the four sampling occassions was determined.

Effect of mulching on plant characteristics and bean fly damage. To determine the effects of mulching on plant characteristics and bean fly damage, bean plants were grown in 24 plastic basins filled with soils of the medium nutrient level described above. Plants in 12 basins were mulched with spear grass (Imperata cylindrica), while the remaining were not mulched. Mulching was done before bean plant emergence. Five days after bean emergence (DABE), all the basins were transferred to a bean field to allow for natural bean fly infestation. At 26 DABE, they were returned to the greenhouse. Data were collected on plant stand, leaf area index, bean fly punctures per leaf, soil temperature and soil moisture content. Soil temperatures were measured at a depth of 6 cm at 28, 35, 42 and 49 DABE using a hand thermometer. Soil moisture content was determined by taking soils from each treatment, sampled at a depth of 6 cm, weighed and thereafter air dried for one week and then reweighed. The moisture content was calculated as;

Soil moisture measurements were at 28, 35, 42 and 49 DABE. Roots/root premodia numbers were established by uprooting six plants at 50 DABE. The plants were carried to the laboratory and the root system washed clean with water, and the number of roots and root premodia were counted.

Data collected were subjected to analysis of variance. Correlations were run to assess the relationship between bean fly damage and plant growth characteristics. All analyses were conducted using the MstatC computer package.

Effect of different nitrogen-fertiliser rates. All plots treated with nitrogen fertiliser had significantly higher leaf area index (LAI) compared to the unfertilised plots (Table 1). However, the differences amongst the individual nitrogen-fertiliser rates on LAI and stem diameter were not statistically significant (P=0.05).

Table 1. Effect of different nitrogen fertiliser rates on leaf area index and stem diameter of bean plants

Fertiliser level (gm)	LAI	Diameter (mm)

0.0	0.51 + 0.02	4.63 + 0.09
1.5	0.60 + 0.01	4.90 + 0.09
3.0	0.58 + 0.01	4.85 + 0.03
6.0	0.62 + 0.00	4.73 + 0.08

C.V (%)	6.00	5.13
L.S.D (P=0.05)	0.06	NS

Application of nitrogen significantly increased the number of bean fly punctures per leaf, being higher with higher level of nitrogen application (Table 2). This was also true of pupae density per plant. It was also observed that the medium nutrient level had more pupae per plant compared to the high nutrient level. All the plots treated with the fertiliser had significantly higher mortality than where no fertiliser was applied (Table 2). Leaf area index was positively correlated with bean fly punctures, pupae density per plant and percentage plant mortality (Table 3). Similarly, bean fly leaf punctures were positively correlated with pupae density per plant, and percentage plant mortality with pupae density.

TABLE 2 Effect of different nitrogen fertiliser rates on number of bean fly punctures per leaf, pupae per plant and percentage plant mortality

Fertiliser level (gm)	Punctures per leaf	Pupae per plant	% Plant mortality

0.00	20 + 1.16	2.50 + 0.16	44.4 + 0.41
1.50	27 + 1.16	4.00 + 0.29	63.9 + 1.80
3.00	22 + 1.16	3.50 + 0.29	58.4 + 0.64
6.00	32 + 1.16	4.50 + 0.17	64.6 + 0.38

C.V (%)	2.0	2.8	9.1
L.S.D (P=0.05)	1.0	0.2	10.6

Table 3. Pearson’s correlation coefficient describing the relation between LAI, leaf punctures, % mortality, bean fly pupae per plant on K-131 bean variety at Kabanyolo

Index	Leaf area index	Puncture per leaf	% Plant mortality

Leaf area index
Puncture per leaf	0.783 ( 0.002)
% plant mortality	0.926 ( 0.000)	0.743 ( 0.005)
Pupae per plant	0.823 ( 0.001)	0.935 ( 0.005)	0.815 ( 0.001)

Effects of mulching. Mulching did not have statistically significant effects on LAI and bean fly leaf punctures (Tables 4 and 5). Nevertheless, mulched plants had slightly higher LAI while bean fly leaf punctures were slightly higher in the unmulched plots. Also mulched plots had higher numbers of roots/root premodia compared to the unmulched (Table 4).

Treatment	LAI	Roots/ Root premodia

Mulched	0.60 + 0.04	56.55 a + 2.20
Unmulched	0.58 + 0.04	46.15 b + 1.20

C.V (%)	9.59	1.93
L.S.D (%)	NS	3.48

Table 5. Effect of mulching on number of leaf puncture, pupae per plant, and percentage plant mortality

Treatment	Leaf punctures	Pupae density	% Plant mortality
Mulched	24.00 + 4.00	3.70 b + 0.08	58.75 b + 0.04
Unmulched	24.50 + 1.10	5.92 a + 0.10	80.00 a + 1.00

C.V (%)	2.98	4.40	3.82
L.S.D (P=0.05)	NS	0.75	9.32

Significant differences were observed in pupae density per plant, with the mulched plots having lower infestation than the unmulched (Table 5). Likewise, percentage plant mortality was significantly higher in the unmulched plants compared to the mulched, unlike soil temperatures which were significantly higher in the unmulched than mulched plots. On the other hand, soil moisture content was significantly higher in the mulched than unmulched plots (Table 6).

Treatment	% Soil moisture content	Soil temperature ºC

Mulched	16.98 + 0.44	21.70 + 1.05
Unmulched	11.63 + 0.42	24.50 + 0.35

C.V (%)	1.53	3.31
L.S.D (P=0.05)	0.77	2.68

The relationships between LAI, pupae density per plant, percentage plant mortality, soil temperature and soil moisture content were statistically significant (Table 7). Percentage plant mortality was positively correlated to pupae density per plant and soil temperature, indicating that as the number of pupae per plant and soil temperature increases the higher the plant mortality. Contrastingly, percentage plant mortality was negatively correlated to soil moisture. On the otherhand, there was a positive correlation between soil moisture content and root/root premodia numbers (Table 7).

Table 7. Pearson’s correlation coefficient describing the relation between leaf area index, % plant mortality, number of bean fly pupae per plant, soil temperature and soil moisture on K-131 bean variety at Kabanyolo

	LAI	% mortality	Pupae per plant	Soil temp (°C)	Soil moisture(%)

% Mortality	0.212 (NS)
Pupae per plant	-0.131	0.935
	(NS)	(0.003)
Soil temp (¡ C)	-0.041	0.919	0.909
	(NS)	(0.005)	(0.007)
Soil moisture (%)	0.172	-0.922	-0.980	-0.967
	(NS)	(0.005)	(0.000)	(0.001)
No. of Roots/Premodia per plant	0.542	-0.698	-0.879	-0.849	0.920
	(NS)	(NS)	(0.014)	(0.024)	(0.005)

Nitrogen fertiliser application increased LAI, bean fly leaf punctures, pupal density, and percentage plant mortality. Mulching also significantly affected pupal density per plant, percentage plant mortality, premodia number, soil temperature and soil moisture content.

Gallagher and Boscoe (1978), Graham (1983), Fukai (1984) and Mackeroon and Waister (1985) associated this increase in leaf surface area to the increase of soluble amino compounds and proteins which allows the leaves to grow larger. Our results also showed that LAI was positively correlated to bean fly leaf punctures, which were in turn positively correlated to pupal density per plant. Bean fly pupae density determines the level of bean fly induced mortality (Karel, 1985; Talekar and Lee,1988). The present study suggests that the bigger the leaf area, the greater the bean fly infestation. Besides the increased surface area for oviposition, the cell walls become thin and succulent which makes them more prone to pest damage. However, Byabagambi and Kyamanywa (1997) noted that whereas there were more bean fly pupae and root damage in nitrogen fertiliser treated plots, the grain yield attained was higher than in the unfertilised crop. It is likely that the increased benefit from nitrogen application compensates for the bean fly damage.

Mulching significantly reduced pupal density per plant and percentage plant mortality. These findings concur with those of Ampofo (1993), Letourneau (1994) and Byabagambi and Kyamanywa (1997). The mulched plots also had lower soil temperature and high soil moisture content. According to Ogenga-Latigo and Khaemba (1986) increased soil moisture content enhances uptake of nutrients by plants and hence improves crop vigour and tolerance to harsh conditions e.g., pest attack. However, Ampofo (1993) suggested that lower temperatures and increased soil moisture conditions do not favour the beanfly.

We concluded that nitrogen fertiliser application probably weakens the cell structure of the bean plant thereby promoting bean fly to lay eggs on weak, succulent leaves. Also larval and pupal establishment and development is easy in the weakened cell wall structures in the fertiliser applied plant tissues. Mulching on the other hand, influenced bean fly damage through its effect of increased soil moisture content, which adversely affects pupal survival, but also encourages better root development and plant crop growth which enhances tolerance to bean fly damage. Therefore a combination of nitrogen fertiliser application and mulching offers a compromising package for bean fly control and increased yields.

The work reported in this paper was supported by Forum grant from the Rockefeller Foundation.