Maruca vitrata (lima bean pod borer)

Round holes are produced in the corolla of the flowers by the larvae; the pods are distorted by larger larvae and affected by frass. Damage is caused by the larvae boring round holes in the corolla; flowers are converted to a mass of brownish frass in 24 hours.

Reviews of control practices are given by Sharma (1998) and Sharma et al. (1999).

Cultural Control

Intercropping studies conducted at the International Centre of Insect Physiology and Ecology (ICIPE) in Kenya over 10 years have identified sorghum and cowpea as the best crop combination in terms of minimizing the population of crop borers, stabilizing productivity and reducing yield loss due to crop borers (Chilo partellus, M. vitrata and Busseola fusca). The maize and cowpea dicrop and the sorghum, cowpea and maize intercrop were also found to be effective. The worst crop combination was an intercrop between maize and sorghum. The incorporation of resistant and tolerant cultivars in an intercropping system offered the added advantage (by reducing the pest attack) to farmers who for good reason had to plant the maize and sorghum dicrop (the worst combination). The use of resistant and tolerant cultivars offered an alternative (Omolo et al., 1993). Karel (1993) in Tanzania found that intercropping beans with maize was useful as a cultural method for controlling pod borers on bean and for higher seed yield of the two crops. Ekesi et al. (1996) found that planting cowpea within the first and second weeks of July would reduce damage by the pest. In Nigeria, Agboh-Noameshie et al. (1997) found that intercropping of cowpea with cassava increased the incidence of M. vitrata while reducing damage caused by other pests.

Host-Plant Resistance

In India, Venkateswarlu and Singh (1999) found that of 16 determinate genotypes tested, the greatest damage occurred to ICPL 94025 and the least to ICPL 4. Of the 16 indeterminate genotypes, Manak and ICPL 91031 received the greatest and least damage, respectively. Damage caused by Helicoverpa and Maruca was greatest in determinate genotype, while pod fly attack was greater in indeterminate genotypes.

In China, Huang (1999) studied 40 varieties of asparagus bean (Vigna unguiculata ssp. sesquipedalis) for their effects on a population of M. vitrata over 3 years. Densities of larvae in the flowers and beans differed significantly between varieties. Xinqing had the lowest larval populations in all 3 years.

In Nigeria, Bottenberg et al. (1998) found that host-plant resistance in TVnu 72 drastically reduced insect populations and damage. Grain yield per hill was high in IT86D-715 and was not affected by intercropping with millet. Seed yield of TVnu 72 was poor and reflected the low yield potential of this accession.

In studies in Karnataka, India, Veeranna (1998) screened 45 cowpea (Vigna unguiculata) genotypes for resistance to M. vitrata. Tolerant genotypes had higher phenol and tannin contents than did susceptible genotypes.

Veeranna and Hussain (1997) screened 45 cowpea genotypes for attack by M. vitrata in Karnataka, India, the most resistant (TVX-7) had a high trichome density (24.41/9 mm²), while the most susceptible (DPCL-216) had a low trichome density (12.82/9 mm²), confirming earlier findings that trichomes are important in reducing attack by the pest.

Saxena et al. (1996) compared damage caused by M. vitrata in determinate (DT) and indeterminate (IDT) lines of pigeon pea (Cajanus cajan), 271 short-duration genotypes were evaluated for percentage pod damage. Mean pod damage of DT and IDT lines was 65-75 and 40-50%, respectively and none of the lines had less than 10% damage. Number of days to 50% flowering in DT lines was 54-84 and was not related to pod damage. In IDT lines, days to 50% flowering was 60-87 days and had a significant negative correlation with M. vitrata damage (r = -0.43), showing that early flowering lines suffered more damage than late flowering lines. In general, pod damage by M. vitrata caused serious flower drop in both DT and IDT types. Recovery from pod damage was poor in DT lines but excellent recovery was recorded in IDT lines ICPL88034, ICPL87113 and MG679. DT lines appear to be more prone to M. vitrata damage because they have clustered inflorescences compared to IDT types which have long fruiting branches and loose inflorescences.

Late-maturing varieties of pigeon pea such as C11 and Berhampur local suffered less pod damage, particularly by Helicoverpa armigera and M. vitrata, than extra-early or early varieties in field trials in Orissa, India, during the 1988 and 1990 rainy seasons; yields of 143-1255 kg/ha were obtained (Sahoo and Patnaik, 1993).

0f 10 mung bean varieties evaluated for seed yield, productivity and pod damage due to M. vitrata at Port Blair, Andaman, India, during 1983-84, ADT2 showed the greatest seed yield (899 kg/ha), followed by ML65, CO3, P104 and P105. ML65 had the highest seed productivity (11.7 kg/ha/day). Pod damage was relatively high, ranging from 29.9% in S8 to 39.2% in CO3. Following S8, the next most resistant varieties were ML65, P101 and P103 (Gangwar and Ahmed, 1991).

The susceptibility of seven medium-duration cultivars of red gram (Cajanus cajan) to infestation by several insect pests (including Helicoverpa armigera and M. vitrata) was determined in the field in Kanke, India during 1985-87. Pusa-855 showed the lowest percentage pod damage (36.3%) over the two seasons, followed by Phule T-14 (43.7%) and ICPL-106 (46.0%). The highest percentage pod damage was recorded in Phule T-20 (51.7%) (Prasad et al., 1989a).

Field studies were conducted in Bihar, India, during the kharif seasons of 1985-86 and 1986-87 to evaluate the performance of eight medium- and late-duration cultivars of pigeon pea to infestation by pod-boring insects (Helicoverpa armigera, M. vitrata, Etiella zinckenella, Clavigralla gibbosa, Exelastis atomosa and Melanagromyza obtusa). The cultivar MTH-8 recorded the lowest pest incidence during both years of the study (25.2 and 25.4%, respectively) and the lowest pod damage (mean of 25.3% for both years); its performance was comparable with that of the cultivars Phule T-17 and MTH-9. Highest pest incidence was recorded on the cultivar BR-65 (44.7%) (Prasad et al., 1989b).

Biological Control

Attempts have been made at biological control of the legume pod borers M. vitrata and Etiella zinkenella by introduction of natural enemies from Trinidad of the related species, Ancylostomia stercorea. The first was made in Mauritius during the 1950s when seven parasitoid species were released. Of these, two became established, Bracon cajani and Eiphosoma dentator. Early claims were made that the addition of these two parasitoids to the fauna had increased the harvestable crop of pigeon pea from 40 to 70% (Greathead, 1971) but these claims were disputed as the pests remain a problem on the island. The reports of successful control in Mauritius led to attempts to introduce the same parasitoids into Hawaii during the 1950s, Sri Lanka in the 1970s and into Fiji during 1967-78 (Cock, 1985). These efforts all failed except for the establishment of Perisierola emigrata in Hawaii, but it had no beneficial impact (Waterhouse and Norris, 1987).

Kumar and Mehto (1996) tested seven insecticides: dusting of each of quinalphos (dust), fenvalerate (dust) and malathion (dust) were used twice during the crop season for the control of Empoasca kerri, Spilosoma obliqua [Spilarctia obliqua] and M. vitrata in Vigna mungo in Bihar, India. The treatment differences were statistically significant.

Neem oil (NO) and different formulations of neem oil with Tetrapleura tetraptera as an emulsifier, including neem oil emulsifiable concentrate (NOEC), neem oil slurry emulsifiable concentrate (NOSEC) and 5% NOEC obtained from the seeds of the neem plant, were tested against M. vitrata on cowpeas in the laboratory. NOEC exhibited a high degree of insecticidal activity to third-instar larvae. All of the treated flowers were protected from larval damage 2 days after treatment compared with the 100% damage on untreated flowers and flowers treated with T. tetraptera solution alone. NOSEC and NOEC also exhibited some insecticidal activity which decreased with a decrease in concentration. NO and NOEC were superior to NOSEC (Jackai and Oyediran, 1991).

In field trials in Nigeria, Emosairue and Ubana (1998) tested two concentrations of neem seed kernel extract (NSKE) and the synthetic insecticide lambda-cyhalothrin for the control of some cowpea insect pests, especially Maruca pod borer, during the 1995 late season. All the treatments significantly reduced the pod and seed damage caused by M. vitrata (P = 0.05). Whereas lambda-cyhalothrin was significantly superior to both concentrations of NSKE, the two neem concentrations were not significantly different. Although yield from lambda-cyhalothrin treated plots was significantly higher than the control, it was not statistically different from the yields of the two neem treated plots. The cost-benefit analysis showed that although insecticide-treated plots gave the highest yield and the highest net gain, the lower concentration of NSKE gave a better cost-benefit ratio.

Integrated Pest Management

Mensah (1997) in a northern Guinea savanna ecological zone of Nigeria on crop mixtures of one row of sorghum alternating with two rows of cowpea found that although the pattern of crop mixture showed evidence of reduction of pests and damage to cowpea, it cannot be substituted for insecticide application. The limited protection offered by the insecticide significantly reduced pest damage and substantially improved grain yield. Intercropping three rows of cowpea alternatively with two or three rows of sorghum and spraying gave a yield advantage of 58 to 69% and was, therefore, the most productive method to be adopted by subsistence farmers.

Alghali (1991) found that chemical control proved unprofitable, and concluded that IPM practices with resistant cultivars and adjustment of the planting date were the most suitable options for residual soil moisture cowpea production in Nigeria.

M. vitrata is one of a group of lepidoptera with pod-boring larvae. It is widespread in tropical areas, especially East and West Africa and India, and most injurious to beans (Phaseolus vulgaris), cowpea (Vigna unguiculata), pigeon pea (Cajanus cajan) and green gram (Vigna radiata). Other pests occurring in conjunction with M. vitrata include the lepidoptera Helicoverpa armigera, Lampides boeticus, Cydia ptychora, Etiella zinckenella and, occasionally, Chilo partellus. Some species of Diptera may also be found in conjunction with M. vitrata. Damage is normally attributed to pod-borers as a complex without an attempt to apportion it to particular species. However, M. vitrata is often regarded as a major pest within the group. Karel (1985) described M. vitrata larvae in Tanzania as more abundant and injurious to pods than H. armigera (causing an average of 31 and 13% damage, respectively). Patnaik et al. (1986) stated that M. vitrata was the dominant pest in Orissa, India; however, Okeyo-Owour and Khamala (1980) did not list M. vitrata among the more important pod-borer pests in Kenya. Loss of yield (of seed) due to the complex of pod-borer larvae was measured as 3.69-8.89% for pigeon pea in Orissa, India (Patnaik et al., 1986); 33-53% for beans in Tanzania (Karel, 1985); 25.7-62.7% for pigeon pea in Kenya (Okeyo-Owour and Khamala, 1980); 45% for early pigeon pea, in untreated trial plots, in Patnagar, Northern India (Ujagir, 1999). Yadava et al. (1988) in Uttar Pradesh, India, found a difference between early varieties of pigeon pea, where M. vitrata was an important member of the complex of species, and late varieties where it was not. The respective yield losses for early and late varieties were 13-13.6% and 26.7-34.8%. The relationship of pod damage to yield loss has been investigated: Patnaik et al. (1986) found that 8.24-15.71% pod damage led to 3.69-8.89% loss of yield; Phookan and Saharia (1987) found no significant relationship between larval density and loss of green gram in Assam, India; Odulaja and Oghiakhe (1993), in field trials in Nigeria, found the best nonlinear model was of the form y = abx where y is the yield loss and x is the percentage flower infestation, larval count or pod damage.