The sugar beet cyst nematodes were not even as much sensitive as the RKN to the cover crop treatments was not variable among the cropping treatments, although were slightly higher in cowpea and marigold cover crops at the ABH sampling of the second year and the ACCP and ABH samplings of the third year. The increase in SCN mainly at the ABH samplings than at other sampling periods, may indicate that broccoli is a host to the SCN. Potter and Olth of actually show that broccoli is a potential host to the cyst nematodes. Infection of broccoli roots and broccoli root gall formation was very minimum and unaffected by the cropping treatments. Based on my current over all findings therefore, the usefulness of cowpea and marigold as offseason cover crops does not confirm their nematode suppression potentials in the subsequent winter broccoli crop.There are various reasons documented for variation in nematode suppressing efficiency of cover crops. Ploeg and Maris state that the life cycle of Meloidogyne incognita complete between average soil temperatures of 16°C and 30°C on tomato, but only at 30°C on marigold . Furthermore, motility of M. incognita J2 and its subsequent root penetration may decrease with decreased soil temperatures below 18°C . These findings suggest that the effectiveness of cover crops to suppress nematodes depends on the condition under which they are utilized. Ploeg and Maris further suggested the need for information on thermal-time relationships of plant parasitic nematodes to predict geographical distributions, nematode population dynamics and effects of cover crops on the subsequent crops. Effectiveness of a cover crop for the purpose of nematode suppression may also depend on the type of target nematode itself.
Wang and McSoreley pointed out that Iron Clay‘, cowpea failed to suppress root-knot nematodes where there were mixed species of Meloidogyne. Ploeg and Maris also identified nematode suppression of marigold being influenced by crop plant variety, nematode species, hydroponic trays and soil temperature. Marigold while suppressive to root-knot nematode, it enhanced the population densities of other nematodes such as stubby-root, spiral and sting nematodes on the other hand. Therefore, the evidence suggests that the type of nematode can determine the effectiveness of a cover crop.Others observed that nematode suppression of cover crops may depend on how the cover crops were utilized. Wang and McSorley observed that cover crop mulch was more effective than live crops. On the other hand, Ploeg and Maris state that live marigold suppress nematodes, because of the release of alpha-terthienyl, a toxic chemical compounds from its live roots that have nematicidal characteristics . These nematicidal compound released by active, living marigold roots may not be available if marigold is used as an organic mulch . Since my research was based on the off season cover cropping system and employed their residues as surface mulch and soil incorporation, the observation of poor or no nematode suppression can be justified. Similarly, Ploeg did not observe any significant suppression from preceding vegetable crops or amending a planting site with marigold plant parts. Furthermore, while cowpea incorporation as a green manure has been observed to suppress Meloidogyne incognita , the suppression was short-lived, and the numbers of M. incognita were not different from a fallow treatment . Another factor determining cover crop effectiveness was the type of the subsequent vegetable crop that may determine the potential incidence of plant parasitic nematodes.
If the subsequent indicator crop is a nematode susceptible plant, it may be possible to detect nematode suppression of cover crops, otherwise, the effects of the cover crops can be masked if the indicator crop is nematode resistant. However, if the vegetable crop isresistant to nematodes by itself, nematode suppression potential of a cover crop could be masked. Accordingly, I may not have observed any significant nematode suppression by the cover crops, because the broccoli used in this research was resistant or a poor host to most nematodes. Most broccoli cultivars contain sulphur compounds such as methanethiol, dimethyl sulphide, methyl thiocyanate, dimethyl disulphide, dimethyl trisulphide, dimethyl tetrasulphide that may be toxic to nematodes . The presence of nematode antagonizing organisms such as bacteria and fungi in a soil may also contribute to the reduction of nematode population densities , regardless of nematode suppressive treatments. Kerry observed that the second-stage juveniles of root-knot nematodes encumbered with spores of the bacterium Pasteuria penetrans are less able to invade the roots of host than the unencumbered nematodes. The most significant outcome of the cover cropping treatment was the enhancement of saprophytic nematodes. Saprophytic nematode populations were significantly enhanced at ACCP sampling of the second year and the ACCI sampling of the third year in the cover cropped plots, relative to the fallow plots. Since these nematode populations became higher at after cover crop incorporation, the increase in saprophytes may have come from the accumulation and decomposition of cover crop residues. The relatively lower saprophytic nematode populations in the fallow plots may have been associated to the lower input of organic matter from such cropping system.Therefore, the results confirm that preceding vegetable crops with cover crop could enhance beneficial saprophytic nematode populations. Saprophytic nematode population density for the first year was not significantly different for the cropping treatments, indicating that a one year cover cropping rotation is not sufficient to enhance populations of free-living nematodes. On the other hand, the increase in saprophyte population with repeated years of cover cropping suggests that there is accumulative effect of the cover cropping treatments.
The results clearly demonstrate that cover-cropping rotations must be repeated for several years in order to provide significant contributions to enhance saprophytic populations. The sharp decline in saprophyte populations at the ABH sampling of the third year might have been due to a complete decomposition and degradation of the organic matter to a level that no longer sustained high saprophytic populations at this stage. An increase in bacterial-feeding nematode population densities following soil treatment with sunn hemp as organic mulch was also observed by Wang and McSorley . Free living bacterial feeding taxa of nematodes constitute more than 60% of the nematode community . The presence of high population densities of saprophytic nematodes may provide an added advantage in soil biology . Saprophytic nematodes are useful in mineralization of plant nutrients and nutrient cycling and can be used as sensitive indicators of ecosystem change . Langat et.al. suggested that bacterivorous nematodes respond quickly to increased food supply. Therefore, cover crops play an indirect role of increasing population levels of the beneficial free living nematodes. The structure of the nematode assemblage offers an interesting instrument to assess changes in soil conditions . Enhancement of saprophytic nematodes and the mineralization and nutrient cycling benefit that such nematodes can provide to the subsequent crop indicates the profitability of cover cropping rotations. In general, the use of cowpea or marigold cover crops as an off-season cropping rotation may not provide a viable alternative as a nematode suppression strategy. Hence, the use of cover crops for nematode suppression must be considered carefully, accounting for the target nematode, how the cover crops are to be used, and the environmental conditions of the field. However, pipp mobile systems these cover crops can be used as off-seasoning cropping rotations to effectively enhance beneficial saprophytic nematode population densities in the subsequent vegetable crop. They do so as their residues decompose supporting nematode food webs. The increase in population levels of saprophytes and feeding on nutrientimmobilizing bacteria and fungi promotes nutrient mineralization and nutrient cycling. It is believed that with more knowledge about the mechanisms stimulating a beneficial nematode community, we may develop cover crop management plans to maximize the desirable effects associated with free living nematodes . Traditional insect pest management approaches utilize pesticides, but these are known tobe environmental pollutants , and in some cases carcinogenic . Insect pests may also develop resistance to insecticides making them inefficient . Those shortcomings of broad spectrum insecticides, encouraged attempts to replace them by ―soft‖ microbial based insecticides such as Bacillus thuringiensis . Yet, insect pests developed resistance to the soft insecticides as well and some ―soft‖ insecticides can be injurious to parasitoids . Therefore, there is an increasing demand for environmentally friendly and economical alternative pest management strategies . Many researchers have suggested crop diversification and cover crops as alternative insect pest management tactics . Cover cropping systems may adversely affect insect pests and, if effective could be used as an alternative insect pest management strategy as they are ecologically benign, minimize reliance on pesticides, reduce chemical exposure, and increase consumer confidence in food production . Although cover crops could potentially interfere with vegetable insect pests, not all cover crops are equally efficient in suppressing vegetable insect pests.
For example, sweet clover cover crop suppressed broccoli pest populations, but not pests of tomato or pepper . There are also concerns that cover crops used as simultaneous inter planting may compete with the main crop for growth resources and reduce vegetable crop yield . Consequently, this research was designed to evaluate the effectiveness of off-season summer cover cropping as an ecologically desirable pest management strategy for the subsequent winter vegetable crop of broccoli. It specifically evaluated the effect of two summer cover crops on population densities of broccoli insect pests and beneficial arthropods.A three-year field study was conducted from 2007-2009 at the University of California South Coast Research and Extension Center in Irvine, CA on a loamy-sandy soil. Three summer cropping treatments were employed: 1) French marigold , 2) cowpea , seeded at 56 kg/ha, and 3) a summer dry fallow as the untreated control. Each treatment plot was 12 m long x 10.7 m wide and laid out into 14 planting rows. The cover crops were direct-seeded in the last week of June in the center of the planting rows of each plot, watered through drip-tubing and grown for three months. The fallow control plots did not receive water during the summer. Each cover crop treatment plot was planted with the same cover crop in each of the three years of study. Plots were separated from each other with a 3 m wide buffer bare ground. The three treatments were replicated four times in a completely randomized design. At the end of the summer cropping period , the cover crops were mowed at the soil line, chopped, and the residues left on the ground. Concurrently, alternate rows of each of the cover crop treatments were incorporated into the soil at about 0.4 m intervals using a hand-pushed rotary tiller in preparation for broccoli transplanting. The fallow plots were not tilled. Plots for cover crop and broccoli planting are shown in Figure 1a. At the beginning of the subsequent cropping season , broccoli seedlings were transplanted in double rows into the tilled strips of the summer cover crop and fallow plots at an inter and intra-row spacing of 13 and 35 cm, respectively . Broccoli transplants were drip irrigated and fertilized with emulsified fish meal at 5 gallons/acre rate. Broccoli was chosen because it is a high-value vegetable crop that is sensitive to weeds, insect pests, nematodes , and requires high soil nutrients . All plot treatments were maintained in the same location for all three years of study in order to assess a cumulative effect of cover crops over time.Plants were non-destructively sampled for major broccoli insect pests beginning 15 days after broccoli transplanting and continuing every two weeks until broccoli harvest. On each sample date, 20 randomly-selected plants per plot were assessed visually for thepresence of insect pests following methods used by Costello and Altieri and Hooks and Johnson . The larvae of the insects actively feeding on the vegetable crop were identified to species level and recorded as the number of individuals per plant. Population density of each species was computed as average number of larvae per broccoli plant.Arthropod parasitoids were assessed by rearing field collected broccoli insect pests in the laboratory, beginning at 21 DAT and continuing once every two weeks until broccoli harvest. Five plants from the interior three rows of each plot were randomly selected and searched for insect larvae. Larvae were collected and placed in clear plastic cups with greenhouse grown broccoli leaves as source of food. Individual larvae were reared in the lab and the fate of each larva recorded.