Total insect population densities in 2007 varied among the sampling weeks , but not the cropping treatments . The total insect population in all treatments started with low density and then developed early and late season peaks. Although differences were not statistically significant, the total insect populations reached higher levels on the cover crop treatments than on the fallow treatment during the early season peak, but that reversed itself during the late season peak . All individual insect pest populations fluctuated significantly throughout the growing season but none were significantly affected by the cover crop treatments . Cabbage loopers and diamondback moths were the two most dominant species and both had bimodal population peaks, which were especially prominent for diamondback moth. Cabbage loopers and diamondback moths had similar population densities in the early season peak, but the diamondback moth was more dominant during the late season peak. Overall the cabbage worm was at much lower densities than the other two species.Leaf damage in 2007 varied among sampling weeks , but not among the cropping treatments . Broccoli leaf damage for all treatments increased early in the growing season peaking in early to mid season as total insect population density increased , and declined shortly from mid to late season .
Since cabbage looper and Diamondback moth make the majority of the total insect pests , pot growing systems broccoli leaf damage for 2007 must have been caused primarily by those two insect pests.Total insect parasitization for 2007 varied among cropping treatments and there was a significant interaction between cropping treatment and sampling week . Total insect parasitization for most of the sampling weeks was significantly higher for the cowpea and marigold treatments, compared to the fallow; however as of the last sample date, there was no significant difference among treatments . Insect parasitism is species specific. Parasitization rates reached very high levels for all three species on at least some sampling dates . Cover crop treatments did not have a significant effect on insect pest parasitism for any of the three species pooled over the season, but the effect for cabbage looper was close to significant . There also was a significant interaction between treatment and sample date for cabbage loopers and cabbage looper had significantly higher levels of parasitism in the marigold plots than in the other plots on the first sample date. Parasitism of cabbage looper was significantly higher in the cowpea plots than in the other plots on the fourth sample date .Total insect population densities for this year varied not only among sampling weeks , but also among cropping treatments with no significant interaction between treatment and sample date .
Total insect population was significantly higher in the cowpea treatment than on either the fallow or marigold treatments; there was no significant difference between marigold and fallow. Total insect population at early crop growth season was relatively higher than at the same time total insect population levels in 2007 , but exhibited a single population peak rather than two. The single seasonal total insect population peak was attained in the middle of the broccoli growing season and then declined . The population peak in 2008 was higher than in either 2007 or 2009. At their peak population levels , total insect pest population was consistently higher for the cowpea treatment compared to the fallow treatment with no significant differences between cowpea and marigold and marigold and the fallow treatment .Within individual insect species, both cabbage loopers and cabbage worms responded significantly for the cropping treatments with populations reaching significantly higher levels on the cowpea treatment than on the other two treatments . Cabbage loopers and diamondback moths both exhibited a single mid-season population peak with cabbage loopers reaching a higher peak than diamondback moths . As in the previous year, the density of cabbage worm was much lower than the density of the other two species .Broccoli leaf damage for 2008 varied among cropping treatments and sampling weeks . Their interaction was not significant.
Damage was significantly higher in the cowpea plots than in the fallow plots and almost significantly higher in the marigold plots than in the fallow plots . Leaf damage rose steadily over the first four sample dates in all plots, continued to rise in the fifth and sixth sample dates in the cowpea plots, but flattened over weeks 5 and 6 in the marigold and fallow plots .Parasitization rates reached very high levels on at least some sampling dates, especially for cabbage looper . Total insect pest parasitization in 2008 differed significantly among cropping treatments and sampling weeks , with no treatment and sampling week interactions . Total insectparasitization levels in 2008 were significantly higher for both cowpea and marigold , respectively compared to the fallow treatment . Among the individual insect species, parasitism did not vary significantly among the treatments for any of the three lepidopteran pests , although there was a significant interaction between treatment and week for diamondback moth. Breaking down parasitism by week for diamondback moth revealed that parasitization was significantly higher in the cowpea plots than in the marigold and fallow plots on one early season sampling date and was significantly higher in the marigold plot than in the fallow plots on the last two sampling dates .Total insect population for 2009 was not significantly variable among cropping treatments , but did vary significantly among sampling weeks . The treatment-sample date interaction was not significant. There were two distinct total insect population peaks . Considering the individual insect population densities, none of the three species differed among the treatments . Both the cabbage loopers and diamondback moths exhibited distinct mid and late-season population peaks whereas, cabbage worm had only an early season peak and was scarce thereafter . However, the diamondback moth had only one population peak in the fallow treatment .Leaf damage in 2009 was significant for the cropping treatments, sampling weeks, and the interaction between cropping treatment and sampling weeks . Early season through the first pest population peak, damage was significantly higher in the cowpea treatment than either marigold or the fallow treatments . On the third sample date, damage also was significantly higher in the marigold plots than in the fallow plots . In mid season there was no significant difference in damage among treatments . Near the beginning of the late season insect pest population peak in 2009, crop damage was significantly greater in the fallow plots than in both the cowpea and marigold plots . There are two peaks of total insect pest density but only a single peak leaf damage , indicating that crop damages do not correspond to insect population densities. Crop damage patterns do not also correspond to the bimodal population peaks observed for cabbage loopers and diamondback moths in 2009 . However, considering the larger individual feeding larval sizes of cabbage loopers, it can be assumed that the cabbage loopers were responsible for most of the broccoli leaf damage. The very low number of cabbage worms indicates that little damage can be attributed to this species.The overall model for total insect parasitization for 2009 was significant among the cropping treatments , with both the cowpea and marigold plots experiencing significantly higher levels of parasitism than the fallow plots. There was also a significant effect of sampling date and an almost significant treatment sample date interaction .
The significant difference among treatments was a group effect of all three species pooled as none of them individually showed a significant effect of treatments on parasitism although the effect of treatment on cabbage looper parasitism was almost significant .In general, planting racks three lepidopteran insect pests; the cabbage looper , the diamondback moth and cabbage worm were observed as the major broccoli insect pests throughout the study years. Overall pooled for three years, total insect population densities and cabbage worms were higher on the cowpea cover crop compared to the fallow system. However, cropping treatments pooled over the three year study had no significant effect on the overall population density of cabbage loopers and diamondback moths. The overall analysis for data pooled over all three years showed significance levels for the cropping treatment and year. Higher total insect population was observed in 2008 than any other year, probably because broccoli growth for this year was more robust making it more attractive to insect herbivores. In all years, total or individual insect population started with low levels during early crop growth season, increased over a period of time and attained either single or multiple population peaks. The knowledge of pest population peaks may help plan insect pest management timing and control strategies. Cover cropping treatments had a significant effect on total insect population densities as well as on cabbage loopers and cabbage worm population densities only in 2008. During this year, the higher insect population levels were attained on the cover crop treatments, particularly the cowpea compared to the fallow treatment. In contrast, insect population densities for 2007 and 2009 were not affected significantly by cover cropping treatments. Therefore, considering the three year data, it can be said that the off-season cover cropping treatments had very little or no effect in suppressing either the total or individual insect pests. If at all, the cover crops rather enhanced pest population densities on the subsequent vegetable crop as was observed in 2008. The greater insect population density on the cover crop treatments for 2008 crops may have been due to better growth, higher nutrition and a broader canopy that was observed in the broccoli crop in 2008 . Based on the three year population dynamics and peaks, the presence of multiple population peaks for the cabbage loopers and diamondback moths may indicate that insect pests are the most important insect pests of broccoli. Sarfraz et al. considered that P. xylostella is a major constraint to brassica crop production, while Furlong et al. recognizes A.rapae as the most devastating pest. These variable categorizations of pest severity could be due to geographic differences and possible year to year variations. In this study, cabbage worms attained far lower population densities that the other lepidopteran pests during all three years.In this study, off-season cover crops had little or no effect on pest population densities. In studies where cover crops were interplanted with the main crop, reduction in pest populations has been reported from several studies. Broad et al. detected a reduced colonization of diamondback moth within the diversified cropping systems, indicating that more insect pests under a mono-cropping system. On the other hand, in a mixed broccoli intercropping system, Hooks and Johnson found higher populations densities of cabbage worm. The more abundant herbivorous insect pest density in monoculture compared to polyculture may be attributed to a ―resource concentration‖ hypothesis where some specialist herbivores may respond more strongly to homogeneous systems than to mixed cropping. These contrasting observations suggest that the success of cover cropping treatments as insect pest suppressant depends not only on cropping diversification, but also on the scale and the timing of the diversification . The early season increase in broccoli leaf damages in all years could be attributed to the influx of colonizing insects. Broccoli leaf damage varied significantly among cropping treatments in 2008 and 2009 but not in 2007. In 2008, broccoli leaf damage became conspicuously higher in the cowpea cover crop plots compared to the fallow plots only late in the season. In contrast, in 2009 the greater damage on cowpea versus fallow plots occurred only early in the season. Broccoli plants abscise damaged or older leaves very frequently and hence the reason why leaf damages may not progressively increase in each year.Numbers of insect damaged leaves in 2007 were lower than the peak leaf damage observed in either 2008 or 2009, but the increase in leaf damage does not necessarily synchronize with the dynamics of insect population densities in most of the crop growing years. Within the insect populations, the fact that cabbage looper and diamondback moth were the majority of the total insect pests, suggests that these insect species must have contributed the most to broccoli leaf damage. Yet, since cabbage looper population density accounted most for the total insect population densities and has larger larval sizes, crop damage can be attributed mostly to cabbage loopers and to a lesser extent to diamondback moth and little to the cabbage worm. Nevertheless, crop pest damage is a cumulative effect of all insect pests; hence consideration of total insect pest population should provide a better depiction of insect and insect management decisions.