| Grubs (Coleoptera: Scarabaeoedea) are all kinds of the floorboard of the beetle larvae.They are omnivorous which damage the underground part of a variety of crops and youngleaves and tender buds of the forest trees, causing great economic loss. This papersystematically investigated the effect of different cropping systems on grub populations andtheir spatial distributions. The results were as follows:1. Effect of different tillage methods on grubsDifferent tillage methods could significantly affect the grub populations, i.e., comparedto crop rotation system, the grubs population density increase after continuous cropping;whilst compared with conventional tillage, no-tillage more prone to infestated by grubs.Vertical distribution of grubs in different tillage methods shows certain regularity. First, thegrubs population density in different soil depth more higher than other tillage methods aftercontinuous cropping. Secondly, the vertical distribution of grubs has seasonal variations,grubs more distributed to shallow layer in soil in their vigorous stage of growth anddevelopment, while grubs quantity increased in deeper soil (>30cm) after autumn when theyprepare for wintering. In addition, the result shows that the space distributions of grubs indifferent crop field all appear the aggregating distribution.2.Occurrence regularity of Maladera orientalis MotschulskyMaladera orientalis Motschulsky is one generation a year in Lanzhou, Gansu Province.They overwinter mainly in adult stage in the40cm soil layer. the overwintered adults firstoccur in mid-April, and they will reach damage peak of plants in early May to early July. InAugust, the adults will dig into the soil for winter. The developments of four stages arefollowed as: egg6~9days (8days in average); larvae53~86days (72days in average); pupae9~16days (of12days in average); adult9months, about280days.Eggs will witness a high hatchability at22~30℃with a fast development. Instead, theywill be dormant below18℃, and their development will decrease over33℃and die above36℃. At4℃,15℃and18℃, the eggs cannot hatch, but the mortality was0. When transferredinto25℃, the eggs will hatch out after7to10days. However, when transferred into35℃,only a small portion can hatch out, and the remainings could not hatch out in25℃placement. These eggs have already turned into yellow from white, indicating that they are dead.The occurrence of M. orientalis is closely related to climatic factors, i.e., when thetemperature is below20℃, the overwintering individuals will not occur no matter that therewill be precipitation or not; when over20℃, their populations are closely related to theprecipitation.The population always witnesses an increase after each rainfall, Gray correlationanalysis shows that the precipitation will significantly affect the occurence of M. orientalis,much overweighting the factor of temperature.Its occurrence and climatic factors have great relevance, when the weather temperature isbelow20℃, no matter the level of precipitation is not unearthed outside; greater than20℃, agreat relationship with rainfall, after each rainfall peak, the amount of Maladera orientalisMotschulsky occurred on the increase. Gray correlation analysis by the learned correlationwith precipitation occurring insect amount is greater than the correlation with temperature.3. Relationship between the population density of grubs and the potatoes yield loss.With the increase in the number of grubs, the potato yield decreases; the relationshipbetween the potato yield loss and the grubs population density could be quantitativelysimulated by quadratic equation (y=-0.0252x2+0.9332x-0.0081(R2=0.9997)), the resultsshowed that potato yield loss caused bya grubs is0.8043%in average. According to theEconomic Threshold (Chiang,1979) of grubs,the average control index decrease progressivelyalong with the increaseing of yield per mu,When the yeild per mu (Y) of potatoes were3000kg,3500kg and4000kg respectively, the control index (ER) were3.4heads/m2,2.9heads/m2and2.5hesds/m2respectively. |
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