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THE RELATIONSHIP f-F DDT TC IMCRE/lSES IN rilTE POPULATIONS 



Ihis subject air/ays comes up whenever we discuss the current spray programs 

 being recommended in Massachusetts, No one has a complete answer, but new ideas 

 and contributing factors are being set forth or discovered all the time, 



Mr. Donald 'i. Davis of California finds that DDT causes mites to become more 

 active and to spread about over the food plant. In his studies this seems to be 

 an important reason v;hy mites on plants treated vfith DDT tend to increase, sometimes 

 very rapidly. Davis worked with a species of mite very closely related to our 

 2-spotted mite and having much thie same habits , 



Normally mites of this group tend to 'gather into small groups and tliese early 

 colonies slowly expand during tlie season. Within the colony there is extreme 

 competition for food. As the number of mites increases, their ability to reproduce 

 falls off and no further build-up occurs. 



Davis' studies indicate that when DDT is applied to mites or to the host plant 

 there is a definite change. The mites become extremely active and there is a tem- 

 porary drop in the egg laying, 



Ihe increased activity causes the mites to travel away from the original colony 

 and thus they become widely dispersed. This relieves the competition for food and 

 increases the reproductive capacity of the population. Thus more eggs are laid and 

 a more widely scattered and higher peak population is the final result. 



This does not mean that killing of predators is not an important factor in 

 mite build-up follovd.ng the use of DDT, However, this does help to understand better 

 why mites are able to increase so rapidly. A mere killing of the predators has not 

 been a satisfactory explanation for this marked increase in reproductive powers, 



— Ellsworth H, liheeler 

 Extension Entomologist 



HOtJE ORCHARD SPRAY SCHEDUIJ: 



The annual revision of the "General Purpose" Home Orchard Spray Schedule is now 

 available throughout the State. Obtain it through your local County Extension 

 Service Offices or at the University of i.Iassachusetts and its branches at u'altham 

 and East Wareham. If you are not a commercial grower and have one or more trees of 

 apple, pear, plnn, peach, cherry, and quince and perhaps some grapes and raspberries, 

 I am sure you will find this Special Circular 178 very useful. 



But remember this I The right material and the right timing for applications is 

 not enough. If you are a home orchardist and vj-ish to bring through a crop of fruit 

 reasonably free from insects and disease injuries, you must resolve to spray 

 thoroughly . Your equipment must be good enough to enable you to drench the fruits 

 and foliage every time you take the trouble to make a spray application. Unless you 

 make this resolve and carry it through, you are going to be disappointed in the end 

 result. It is lack of thorough coverage rather than inferior materials or improper 

 timing which all too often causes the home orchardist to fail, 



YOiy not make sure you have the infonnation contained in this Special Circular 

 for the home orchardist. If you are sincere in your desire to grow some reasonably 

 good fruit, follow the schedule of applications suggested — and above all — do a 

 good job , 



— Ellsworth H. './heeler 

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