PESTS AND PESTICIDES 



ample, where a broad-spectrum pes- 

 ticide is used (and natural enemies 

 of the pest are also eliminated) or 

 when high dosages of a selective ma- 

 terial are used that kill off high per- 

 centages of the pest (and thus starve 

 out the natural enemies by eliminat- 

 ing their food), the pest populations 

 can recover quickly without hin- 

 drance of natural enemies. 



This destruction of natural ene- 

 mies, as an unfortunate side effect 

 of pesticide usage, has two main 

 consequences. First, the target pest 

 may quickly recover from the impact 

 of pesticide usage and resurge to even 

 higher levels. (See Figure X-13) Sec- 

 ond, the resurgence of unleashed 

 secondary pests may occur shortly 

 after the application of the triggering 

 pesticide, or later in the growing sea- 

 son, or even in a subsequent season. 

 In cotton in California, for example, 



we have had serious outbreaks of 

 such unleashed secondary pests as 

 beet armyworm, cotton-leaf perfora- 

 tor, and cabbage looper. These sec- 

 ondary pests may be more destructive 

 and more difficult to control than the 

 original target pest. Re-establishing 

 the effectiveness of natural enemies 

 may require two or more years. 



Health Hazards — It is obvious that 

 we should not knowingly use pesti- 

 cides in ways that would constitute 

 a risk to human health. Such hazards 

 can occur to the individual applying 

 the pesticide, or to persons entering 

 the treated area either during the 

 application or at some appreciable 

 time later, or to persons exposed to 

 the residues of the pesticide on or 

 in the harvested crops, or in other 

 more subtle ways. When such a 

 hazard is discovered, procedures 

 should be taken to avoid the risk — 



Figure X-13 — RESURGENCE OF CALIFORNIA RED SCALE 



The table shows differences in the density of California red scale between trees 

 left under normal biological control and some experimental citrus groves treated 

 with DDT. The initial counts, comparable in both sets, were made just before DDT 

 was applied; the low ratings (mostly 10 or less) indicate that the scale was scarce 

 and under an excellent degree of natural control. After one or two seasons of 

 treatment with DDT, however, red scale was far more common, whereas the scale 

 on the untreated trees was evidently held in check by natural enemy activity. 

 Ratings above 50 to 100 begin to cause visible, and economically unacceptable, 

 twig and branch kill. 



e.g., proper masks and clothing for 

 applicators, minimum time period af- 

 ter treatment before the treated area 

 can be entered, or minimum time 

 after treatment before harvest. In 

 those instances where the risk can- 

 not be avoided, then use of that 

 particular pesticide should be cur- 

 tailed and a suitable alternative con- 

 trol measure sought. This is not to 

 suggest that all uses of the pesticide 

 be banned but rather that the par- 

 ticular hazardous uses be eliminated. 

 Furthermore, when alternatives are 

 considered, all aspects, both positive 

 and negative, should be carefully 

 weighed. 



Limitations of Substitute Mate- 

 rials — In the past year or more, there 

 has been considerable public pressure 

 to eliminate all persistent pesticides. 

 Let us not forget that, if this comes 

 about, society is losing valuable tools 

 for pest control and that there are 

 other problems associated with many 

 of the substitute materials. Basically, 

 each compound should be considered 

 individually as to its peculiar risks on 

 the basis of its specific characteristics 

 together with the exact manner of 

 dosage and place of application. 



To date, the substitutes have usu- 

 ally been either organophosphorus 

 compounds or carbamates, though it 

 is difficult to generalize because there 

 are so many exceptions. The sub- 

 stitute materials used so far have 

 shown, first, a frequent pattern of 

 higher acute toxicity, with associated 

 greater immediate risk to man, live- 

 stock, and wildlife. Secondly, these 

 substitute compounds frequently have 

 produced serious damage to honey 

 bees and other necessary pollinators; 

 their impact on the California bee- 

 keeping industry has been disastrous. 

 Thirdly, they have had a severe im- 

 pact on insect natural enemies. As 

 noted above, elimination of natural 

 enemies from treated areas frequently 

 permits rapid resurgence of the target 

 pests and outbreaks of previously in- 

 nocuous species. Finally, the short- 

 lived nature of the substitute mate- 

 rials together with their side effect 



355 



