CACTUS-FEEDING INSECTS AND MITES 145 



is extended to 4 to 6 months. In Australia crawlers appear at intervals 

 throughout the winter. At Tucuman, Argentina, reproduction has 

 been observed in June, July, and August, but the July crawlers im- 

 mediately succumbed to a drop in temperature; increase was not 

 general, however, until October. At Uvalde, Texas, crawlers have 

 appeared in late November, but not in December and January; in 

 February and early March reproduction may occur whenever a period 

 of mild weather occurs; thus in 1932 crawlers were produced in the 

 first week of February and again in the last week of February and the 

 first few days of March, but not in large numbers until the last week 

 of March. 



THEIR EFFECT ON HOST PLANT 



Experiments in Australia have shown that cochineal is negatively 

 phototropic and makes more rapid progress when protected from direct 

 sunUght. The cultures are more numerous on the under surface of 

 prickly pear joints and on plants growing in the shade. Rain is a second 

 important factor, influencing the preference for sheltered situations. 

 Heavy falls of rain wash the crawlers and even the well-grown cultures 

 from the plants, and, futhermore, by destroying the protective woolly 

 covering of the developing and mature females, expose the insects to 

 the depredations of enemies. Thus, dry weather aids the increase of 

 cochineal, and thunderstorms and other heavy rains retard progress. 

 Wind is important in the spread of cochineal, but also has an adverse 

 effect, as the majority of wind-borne crawlers are deposited away from 

 food plants and are lost. 



Cochineal feeds on ail aerial portions of the plant, but in general 

 prefers young growth, developing fruit, and detached joints on the 

 ground. When the infestation is heavy, the infested joints shrivel 

 up and fall off. Small seedlings are very susceptible to the attack. 

 The effect on the fruit is to cause premature ripening and falling off 

 before the seeds are mature. Although the more succulent growth is 

 preferred, cultures are often found on the woody basal joints. The 

 so-called Texas strain of D. opuntiae has been noted in Australia for its 

 infestation of the basal stems of Opuntia stricta and the main stem of 

 the tree pear 0. tomentosa; on 0. stricta, a heavy ring of colonies often 

 occurred around the junction of basal joints; these habits are peculiar 

 to the Texas strain. 



Frequently the damage caused by cochineal is out of proportion to 

 the insect population. The falling off of upper joints when the insect 

 colonies are mainly on the lower stems is not uncommon. Examination 

 has proved that the insect's proboscis penetrates into the phloem of 



263-417—68 11 



