**5.3 Increasing CO2 levels**

Increased CO2 levels can directly lead to increased crop harvests, but any increase in yields can be partially or fully offset by losses caused by insects, pathogens and weeds. For example, in North America, cabbage loopers, *Trichoplusia ni* are observed to ingest a higher amount of leaves under higher CO2 levels, which is believed to be due to the decreased levels of nitrogen observed in cabbage leaves that grown under these conditions [76].

#### **5.4 Extreme weather actions**

Extreme weather conditions can unpredictably affect interactions between crops, pests and diseases, likely leading to the failure of some crop protection strategies and subsequent reductions in yields. For example, *Trichogramma* 

*Climate Change and Its Potential Impacts on Insect-Plant Interactions DOI: http://dx.doi.org/10.5772/intechopen.98203*

*evanescens* populations were so reduced in May 1993 by exceptionally dry and warm weather conditions in Slovakia that no record of active parasitism of European corn borer eggs was reported that year [77]. In hurricanes, intense air streams can also move fungal spores or insects from overwintering sites to places where additional problems can be caused. Winds associated with Hurricane Wilma thus spread citrus canker widely in Florida, killing 170,000 acres of fruit trees grown commercially [78]. Ecosystems affected by extreme climate events are also automatically more fragile and vulnerable to invasions of space by aliens and indigenous organisms.
