Using Parasites and Predators 



to Control Insects on Ornamentals 



Patricia J. Vittum 



Drawing: John Weaver 



M 



lany customers have become increasingly con- 

 cerned about the use of pesticides in plant pro- 

 duction. They are concerned about a variety of 

 environmental issues (run-off of pesticides or fer- 

 tilizer into surface water, leaching of materials into 

 ground water, exposure of people and pets). As a 

 result, some of these people are beginning to 

 look for alternatives, including buying plants 

 which have been grown without the use of pesti- 

 cides. While this may appear to be a simple solu- 

 tion, at least in the eyes of the consumer, most 

 growers do not feel they can maintain plant qual- 

 ity without the use of pesticides. But there are 

 some alternatives which may enable a grower to 

 reduce his/her dependence on traditional pesti- 

 cides, particularly insecticides. In the past several 

 years many university researchers have been in- 

 vestigating the use of parasites and predators to 

 control pest insects. 



PARASITES (or more accurately, parasitoids) are 

 insects which parasitize other insects. An adult fe- 

 male parasite will lay an egg on or in the body of 

 the target insect. The egg hatches into a maggot 

 or similar immature stage, which burrows into the 

 body of the target and feeds on the internal tis- 

 sue. Normally, the parasite passes through several 

 molts inside the body of the target before it 

 emerges as an adult, ready to repeat the process. 

 Each individual parasite accounts for the death of 

 only one "victim", but usually the females lay lots 

 of eggs, so the population of parasites can be 

 very effective. 



PREDATORS are insects or close relatives (for ex- 

 ample, spiders and some mites) which can seek 

 and destroy target insects. Some predatory insects 

 have chewing mouthparts, while others have spe- 

 cially designed sucking mouthparts which suck out 

 the internal tissues of the "victim". Each predator 

 is mobile and will find several victims during its 

 lifetime. In fact, some predators will account for 

 the death of ten or more target insects per day. 

 Some parasites and predators can be pur- 



chased for release in greenhouses or nursery set- 

 tings. However, certain basic concepts need to be 

 understood before a grower attempts to use para- 

 sites or predators to control pest insect popula- 

 tions. First of all, sometimes there is a "lag time" 

 between the time a parasite or predator is re- 

 leased and when that agent begins to have a no- 

 ticeable effect on the target population. Usually a 

 release is made when there is a fairly large popu- 

 lation of the pest. This population is probably 

 thriving and it will take a while (perhaps as much 

 as a couple weeks) before the predator or para- 

 site can begin to "catch up" with the targets. Then 

 there is a period when the parasite or predator 

 has a strong impact on the targets, reducing the 

 population quickly and substantially. 



However, sometimes the beneficial insect is too 

 successful and destroys virtually all of the pest in- 

 sects. There is nothing left for the beneficial in- 

 sects to feed on and they ultimately starve. Invari- 

 ably, the pest insect finds its way back into the 

 greenhouse and re-establishes. As a result, follow 

 up releases are usually necessary in response to 

 each build up of the pest population. This, of 

 course, is no different than using standard insecti- 

 cides, in that repeat applications are usually nec- 

 essary there as well. 



Second, most beneficial insects and mites are 

 sensitive to the traditional insecticides and miti- 

 cides which are used in greenhouses and nurser- 

 ies. If a grower uses a parasite to control a white- 

 fly population and then a thrips population builds 

 up, the grower may use a standard insecticide to 

 control the thrips. Unfortunately, most of the para- 

 sites will also be killed by such an application. So 

 balancing the use of parasites and predators in a 

 full scale greenhouse or nursery operation can be 

 very challenging. There are ways to coordinate ef- 

 forts (for example, timing applications when para- 

 sites or predators are not present or are in less 

 susceptible stages) to give the beneficial insects 

 the best chance of survival. 



Third, most beneficial insects have particular 



June & July 1993 

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