Ask Us About Insects

September 23, 2008 - 10:04

Q How many thrips should be on a yellow sticky card before I spray?

A Good question. There is no hard and fast rule, but there are some recommendations that have been made by researchers working with thrips. First, it really depends on the crop and how many cards you place within the greenhouse. At two traps per 10,000 square feet of homogenous production, research suggests that treatment thresholds are reached if you observe 25 (sensitive varieties) to 50 thrips per sticky card per week. If monitoring with eight traps per 10,000 square feet, then consider treating if you observe 10 to 20 thrips per card per week. In more sensitive crops, consider treating if an average of five to 10 thrips per card per week are present. The numbers will change if you are worried about Tomato Spotted Wilt Virus or other tospoviruses. Very few thrips can be serious under those conditions and will need immediate attention, and rather than preventive treatments, exclusion screening might be a better option. A solid record of trap catches is a good way to determine your own treatment thresholds. Correlate trap catches with plant quality, and fix treatment thresholds at a level that will prevent damage and poor quality.

Q I’ve heard that worm castings will protect my plants from whitefly infestations. Is that true? Should we be adding worm castings to our media mixes?

Because there is such a push to become more sustainable lately, I am getting more and more questions like this from varying audiences. A simple answer to these two questions: No. If you mix your own media, and worm castings are available to add to your mix, then it could certainly be a part of your media — although it may be an expensive one. Worm castings are without a doubt beneficial to a healthy soil, adding nutrients and beneficial microbes, but they lack insecticidal properties. Similar benefits can be obtained from properly prepared compost. The ornamental industry has quality potting media options that are quite healthy and beneficial to the plant, so adding something with a dubious insecticidal effect is not wise. I can’t wait to get the e-mails I expect from the wormophiles out there who swear by worm castings.

Q Can I start rearing beneficials in my own greenhouses? Are there any restrictions?

Yes, you can, and there are no restrictions to my knowledge. However, it’s not easy. This question led to quite a discussion in a recent audience about the risks versus the benefits of mass rearing your own beneficials. The grower was plagued by leafminers, mites and thrips, and he was willing to attempt rearing his own wasps and predatory mites. For those of us with experience rearing insects and beneficials, we can admit that it is not an easy process, and there are many pitfalls, such as knowing how many pests to introduce to produce the required number of beneficials. In addition to the numbers game, there are also physical factors to consider in rearing, such as temperature, humidity and what kind of cage or facility to rear them in. Success will require complete dedication to the effort. There is, however, a lot of scientific literature out there to get you started if you are willing to make an attempt. It will be important to correctly identify the pest species you are dealing with because not every beneficial will work with every pest. In my opinion, it is best left to the experts, so forget the headaches and purchase them. You will also get some pretty good advice or support from the suppliers.

About The Author

Jim Bethke is floriculture and nursery farm adviser with the University of California Cooperative Extension in San Diego County.

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