FEATURE ARTICLE

You Get What You Pay For: Quality Control of Natural Enemies

Augmentative biological control is the periodic release of natural enemies to control an insect pest. In the US, augmentative biological control is used in greenhouses, home gardens, horticultural crops and interiorscapes. The majority of natural enemies used in augmentative programs are produced commercially. In North America, there are about 130 suppliers, selling 100 species of arthropod natural enemies.

One of the most common questions people ask when they buy a natural enemy is "Am I getting the number I paid for?" Grammar aside, this is a very important question, as the chances for control are very much related to the number of natural enemies released. To assess this, a group of biological control specialists from Purdue (yours truly), Iowa State (John Obrycki) and University of Wisconsin (Dan Mahr) conducted a 3 year study of four species of commercially available natural enemies. We selected the whitefly parasite Encarsia formosa, the egg parasite Trichogramma pretiosum and two predators, Chrysoperla carnea (green lacewing) and Hippodamia convergens (lady beetles).

We ordered 5 shipments of each natural enemy from 4 companies (here identified by number). We ordered natural enemies at 3 different times/places; Purdue in November-December (1991), Iowa State in January-May (1993) and Wisconsin, May-October (1994). Paid cooperators ordered the natural enemies at each location. Each cooperator was told not to disclose they were ordering natural enemies for evaluation. Indeed we had a "cover story" for cooperators sort of a "Mission Impossible" approach to quality control! Parasites were shipped as pupae inside hosts glued to small cardboard cards. Lady beetles were shipped as adults within cardboard cartons or cloth bags containing straw-like packing material. Lacewing eggs were shipped inside several types of small vials or cups, usually containing rice hulls or another carrier. For each shipment, we estimated the number of natural enemies received versus the number of natural enemies ordered. We then computed an "O:E ratio" to relate the numbers received ("O", for "observed") versus the numbers ordered ("E", for "expected"). If you got what you asked for, the O:E ratio would equal 1. If you got less than you asked for, the O:E ratio would be <1; an O:E> 1 means you got more than you asked for. By using the O:E ratio, we could "correct for" the differences in the numbers of natural enemies commonly ordered (e.g., tens of thousands of Trichogramma, vs. hundreds of Encarsia). We then compared O:E ratios for each natural enemy from each company and location. We used statistical analyses (two-way ANOVA for you statistical buffs) to compare O:E ratios across companies and locations. Since we ordered natural enemies at different times at the 3 locations, locations also represent time-of- year differences in O:E ratios. (We measured more than O:E ratios, but space precludes discussion here the interested reader can contact me for a copy of the full report.)

Table 1 shows the results of the study. For Encarsia, 38 shipments were processed; 12-14 shipments from each of three companies (3, 6, 12). Average O/E ratios for the three sites ranged from 0.8 to 2.8. Statistical analysis indicated location and company differences, suggesting that you'd receive different numbers of Encarsia depending on when you buy and from whom you buy them. For Trichogramma, 43 shipments were processed; 13-16 shipments from each of three companies (4, 7, 11). Average O/E ratios ranged from 0.6 to 2.4. Statistical analysis again indicated location and company differences in O/E ratios when and from whom you buy has an impact on the numbers you'd receive. For lady beetles, 54 shipments were processed; 12-14 shipments from each of four companies (2, 5, 8, 9). O/E ratios ranged from 0.6 to 1.3. Analyses indicated location, but not company, effects on O:E ratios. Thus when you order is important, not from whom you order. Finally, 39 shipments were processed for Chrysoperla; 12-14 shipments from each of three companies (4, 7, 8). O/E ratios ranged from a low of 0.4 to a high of 2.2. For lacewings we noted company, but not location (time) differences in O/E ratios.

Table 1. Observed: Expected (O:E) ratios of four natural enemies shipped by various companies to three locations. 3-5 shipments were evaluated per species/location.
SpeciesLocationO:E ratios (Standard Error)
Company Number
Encarsia formosa3612
Purdue 2.77 (0.34) 0.84 (0.17) 2.56 (0.20)
Iowa State 1.60 (0.24) 1.82 (0.09) 1.54 (0.13)
Wisconsin 1.51 (0.23) 1.71 (0.07) 1.72 (0.07)
Company Number
Trichogramma pretiosum4711
Purdue 0.88 (0.04) 1.24 (0.28) 2.44 (0.34)
Iowa State 0.63 (0.07) 0.94 (0.10) 0.91 (0.16)
Wisconsin 0.96 (0.09) 1.19 (0.10) 0.92 (0.14)
Company Number
Hippodamia convergens2589
Purdue 0.88 (0.05) 1.32 (0.48) 0.89 (0.04) 1.11 (0.20)
Iowa State 1.12 (0.06) 1.09 (0.11) 1.13 (0.04) 1.27 (0.07)
Wisconsin 0.76 (0.02) 0.62 (0.12) 0.88 (0.03) 0.89 (0.05)
Company Number
Chrysoperla carnea478
Purdue 2.20 (0.89) 0.65 (0.50) 0.53 (0.23)
Iowa State 1.21 (0.10) 0.85 (0.12) 0.72 (0.14)
Wisconsin 1.03 (0.22) 0.42 (0.08) 0.53 (0.23)

So, what does the study tell us about the "quality" of commercially available natural enemies? First, I think it's clear that evaluations should be done periodically and that comparisons be made over time. For example, the O/E ratios of lady beetles for Wisconsin were consistently lower than the other sites (Table 1). Since adult lady beetles are collected in autumn (in California) and then placed in cold storage until shipping, mortality of adults held over six months before being shipped to Wisconsin may have resulted in the lower O/E ratios there. Also, location (time) effects for both parasite species may also reflect changes in production procedures or changes in the intrinsic properties of the natural enemies over time. Secondly, company effects for three of four natural enemies suggests that future evaluations continue to examine differences among companies. While O:E ratios for Encarsia were mostly > 1 (i.e., you got more than you paid for), for Trichogramma and lacewings O:E ratios tended to be < 1 (i.e., you got less than you paid for). Knowing the O:E ratios helps you decide how many natural enemies you need to order to get the number you need to control your pest problem. [FYI- Encarsia sold in the US are produced by one company, Koppert of Holland. The "good" O:E ratio for Encarsia probably reflects the effort (mostly) Europeans workers spent refining the quality control of this "product".] The relatively low O:E ratios for the other species could probably be improved in a similar fashion.

Ok, so what tips do I have for those purchasing natural enemies? The first question you need to ask is, "Do I need natural enemies?" If you do not know what pest you have and know it is, or soon will be, a problem, put your checkbook away. Once you know you need natural enemies, you have to order the right natural enemy to do the job. Your local Extension service and the companies selling natural enemies are good sources of information (see below to order a catalog of companies). Order from a reputable firm; one that either specializes in natural enemies or has been selling them for a long time. Your local Extension Service may/should know, but if not, call the company (most have 800 numbers) and ask them! Companies usually provide lots of (free!) information right over the phone, so ask away. They should be able to tell you what natural enemies they have in stock, what they are used for, how many to use, and under what conditions you should use them (e.g., when and how to release). Finally, once you receive your natural enemies, look at your order. Did it come on time (most of ours did)? If you ordered eggs (e.g. for lacewings), do you see larvae crawling around in the shipment (we did)? If you ordered parasites, do you see lots of "mini-wasps" flying out of the shipping container (we didn't)? A small magnifying glass will come in handy, but you can count natural enemies such as lady beetles. If you suspect you did not get what you paid for, call the company. Our experience has been that they are friendly and will replace shipments. If neither, use someone else next time! Using natural enemies in augmentative biological control will require a bit of experimentation on your part. Regardless if you're a home gardener or own a production greenhouse, you are used to experimenting with plant varieties, fertilizers, watering schedules, etc. What works and what doesn't is somewhat dependent on your situation. So, too, for natural enemies! Evaluation of quality control will give you, the end-user, that confidence to use natural enemies to control insect pests. To know what companies sell natural enemies, request a free copy of the publication Suppliers of Beneficial Organisms in North America by C. D. Hunter, from:

- Bob O'Neil, Purdue University


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