Tomatoes Love Basil...Or Do They? Ask the Snarky Farmer!

 It smells like Friday.  Time to...Ask the Snarky Farmer!

Paul in Sacramento writes: 

"I have a conjecture that I need to have validated, or otherwise. Could it be that the proximity of my basil plants to the tomato plants has anything to do with the lack of tomato hornworms? The vapors from the basil are wonderfully aromatic. Have I, in any way, gotten something right without even trying?"

 

Paul:
Here is a link to a rather lengthy, scholarly paper from West Virginia University on the subject of companion planting, including the relationship of tomatoes and basil.

To sum up the research briefly: No, Paul, you haven't, at least not in the way you intended. But you may have encouraged the tomato plants to grow stronger.


The longer answer,  from that PhD dissertation:   
"The earlier data showed that there is no scientific evidence that the odors from highly aromatic plants can actually deter pest insects. This, therefore, brings into question how these aromatic plants produce their effects. The studies reported here showed the relationship of tomato to basil to be one of domination, with a net benefit accruing to the gardener from basil’s ability to survive and produce some yield without compromising tomato yield. Although companion planting has significant effects on some insect pests at the garden scale, these effects are not simply due to the combination of certain companion species. High density plantings can optimize land use efficiency and diculture advantages, but may lead to reductions in crop quality, or management difficulties. The combination of a dominant and subordinate crop results in larger dominant plants and smaller subordinate plants than in equivalent monocultures grown at the same density. Such combinations offer yield advantages when the increase in dominant plant weights exceeds the reduction in subordinate plant weights, suggesting that the two crops draw on slightly different resource sets. Combinations with high subordinate crop proportions may offer the greatest yield advantages."

Another interesting facet of that study asks the question that many gardeners assume as gospel: 
Do tomatoes, when planted with basil, taste better?

If you are squinting to try to figure out that chart, allow me to interpret the results of that test: there is little, if any taste difference. 


But Paul, you still are asking a good question. Where did the tomato hornworms go? I have heard from many gardeners that hornworms just aren't the problem they used to be. Again, that study hints at a possible reason:

"Ground beetles and harvest spiders prey on caterpillars at a higher rate in polycultures" (Dempster and Coaker 1974).

 Because of shrinking yard size and the influence of such books as Mel Bartholomew's "Square Foot Gardening", more and more backyard gardeners are cramming lots of different vegetables together in a planting bed: a polyculture.


And yet another "garden good guy" (although many think ill of this creature), the European paper wasp, is at work on hornworms, according to Dr. Whitney Crenshaw of Colorado State University:

"The European paper wasp has been huge out here in recent years. It took over in a phenomenally short period of time. In addition to the stinging/nuisance issues (complicated by its very close superficial similarity to the western yellowjacket), the insect has decimated backyard Lepidoptera. It has had such an impact that I no longer discuss cabbageworms or hornworms in Master Gardener programs – they are too rare. Also, I usually drop butterfly gardening as a topic, as they eat the butterfly larvae."


And don't forget the help tomato growers get from the ubiquitous (and hungry) backyard birds. Build it (a bird-friendly garden), and they will come. So what if you lose a few tomatoes along with the worms?