Bt is Bacillus thuringiensis, a bacterium naturally found in the soil used to control pests, is the most widely used biopesticide worldwide (1). A biopesticide is a pesticide derived from a biologically occurring, natural source, such as from an animal, plant, bacteria, or mineral. Although the use of Bt can be traced back to the early 1900s the first commercial use was in France in 1938 (1, 2) and was first registered for use in the United States by the Environmental Protection Agency in 1961 (3). B. thuringiensis is a rod-shaped bacterium that switches between normal vegetative growth and a sporulation state in which round spores are formed (4). When spores are formed, the bacterium also produces crystalline proteins (Cry family of proteins) which are toxic to certain insects (5). When the insect ingests the Bt, for example, while eating a leaf sprayed with the Bt-containing pesticide, the crystalline proteins bind to specific receptors on the epithelial (skin) cells of the insect’s gut, the cry proteins then form a pore in the cell, causing them to lyse (rupture) due to osmotic shock (6). Other organisms, such as humans, other mammals, birds, earthworms, and most other insects do not have the receptor necessary for binding and are therefore not harmed (2, 3).
Different subspecies of Bt are specific for different types of insects. For example, Bt kurstaki (Btk), the most common and easiest to find Bt, is used to control Lepidoptera (butterfly and moth) larvae such as cabbage worms, cabbage loopers, gypsy moth, tobacco hornworm, and more (2). It has no effect on beneficial insects like honeybees or ladybugs (3) but should only be applied to plants that butterflies, such as monarchs, are unlikely to visit, for example, brassicas.
Bt israelensis (Bti) is used to control mosquitos and fungus gnats (Dipterans or flies) by targeting the larvae of those insects (2) and is commercially available as Mosquito Bits or Mosquito Dunks. Mosquito Bits are in pellet form and can be sprinkled in standing water to kill mosquito larvae. I have also mixed the pellets into my plant water when I have a fungus gnat outbreak in the soil of my house plants. Simply watering your plants soaks the soil with the Bti allowing the pesticide to control the larval stage. Mosquito Dunks are larger and float on water and slowly release the Bt as they dissolve over time and can also be used in standing water.
Bt aizawai (Bta) is used to control certain moth larvae species, especially those that eat grains (6). This formulation appears available for commercial farms but not for the average homeowner to purchase.
Bt tenebrionis and Bt san diego are used to control beetles such as the Colorado potato beetle (6). This formulation also appears available for commercial farms but not for the average homeowner.
Bt is a very safe pesticide (2, 3) especially when compared to other non-organic synthetic pesticides and even some organic pesticides. Bt is also very effective against specific pests (6), but its mechanism of action does have some disadvantages (see below). Bt is also a very host-specific pesticide (2, 3, 6), particularly when used according to the label instructions, which reduces off-target effects.
Because the larvae need to eat the Bt for it to be active, plants will sustain more damage until the toxin takes effect, usually within a few days. The Bt spores are also sensitive to UV light (sunlight will break them down) (7) and may wash off the leaves, particularly after a hard rain. The typical half-life of Bt on foilage in field conditions is 1-2 days (7), therefore it is recommended that plants be sprayed weekly or following a hard rain or overhead irrigation. There is also the possibility of off-target effects however, Bt is much more host-specific than many other pesticides on the market, even other biopesticides. When used carefully on specific plants off-target effects are minimized.
One last use for Bt has been introducing the gene into the plant itself, making a transgenic crop. This eliminates the need for spraying, instead, the plant cell itself produces the insecticidal protein. The use of Bt in transgenic crops is beyond the scope of this blog post and I will likely address the topic in the future. However, in the United States, the most common Bt crops are corn and cotton, but potato and tobacco have also been modified. To my knowledge these crops are not available to the average home gardener but only to commercial farms.
There are a ton of pests that can affect your garden. I will touch on a few that have been the biggest problems for me both currently and in the past. This year I have been lucky that we had very few bug problems. However, this is most likely due to the fact that this was our first year gardening in this spot. It sometimes takes pests a year or two to find your garden. My biggest problem this year has been slugs. This may be because we had an excessive amount of rain in the early part of the growing season. Other pests that I have had problems with include potato beetles, flea beetles, squash bugs, cucumber beetles, and mice. Putting down lots of mulch or cardboard provides great benefits in weed control and retaining moisture however I have found that this increases the slug and mouse population as both tend to live under the mulch and cardboard. However, I believe the advantages of mulching outweighs any disadvantages. We set traps for the mice but insect pests can be harder to control, particularly if you try to avoid chemical control methods. I have detailed some of our biggest insect pest problems below and possible methods of control.
It can be tough to know if you have a slug problem as they are not generally out during the day so they can be hard to see. However, if you find you have lots of holes in your plants particularly cabbage, broccoli, and cauliflower then you may have the slug problem. I have also found slugs on my beans, corn, peppers, and other vegetables. To determine if you have a slug problem check out your plants at night or in the early morning particularly when it is humid out. I found hundreds of slugs coating my plants this year when I went out at night. There are many methods to get rid of slugs. One of the more popular ones is beer or yeast traps. I have tried mixing yeast with some flour and water however this did not attract any slugs. But different slugs may be attracted to different baits, including the commercially available ones. I plan on trying straight beer this week to see if that is more effective and will update this post after. The take home message is if you are using a bait try different ones until you find one that works. Other solutions include chemical control methods such as pesticides or slug pellets. However, many animals such as birds and frogs eat slugs so if you decide to go the chemical route ensure that the pesticide will not affect the animals that eat the chemically treated slugs. My favorite method, which is more labor intensive, includes handpicking the slugs from the plants or from underneath cardboard or wooden boards that you lay down around your plants. Wear gloves if you don’t like the slime! If you want to pick them directly from the plants you must go at night or early morning when the slugs are out. If you mulch or lay down cardboard or wooden boards around your plants you can simply lift up these boards and cardboard during the day to look for slugs. I have collected hundreds of them by picking them directly from the plants or from the cardboard I lay down to mulch. A bonus for us is that we use the slugs as bait when fishing in our pond!
Slugs hiding under cardboard mulch during the day.
Damage to cabbage caused by slugs
2. Potato Beetles
The Colorado potato beetle is a huge pest for potatoes and other related plants. As this was our first-year gardening in this spot we had very few pests including potato beetles however one day I found a few adult beetles on our potato plants. Unfortunately, I did not get a picture of the potato beetles but a simple internet search should show a round, striped beetle that is fairly distinctive. I simply hand-picked the beetles I saw from the plants and thought I had won the battle! However, several weeks later I noticed our plants were getting devoured by the potato beetle larva (see picture below). Likely the adult beetles had laid eggs resulting in the larva several weeks later. The potato beetle larva are rather ugly pinkish bugs that can quickly destroy your plants. Again, I simply hand-picked the larva but they did do significant damage to some of the plants before I noticed them. You can use chemical pesticides against potato beetles but if possible, I prefer to avoid chemicals. Hand-picking potato beetles is possible as long as your number of plants are small and you keep an eye on your plants throughout the season. I have also read that due to widespread use of pesticides many potato beetles have become resistant to pesticides (University of Minnesota extension), another reason not to use them.
Potato beetle larvae eating a potato plant
3. Flea Beetles
I had minimal issues with flea beetles this year however I have had significant issues with them in the past and I am sure I will see them in my garden in the near future. Flea beetles are tiny black beetles that jump (like fleas!) when you get near them. These beetles can be found on a wide range of plants but I have had the most trouble with them in leaf lettuce mixes, radishes, turnips, spinach, and other similar vegetables. Particularly arugula, mustard greens, and radishes tend to be devoured by them. If you find your plants have very tiny holes in them you may have a flea beetle problem. One of the most effective methods is to cover your plants early with row cover. Row cover is a cloth that allows light through but has tiny holes that do not allow the bugs to pass through it. However, I find row cover to be a pain because you have to cover your crops as soon as the plants emerge, before the bugs arrive, and remove it to harvest or weed. However, flea beetles are most active and do the most damage in spring, so once your plants are big enough you can remove the row cover and a little damage by the beetles generally will not affect your harvest. Another method I have tried with variable success is to plant another plant nearby such as mustard greens, which flea beetles love, to lure them away from your desired crop. However, I find that although this works, you are helping the population to grow more by feeding them extensively. You can always choose to treat the bait plant with insecticide to kill the flea beetles. These bugs are so small and so fast, this is one case where hand-picking them is not an option. A third, and I think best option, is to do much of your cold weather crop planting in the fall rather than the spring (see my earlier post on fall gardening) as flea beetles are mostly active in the spring. Therefore, I have many fewer issues with these beetles in mid to late summer and fall.
4. Cucumber Beetles
There are two types of cucumber beetles. I have mostly had trouble with the striped cucumber beetle but a spotted one also exists. Both are yellow bugs with black stripes or black spots. These beetles eat cucumber plants but also squash, melons, and other related plants. The striped beetles overwinter and emerge in early summer and can completely destroy young plants. The spotted beetles do not overwinter and so they tend to arrive in later summer and cause fewer problems. I did not have problems with either beetle population this year, likely because my garden is new and they haven’t found it yet. But, in the past, particularly in community gardens, I have had to replant my cucumbers and squash because the first round was completely eaten by these bugs. In addition to eating the leaves they may also eat the flowers and fruit. The striped beetles will also transmit bacterial wilt disease to your plants so it is important to monitor your plants for any infestations. The best way to control these beetles is with an insecticide but if possible use a “natural” insecticide that is less likely to kill good bugs such as native ladybugs that feed on aphids. Neem oil is a popular “natural” insecticide although in general it will not be as effective as a non-plant based one. Another alternative is to plant cucumbers or squash plants early in the season a couple weeks prior to planting your actual crop. Also plant these a little distance away from where you plan to plant your actual crop. These early plants will attract any beetles to them. Then hand-pick or treat these plants with insecticide to kill the beetles. Also, never apply any insecticide when your plants are flowering as this will also kill any honeybees that are attracted to your flowering plants. For more information on cucumber beetles and other pests, I have found that the University of Minnesota extension has a lot of good resources.
There are obviously many more insects that cause problems in gardens as well as bacterial and viral diseases. In the future I hope to expand more on these topics. If you have any questions or comments please feel free to contact me or comment below.
In 2016, my family and I moved from the New York City area to small town Wisconsin. Our move, this website and blog (and our previous Etsy store) is the result of our desire over the past several years to simplify our lives, increase our quality of life, reconnect with nature, and enjoy a more self-sufficient life. I grew up as a country kid in central Pennsylvania working on my grandfather's fruit farm and as a corn "de-tassler" at a local seed farm. My background is in biology where my love of nature originated. I am a former research scientist and professor and have now transitioned to a part-time stay-at-home mom, self-employed tutor, and small business owner. Thank you for taking the time to check out my site.