You already have cucumber plants in the ground? That might be part of the problem. When you plant a little on the early side, the cucumber plants are in soil that is cooler than they like and grow slower, stay a little weaker and are very vulnerable to diseases and pests because they are less vigorous. It is recommended that you wait until soil temperatures are staying at 60 degrees and nighttime temperatures are staying above 55 degrees consistently before you plant cucumbers. This may sound like a nit-picky kind of point to make, but I have found that when I put plants into the ground when the soil is too cool or the nighttime temperatures are too low for them, they struggle and pests always flock to struggling plants. Those pests are voracious eaters early in the season as they try to establish their own populations so early plants are a prime target for them.

Once you have white flies, they may be hard to control and almost impossible to eliminate. There are some things you can do but success can be elusive. They reproduce in as little as 20-30 day cycles so you have to be vigilant because about the time you eliminate the ones you have, new ones start hatching.

If the seedlings were in a greenhouse or in flats on your patio or whatever, then using sticky yellow traps to catch them might be the easiest method. It will knock down their numbers quite a bit. I don't know how effective yellow sticky traps are out in a garden. I'd be worried that they'd trap and catch too many beneficial insects in the garden so I don't use them that way.

If the cumcumber plants already are in the ground, you can scout the undersides of the leaves for the whiteflies and their eggs. Remove all you find. If a leaf is just covered in whiteflies, remove the entire leaf and dispose of it in the trash. You can try spraying the undersides of the leaves with neem oil or with a purchased insecticidal soap product. Both work somewhat well. It is just that whiteflies are so persistent that nothing organic totally eliminates them. You also can use a small handheld vacuum or a shop vac (if your extension cord is long enough to reach the garden) to suck the pests off the leaves. Then, open the shop vac away from the garden and dump the insects quickly into a bowl of soapy water to drown. Or, put a couple of inches of water with a couple of drops of soap added to it in the bottom of the canister of the wet-dry shop vacuum before you vacuum the pests off the leaves.

I plant tons of plants that have small flowers to attract beneficial insects to my garden. For the most part, the beneficial insects control the pest insects like whiteflies. Green lacewings are particularly voracious eaters and they like whiteflies. So do lady bugs. You can plant flowers like sweet alyssum, dill, yarrow, chamomile, verbena bonariensis, ammi, tansy, feverfew, catmint, veronica, catnip, lemon balm, etc. to attract beneficial insects. Some people feel like nasturtiums help repel whiteflies. I have no idea if that is true, but I plant tons of nasturtiums in my garden as companion plants and I do feel like they are helpful. You can order green lacewings and put them in your garden, but if you don't have a large enough food supply for them, they'll leave. Of course, they'll likely gobble up the whiteflies and other pests before they leave. I have had green lacewings all over the property for a good month now, so there must be a lot of something for them to eat because some years I barely see them at all, and certainly not this early. The lady bugs have been out and active very early too.

Finally, consider your cultural practices. Pest insects often attack plants that are stressed, and the cause of the stress often is something that is going on in the soil itself and which then affects the plants. Be sure you aren't feeding your plants high-nitrogen quick-release fertilizers because plants that are overfed nitrogen, especially early in the season when they are small, become magnets for pest insects. Whiteflies also are particularly attracted to plants grown in soil that suffers from magnesium deficiency or from a phosphorus deficiency.

I usually just let the beneficial insects take care of pests like whiteflies and never have much of an issue with them, so I haven't tried Spinosad on them and have no idea if it would kill them, but in general, Spinosad has been very effective on pest insects. I just don't use it much because it is a broad-spectrum pesticide so it can kill beneficial insects and pollinators.

As a last result you could try a pyrethrin product. I personally avoid using any pyrethrin product except as a drastic last result because they are more dangerous than I'd like, but they work. I keep a bottle of Take-Down Spray in my potting shed and use it so little that one bottle lasts about 5 years. It is a blend of pyrethrim and canola oil. It probably would be fairly effective on whiteflies.....at least as effective as any other control.

If you prefer a synthetic control to an organic one, someone who uses chemicals will have to tell you what works for them.

There's a couple of things I'd do if I had a persistent whitefly issue (even one that has developed only in the last year or two). I probably would use silver, reflective mulch underneath the plants (you can use the redneck version....heavy-duty alumnum foil) and as soon as the seed had emerged from the soil, I'd cover the plants with a lightweight floating row cover and I wouldn't remove it until the cucumbers were beginning to flower. That would keep the whiteflies off of the plants while they were small and more vulnerable.

Since your whitefly issue developed last year and you're already seeing them this year, they must have overwintered on some sort of plant. At the end of the season, destroy all the dead foliage where pests may hibernate or hide. You can run over the dead foliage with a lawn mower and chop it up into smaller pieces and put it in your compost pile. Build the pile and keep it going as a hot, working compost pile so the heat created by the pile will kill any pests attempting to overwinter in it, and so the heat will break down all the plant matter before spring.

Good luck with the whiteflies and with the cucumbers. At least it is very early in the season so you can start over with new plants if the current plants are too heavily infested to save.

Finally, if this is the sort of infestation that just totally ruins your garden and makes you crazy, you can buy and release whitefly predators. Normally I wouldn't even recommend that for a problem this early in the year because often the pest insects show up first, and then the predator (beneficial) insects show up to devour them a few weeks later. Buying and releasing predator insects can be expensive and I usually consider it a last resort. However, since you had whiteflies last year, you might feel like buying and releasing the whitefly predators would be worthwhile. I'll find someone who sells the whitefly predators and link them below.

Dawn

Here is a link that might be useful: Whitefly Predators at Arbico Organics

We have had a pretty good attack of white flies in our greenhouse. I have been using a combination of neem oil and sticky traps and have kept them at a small population.

here is a how to on making your own sticky traps.

Southern Agriculture sells the Sticky gel in the pest section, about $10 here.
Walmart sells a huge Styrofoam bright yellow poster board.

we cut the board into whatever size we wanted and spread the sticky gel on it. Works just like the commercial traps and I can make a ton of them cheap.

Mike

Thanks Dawn. Good news is they're not white flies... yet. I think they're leaf hoppers instead.
I rotate my plants and try to eliminate stress but the bugs were everywhere last year. I used homemade insecticidal soap trying to save the cukes, but was afraid I would kill the lacewings and ladybugs. Does tilling help prevent them the next year?
I love to use a shopvac on squash bugs then squash then in the drive way.

Thanks Dawn. Good news is they're not white flies... yet. I think they're leaf hoppers instead.
I rotate my plants and try to eliminate stress but the bugs were everywhere last year. I used homemade insecticidal soap trying to save the cukes, but was afraid I would kill the lacewings and ladybugs. Does tilling help prevent them the next year?
I love to use a shopvac on squash bugs then squash then in the drive way.

You're welcome, though I'd say leaf hoppers aren't much of an improvement over whiteflies. : )

Tilling might help if you're dealing with a pest that overwinters in the ground or in garden debris. Ideally you rototill during cold weather so the overwintering insects are exposed to temperatures that are cold enough to kill them.

I don't use insecticidal soap often and try to spray it only on the actual pest I want to kill. I just walk around with a spray bottle in my hand and spray whatever pest is making me crazy....usually it is blister beetles or grasshoppers in August. I try to tolerate most insects and just let the beneficial ones take care of the pest ones. That works most of the time. It works least well when prolonged heat and drought have combined to allow something....like blister beetles or grasshoppers....to reproduce until they reach horrifying levels. The last two summers have been great for the pests and hard on the gardeners and their gardens.

Thanks Dawn. Good news is they're not white flies... yet. I think they're leaf hoppers instead.
I rotate my plants and try to eliminate stress but the bugs were everywhere last year. I used homemade insecticidal soap trying to save the cukes, but was afraid I would kill the lacewings and ladybugs. Does tilling help prevent them the next year?
I love to use a shopvac on squash bugs then squash then in the drive way.